| 1. |
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| 2. |
- Guo, Lei, et al.
(författare)
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Exponential Stability of General Tracking Algorithms
- 1995
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Ingår i: IEEE Transactions on Automatic Control. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9286 .- 1558-2523. ; 40:8, s. 1376-1387
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Tidskriftsartikel (refereegranskat)abstract
- Tracking and adaptation algorithms are, from a formal point of view, nonlinear systems which depend on stochastic variables in a fairly complicated way. The analysis of such algorithms is thus quite complicated. A first step is to establish the exponential stability of these systems. This is of interest in its own right and a prerequisite for the practical use of the algorithm. It is also a necessary starting point to analyze the performance in terms of tracking and adaptation because that is how close the estimated parameters are to the time-varying true ones. In this paper we establish some general conditions for the exponential stability of a wide and common class of tracking algorithms. This includes least mean squares, recursive least squares, and Kalman filter based adaptation algorithms. We show how stability of an averaged (linear and deterministic) equation and stability of the actual algorithm are linked to each other under weak conditions on the involved stochastic processes. We also give explicit conditions for exponential stability of the most common algorithms. The tracking performance of the algorithms is studied in a companion paper.
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| 3. |
- Guo, Lei, et al.
(författare)
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Necessary and Sufficient Conditions for Stability of LMS
- 1996
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In a recent work (7), some general results on exponential stability of random linear equations are established which can be applied directly to the performance analysis of a wide class of adaptive algorithms including the basic LMS ones without requiring stationarity independency and boundedness assumptions of the system signals The current paper attempts to give a complete characterization of the exponential stability of the LMS algorithms by providing a necessary and sucient condition for such a stability in the case of possibly unbounded nonstationary and non ?mixing signals The results of this paper can be applied to a very large class of signals including those generated fromeg a Gaussian process via a timevarying linear lter As an application several novel and extended results on convergence and tracking performance of LMS are derived under various assumptions Neither stationarity nor Markov chain assumptions are necessarily required in the paper
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| 4. |
- Guo, Lei, et al.
(författare)
-
Necessary and Sufficient Conditions for Stability of LMS
- 1995
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Guo and Ljung (1995) established some general results on exponential stability of random linear equations, which can be applied directly to the performance analysis of a wide class of adaptive algorithms, including the basic LMS ones, without requiring stationarity, independency, and boundedness assumptions of the system signals. The current paper attempts to give a complete characterization of the exponential stability of the LMS algorithms by providing a necessary and sufficient condition for such a stability in the case of possibly unbounded, nonstationary, and non-φ-mixing signals. The results of this paper can be applied to a very large class of signals, including those generated from, e.g., a Gaussian process via a time-varying linear filter. As an application, several novel and extended results on convergence and the tracking performance of LMS are derived under various assumptions. Neither stationarity nor Markov-chain assumptions are necessarily required in the paper.
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| 5. |
- Guo, Lei, et al.
(författare)
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Necessary and Sufficient Conditions for Stability of LMS
- 1997
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Ingår i: IEEE Transactions on Automatic Control. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9286 .- 1558-2523. ; 42:6, s. 761-770
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Tidskriftsartikel (refereegranskat)abstract
- Guo and Ljung (1995) established some general results on exponential stability of random linear equations, which can be applied directly to the performance analysis of a wide class of adaptive algorithms, including the basic LMS ones, without requiring stationarity, independency, and boundedness assumptions of the system signals. The current paper attempts to give a complete characterization of the exponential stability of the LMS algorithms by providing a necessary and sufficient condition for such a stability in the case of possibly unbounded, nonstationary, and non-φ-mixing signals. The results of this paper can be applied to a very large class of signals, including those generated from, e.g., a Gaussian process via a time-varying linear filter. As an application, several novel and extended results on convergence and the tracking performance of LMS are derived under various assumptions. Neither stationarity nor Markov-chain assumptions are necessarily required in the paper.
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| 6. |
- Guo, Lei, et al.
(författare)
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Performance Analysis of General Tracking Algorithms
- 1995
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Ingår i: IEEE Transactions on Automatic Control. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9286 .- 1558-2523. ; 40:8, s. 1388-1402
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Tidskriftsartikel (refereegranskat)abstract
- A general family of tracking algorithms for linear regression models is studied. It includes the familiar least mean square gradient approach, recursive least squares, and Kalman filter based estimators. The exact expressions for the quality of the obtained estimates are complicated. Approximate, and easy-to-use, expressions for the covariance matrix of the parameter tracking error are developed. These are applicable over the whole time interval, including the transient, and the approximation error can be explicitly calculated.
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| 7. |
- Kim, Karl, et al.
(författare)
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Modeling the causes and consequences of collisions with utility poles
- 1998
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Ingår i: Proceedings of Traffic safety on two continents. Conference in Lisbon, Portugal, September 22-24, 1997. - Linköping : Statens väg- och transportforskningsinstitut. ; , s. 153-167
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 8. |
- Knobe, Karin, et al.
(författare)
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Probing the activation of protein C by the thrombin-thrombomodulin complex using structural analysis, site-directed mutagenesis, and computer modeling
- 1999
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Ingår i: Proteins. - 0887-3585. ; 35:2, s. 218-234
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Tidskriftsartikel (refereegranskat)abstract
- Protein C (PC) is activated to an essential anticoagulant enzyme (activated PC or APC) by thrombin (T) bound to thrombomodulin (TM), a membrane receptor present on the surface of endothelial cells. The understanding of this complex biological system is in part limited due to the lack of integration of experimental and structural data. In the work presented here, we analyze the PC-T-TM pathway in the context of both types of information. First, structural analysis of the serine protease domain of PC suggests that a positively charged cluster of amino acids could be involved in the activation process. To investigate the importance of these basic amino acids, two recombinant PC mutants were constructed using computer-guided site-directed mutagenesis. The double mutant had the K62[217]N/K63[218]D substitution and in the single mutant, K86[241] was changed to S. Both mutants were activated by free thrombin at rates equivalent to that of wild-type PC (wt-PC) and they demonstrated similar calcium-dependent inhibition of their activation. The K86[241]S mutant and wt-PC were activated by thrombin bound to soluble TM at a similar rate. In contrast, the K62[217]N/ K63[218]D mutant was activated by the T-TM complex at a 10-fold lower catalytic efficiency due to a lowering in k(cat) and increase in Km. Molecular models for PC and thrombin bound to a segment of TM were developed. The experimental results and the modeling data both indicate that electrostatic interactions are of crucial importance to orient PC onto the T-TM complex. A key electropositive region centered around loops 37[191] and 60[214] of PC is defined. PC loop 37[191] is located 7-8 A from the TM epidermal growth factor (EGF) 4 while the loop 60[214] is about 10 A away from TM EGF4. Both loops are far from thrombin. A key function of TM could be to create an additional binding site for PC. The Gla domain of PC points toward the membrane and away from thrombin or the EGF modules of TM during the activation process.
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| 9. |
- Lei, Aurora Xiaomeng, 1961
(författare)
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Dynamic Characteristics of Floating Breakwaters
- 1996
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The dynamic behaviour of interconnected floating breakwaters has been studied theoretically and experimentally. For this type of shore-protection system, problems that must be dealt with include wave loads on the breakwaters caused by the interaction between the bodies and the waves, motions of the moored breakwaters caused by the waves, changed wave fields in front of and behind the breakwaters, and forces on the mooring cables and in interconnections between breakwater units. These problems have been solved in two major steps. The first step consists in solving the hydrodynamic forces in the frequency domain by means of linearized diffraction theory. The boundary value problem involved in obtaining the velocity potential on or around the bodies is solved numerically by a panel method. The second step deals with the dynamic response problem. The equation of motion is extended to include the interaction between the bodies conveyed by mechanical couplings and the interaction between a body and its mooring cables. Some significant nonlinear effects are also taken into account in the dynamic modelling, such as wave drift force, viscous damping, and drag force on the mooring cables. Owing to the facts that restoring forces imposed by the mooring cables are highly non-linear even for a small motion, and that the non-linear stiffness for a complex cable arrangement cannot be linearized by any simple method, the Newton-Raphson method is applied with an iteration technique for the resulting non-linear systems of equations. Based on the theories presented, a computer program FB (floating bodies) was developed for estimating the wave loads. An extension of the FB program has been made in conjunction with a well-developed dynamic mooring analysis program, MODEX. The program package should be of practical use for predicting the wave loads, system response, wave field near the objects, and the dynamic performance of the cables. Experiments on box-type floating breakwaters were carried out in a model basin with waves of different heights and periods. Results from model tests are presented for motions, cable tensions and wave attenuation for regular beam and oblique waves. The 3-D motion tests were performed using a camera and video processor-based measurement system, MacReflex. Data aquisition for waves and forces was carried out by another system, MP100. Results indicate that the dynamic model should prove to be a useful tool in the design of floating breakwaters and offshore structures.
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| 10. |
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| 11. |
- Lei, B., et al.
(författare)
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Nitridation study of reaction-bonded silicon nitride in situ by high temperature X-ray diffraction
- 1997
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 17:9, s. 1113-1117
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Tidskriftsartikel (refereegranskat)abstract
- The reaction-bonded silicon nitride (RBSN) nitriding process has been studied using a high temperature X-ray diffractometer (HT-XRD) under isothermal conditions in the temperature interval 1300-1400°C. With HT-XRD, the nitridation reaction and phases formed could be monitored almost instantaneously at temperature. The experimentally observed kinetics of the nitriding reaction were found to be in fair agreement with a theoretical model which predicts that the nitriding reaction occurs predominantly by Knudsen diffusion of nitrogen molecules through channels in a layer of growing Si3N4. However, no single rate law is likely to describe the whole nitridation process. Observation of the microstructure after nitridation indicates that the process occurs partly by reaction of Si vapour with nitrogen gas but that inward diffusion of nitrogen into particles also contributes significantly to the overall nitridation. The phase analysis showed that α-Si3N4 formation predominates over β-Si3N4 formation but the proportion of β-Si3N4 increases as nitridation continues
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| 12. |
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| 13. |
- Lei, B.Q., et al.
(författare)
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Mechanisms and kinetics of nitridation in silicon powder compacts
- 1995
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Ingår i: Ceramic materials and components for engines. - : World Scientific Publishing Co Pte Ltd. - 9810219059 ; , s. 189-193
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Konferensbidrag (refereegranskat)abstract
- Due to the strongly exothermic nature of silicon nitridation, a clear understanding of the nitridation process and an effective control regime, especially when the nitridation involves a rather large mass of silicon are of commercial interest. The nitridation process was investigated by reacting "large masses" of silicon powder compacts (about 1.5 kg) in a nitriding atmosphere in which some auxiliary gases such as Ar or He were added. Because of the exothermic heat, the reaction kinetics were found to be influenced by the amount of silicon in the nitridation cycle. The effect of the additional gases (Ar and He) is enhanced as the mass of silicon loaded in the furnace increases. The kinetics and the corresponding mechanisms of the whole nitridation cycle are discussed and a mechanism for the second reaction maximum is proposed. 22 refs.
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| 14. |
- Lei, Bingqiang
(författare)
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The silicon nitride reaction-bonding process
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The so called Reaction-Bonded Silicon Nitride (RBSN) materials are usually produced through the chemical reaction of silicon powder compacts in a nitriding atmosphere. This nitriding reaction is strongly exothermic and much heat is generated, which may cause to overheating, even melting of the powder compacts so to hinder the further reaction due to the reduction of specific area of the powder compacts, especially when the nitridation involves a rather large mass of silicon. The commercial process of reaction-bonded silicon nitride is usually slow and energy consuming. In order to optimise the production process and to achieve better properties of the materials obtained, mechanism of the enhanced nitridation needs to be further understood. In this thesis, a brief overview is presented of the current state of knowledge concerning the so called Reaction-Bonded Silicon Nitride (RBSN), including relevant crystallography, thermodynamics and kinetics on gas-solid reactions. The work concerns the reaction of silicon powder compacts with a nitrogen atmosphere. The kinetics and mechanisms of the nitridation process of silicon powder compacts have been studied by means of experiment and modelling. The majority of experiments have been carried out on pilot scale batches of silicon powder compacts in nitrogen environment. The XRD, HTXRD (high temperature X-ray diffraction) and SEM have been used to study the kinetics and mechanisms of the nitridation process. The experimental work has confirmed the existence of two reaction rate maxima at two different temperatures and has revealed the effects of selected process variables on these reactions, and explanation for the second reaction regions has been given in terms of possible reaction mechanism. The effect of the auxiliary gases (hydrogen, argon and helium, which were added to the nitriding atmosphere in order to minimise the overheating of the powder compacts) on the nitridation process has been discussed, relating with the sizes of the powder compact and the amount of silicon mass in the nitridation. Based on the understanding of the nitridation process, an advanced computer program that can monitor the gas pressure and temperature and can control the rate of reaction by tailoring both the supply of nitrogen gas and heating rate according to the instantaneous nitridation rate has developed. The nitridation has been investigated by high temperature X-ray diffraction and in situ information of the reaction kinetics and mechanism has been obtained. The insights obtained have formed the foundation for the creation of two theoretical models, one being a quantitative description of the solid-gas reaction as a complex combination of reaction steps, concerning the multiple reaction steps and many factors such as the changes of the specific area of the particle, the corporation of reaction and sintering and the volumetric change due to the reaction, etc.; and the other predicting the development of internal stresses arising when a reaction product forms on particle surface in a solid/gas reaction.
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| 15. |
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| 16. |
- Li, W.B., et al.
(författare)
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A kinetic model for reaction bonding process of silicon powder compact
- 1997
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 17:9, s. 1119-1129
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Tidskriftsartikel (refereegranskat)abstract
- In order to obtain detailed information about the kinetics and the reaction nature of a complex reaction process like reaction bonding of silicon nitride, mathematical modelling of the process is necessary. The previous quantitative models for this process have been based only on the mechanism that the nitrogen diffuses through the solid silicon nitride without taking into account the multiple reaction mechanisms. In the present study, a comprehensive kinetic model, which is based on analysis of the multiple mechanisms in a silicon powder compact reacting with nitrogen gas and forming silicon nitride, is constructed for a solid-gas reaction bonding process with specific application to the reaction-bonding of silicon nitride. The model will incorporate the rate equation for each mechanism into a constitutive equation from which more complete information of process kinetics can be predicted. The results predicted by the present model have been compared with previous experimental results and satisfactory agreement obtained
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| 17. |
- Li, Wen Bin, et al.
(författare)
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Stresses developed in reaction-bonded ceramics
- 1995
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Ingår i: Ceramic materials and components for engines. - : World Scientific Publishing Co Pte Ltd. - 9810219059 ; , s. 465-468
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Konferensbidrag (refereegranskat)abstract
- For a number of reaction-bonded ceramics, the reaction starts from the surface of the particles, forming a shell of products surrounding a reactant core. The reaction is continued by inward growth of the shell by means of diffusion of the reactant elements from the particle surface to the interface between the shell and core and vice versa. Since there exists a volumetric change due to the reaction, the products forming at the shell/core interface will be severely constrained. Strain and stress fields are then built up within the particles. A physical and mathematic model to calculate the strain and stress during and after the reaction bonding process is constructed. The preliminary results of estimation of stresses within the particles and their possible effect on reaction kinetics are presented and briefly discussed
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| 18. |
- Li, Wen Bin, et al.
(författare)
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Stresses developed in reaction-bonded ceramics
- 1999
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 19:3, s. 277-283
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Tidskriftsartikel (refereegranskat)abstract
- A physical model is presented that predicts the stress distribution created in a particle during its reaction with a surrounding reactant to form a uniform layer of reaction product on its surface, when the reaction involves a volume change. The results of the model are applied specifically to the case of silicon reacting with nitrogen to form Si3N4. The model predicts the generation of a high, tensile hydrostatic stress in the Si core as well as high tensile radial stress and compressive tangential stress in the nitride layer. Although the model is restricted to elastic deformation only and therefore predicts unrealistically high stresses in some cases, the results are anyway of relevance in the consideration of possible non-elastic processes such as creep and fracture and also in assessing the possible effect of stress on the reaction equilibrium. It is predicted that the nitride reaction layer would fracture during the nitridation process. A second model is also presented predicting the residual stresses arising during cooling of a partially reacted particle as a result of the difference in thermal expansion of the reactant core and the reaction product layer. In the case of the reaction of silicon to silicon nitride these thermal expansion mismatch stresses are significant but small compared to the stresses due to the chemical reaction.
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| 19. |
- Ljung, Lennart, 1946-, et al.
(författare)
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Classical Model Validation for Control Design Purposes
- 1996
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Model Validation is at the heart of the System Identification process. Recently, much renewed interest has been expressed in so called "identification for control". This means that the design variables associated with the identification process are tailored to achieve models that are well suited for control design purposes. A separate, but closely related issue is to devise validation tests that give information about the model's quality and suitability for control design. This paper shows and discusses how a basic and classical residual test gives such information.
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| 20. |
- Ljung, Lennart, 1946-, et al.
(författare)
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Classical Model Validation for Control Design Purposes
- 1997
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Ingår i: Mathematical and Computer Modelling of Dynamical Systems. - : Informa UK Limited. - 1387-3954 .- 1744-5051. ; 3:1, s. 27-42
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Tidskriftsartikel (refereegranskat)abstract
- Model Validation is at the heart of the System Identification process. Recently, much renewed interest has been expressed in so called "identification for control". This means that the design variables associated with the identification process are tailored to achieve models that are well suited for control design purposes. A separate, but closely related issue is to devise validation tests that give information about the model's quality and suitability for control design. This paper shows and discusses how a basic and classical residual test gives such information.
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| 21. |
- Ljung, Lennart, 1946-, et al.
(författare)
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Estimating the Total Model Error from Standard Model Validation Tests
- 1996
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Ingår i: Proceedings of the 13th IFAC World Congress. - 0080429106 ; , s. 133-138
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Konferensbidrag (refereegranskat)abstract
- The problem of assessing the quality of a given, or estimated model is a central issue in system identification. Classical model validation procedures are considered. We discuss the principles by which we reach confidence in a model through such validation techniques, and also how the distance to a “true” description can be estimated this way. In particular we stress how the typical model validation procedure gives a direct measure of the model error of the model test, without referring to its ensemble properties. Several model error bounds are developed for various assumptions about the disturbances entering the system.
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| 22. |
- Ljung, Lennart, 1946-, et al.
(författare)
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Estimating the Total Model Error from Standard Model Validation Tests
- 1995
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Ingår i: Proceedings of the 34th IEEE Conference on Decision and Control. - 0780326857 ; , s. 1651-1656 vol.2
-
Konferensbidrag (refereegranskat)abstract
- The problem of assessing the quality of a given, or estimated model is a central issue in system identification. Classical model validation procedures are considered. We discuss the principles by which we reach confidence in a model through such validation techniques, and also how the distance to a “true” description can be estimated this way. In particular we stress how the typical model validation procedure gives a direct measure of the model error of the model test, without referring to its ensemble properties. Several model error bounds are developed for various assumptions about the disturbances entering the system.
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| 23. |
- Ljung, Lennart, 1946-, et al.
(författare)
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The Role of Model Validation for Assessing the Size of the Unmodeled Dynamics
- 1995
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The problem of assessing the quality of a given or estimated model is a central issue in system identication Various new techniques for estimating bias and variance contributions to the model error have been suggested in the recent literature In this contribution classical model validation procedures are placed at the focus of our attention We discuss the principles by which we reach condence in a model through such validation techniques and also how the distance to a true description can be estimated this way In particular we stress how the typical model validation procedure gives a direct measure of the model error of the model test without referring to its ensemble properties Several model error bounds are developed for various assumptions about the disturbances entering the system
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| 24. |
- Ljung, Lennart, 1946-, et al.
(författare)
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The Role of Model Validation for Assessing the Size of the Unmodeled Dynamics
- 1997
-
Ingår i: IEEE Transactions on Automatic Control. - : IEEE. - 0018-9286 .- 1558-2523. ; 42:9, s. 1230-1239
-
Tidskriftsartikel (refereegranskat)abstract
- The problem of assessing the quality of a given, or estimated, model is a central issue in system identification. Various new techniques for estimating bias and variance contributions to the model error have been suggested in recent literature. In this contribution, classical model validation procedures are placed at the focus of our attention. We discuss the principles by which we reach confidence in a model through such validation techniques and also how the distance to a “true” description can be estimated this way. In particular, we stress how the typical model validation procedure gives a direct measure of the model error of the model test without referring to its ensemble properties. Several model error bounds are developed for various assumptions about the disturbances entering the system.
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| 25. |
- Ramström, Olof, et al.
(författare)
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Recent Advances in the Use of Molecularly Imprinted Materials in Separation and Synthesis
- 1998
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Ingår i: Molecular and Ionic Recognition with Imprinted Polymers. - Washington, DC : American Chemical Society. - 0097-6156. - 9780841235748 - 9780841216853 ; 703, s. 82-89
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Bokkapitel (refereegranskat)abstract
- Molecularly imprinted materials, prepared using self-assembly imprinting protocols using only non-covalent interactions, can be used as chromatography media in aqueous phase. The recognition properties are highly dependent on the interacting species used in the imprinting protocol. Ionic interactions, together with strong hydrogen bonding, represent useful means of obtaining recognition. With increasing levels of water in the recognition media, the hydrophobic effect comes into play. Molecularly imprinted materials can furthermore be used as auxiliary agents in enzymatic syntheses in water-saturated organic phases. When materials molecularly imprinted for the reaction product were applied to the thermolysin-catalyzed aspartame synthesis, an increase in yield was observed. Introduction of a thermodynamic trap provided by the molecularly imprinted matrices allowed a non-favorable equilibrium for the thermolysin reaction to be pushed in the forward direction.
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| 26. |
- Ramström, Olof, et al.
(författare)
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Screening of a combinatorial steroid library using molecularly imprinted polymers
- 1998
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Ingår i: Analytical Communications. - : Royal Society of Chemistry (RSC). - 1359-7337 .- 1364-5536. ; 35:1, s. 9-11
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Tidskriftsartikel (refereegranskat)abstract
- Molecular imprinting is an attractive method for producing highly selective adsorbents, and several new and potentially useful applications based on molecularly imprinted polymers (MIPs) have been described in recent years. In this article, we introduce a new field of application in which the selectivities of imprinted materials can be gainfully employed as binding matrices in the screening of combinatorial libraries. MIPs prepared against androstenone structures could be efficiently employed to select these compounds from a combinatorial library.
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| 27. |
- Shen, Lei
(författare)
-
Anticoagulant protein C--structural and functional studies
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Anticoagulant protein C is a vitamin K-dependent factor circulating in plasma as a zymogen of a serine protease. It is converted to its active form (APC) by the thrombin-thrombomodulin complex on the surface of endothelial cells. APC inhibits the generation of thrombin by selective proteolytic inactivation procoagulant factors Va and VIIIa. Protein S, another vitamin K-dependent protein, serves as a cofactor to APC in the down-regulation of the coagulation cascade. In the studies upon which this thesis is based, we have tried to elucidate the structure and function of the protein C molecule. We found intact factor V to be yet another cofactor to APC. Human plasma factor V was demonstrated to stimulate the inactivation of the FVIIIa-FIXa complex in the presence of APC and protein S in a purified system. Thus, factor V also manifests anticoagulant properties. The species-specific interaction of APC, protein S and factor V was studied in systems containing proteins from human and bovine origins. The results suggest the species specificity of APC to be due to the interaction between APC and protein S, but not factor V. Vitamin K-dependent proteins contain 9-13 g-carboxyglutamic acids in the Gla domain, which are crucial for membrane and calcium binding. Despite very high levels of amino acid sequence identity, naturally occurring vitamin K-dependent proteins appear to be characterized by a wide (about 1000-fold) range of membrane affinity. The recombinant bovine protein C mutant H10, and human protein C mutants, G11E32D33, Q10G11E32D33 and G11N12E32D33 were demonstrated to manifest greatly enhanced membrane affinity and anticoagulant activity, as compared to their respective wild type forms. Loop148 in the serine protease domain of bovine protein C lacks four amino acids, as compared with the corresponding loop in human protein C. Study of recombinant human protein C containing bovine loop-148 and bovine protein C with human loop-148 indicated human protein C with bovine loop-148 to manifest increased catalytic activity against small synthetic and natural substrates. This suggests loop148 to be crucially involved in the catalytic activity of the enzyme. Using a recombinant human protein C mutant (K62N/K63D), we studied the interaction of APC with protein C inhibitor (PCI) in the absence and presence of heparin. The results suggest that residues K62 and K63 not only interact directly with PCI but are also involved in heparin binding.
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| 28. |
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| 29. |
- Wei, Lei
(författare)
-
Guinea pig osteoarthrosis : morphological and biochemical studies
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hartley guinea pigs spontaneously and reproducibly develop an arthropathy mimicking human primary osteoarthrosis. The morphological lesions first appear between 6 and 12 months of age, when it is confined to the non-meniscus-covered central compartment of the medial tibial plateau without initially involving the lateral side. The knees of these animals are constitutionally in varus position, so the load is mainly carried by the medial compartment and by its non-meniscus-covered parts in particular. An early change, while the cartilage surface is still intact, was a reduction in volume density of chondrocytes in the superficial zones. Cell clustering, hypertrophy and intensive staining of the extracellular matrix were found adjacent to areas of cartilage destruction, probably indicating reparative processes. The concentration of proteoglycans in the articular cartilage was higher in areas presumably subjected to a higher load, and in healthy cartilage the concentration increased with age and body weight. The highest concentrations were found just before osteoarthrosis became morphologically evident, and the concentrations then decreased. Simultaneously, there was a reduction in collagen concentration and an increased content of tissue water. Surgical redistribution of load by femur valgus osteotomy or unilateral tibia amputation altered the natural history of the arthropathy: osteotomy induced osteoarthrosis on the lateral condyle, while it reduced the changes on the medial side. Amputation did not completely prevent the development of fibrillation. After intervention, the extent of fibrillation was correlated to the proteoglycan concentration in the tissue. Thus, the chondrocytes seem to modify the composition of the extracellular matrix to balance variations in load within a certain range. Hypothetically, with higher load, the rate of proteoglycan synthesis rise, thus increasing the swelling pressure of the tissue. After continued excessive load, the chondrocytes lost this ability. As a result of reduced proteoglycan concentration, the cartilage became less resistant to mechanical stress.
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| 30. |
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| 31. |
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| 32. |
- Ye, Lei, et al.
(författare)
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A new application of molecularly imprinted materials
- 1998
-
Ingår i: Journal of Molecular Recognition. - 0952-3499. ; 11:1-6, s. 75-78
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the possibility of shifting a thermodynamically unfavourable enzymatic equilibrium towards product formation via the addition of a highly specific adsorbent. The commercially interesting enzymatic condensation of Z-L-aspartic acid with L-phenylalanine methyl ester to the sweetener aspartame was chosen as the model system. Extremely stable and specific adsorbents for the product Z-L-Asp-L-Phe-OMe (Z-aspartame) were prepared using the emerging technique of molecular imprinting. A considerable increase (40%) in the yield of product was obtained when such adsorbents were present during the enzymatic reaction. The message of this investigation is that the use of such specific, sterilizable adsorbents should be considered for enzymatic processes to increase the yield. Finally, the direct isolation of a product formed by the retrieval of the adsorbents carrying the product can be envisaged, especially if the adsorbents are magnetic.
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| 33. |
- Ye, Lei
(författare)
-
Artificial receptors: New opportunities for the exploitation of molecularly imprinted polymers
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Molecular imprinting, method for creating artificial receptors that are mimics of biological antibodies and receptors, is based on the concept of creating substrate-specific recognition sites in polymeric matrices by means of template polymerisation. The polymeric receptors produced display favourable binding characteristics, such as high affinity and specificity towards the substrate molecules. Compared to natural antibodies and receptors, imprinted polymers are much more stable and can therefore be utilised under considerably harsher conditions. In the researches described in this thesis, non-covalent interactions are the major force driving the assembly of imprinted receptors. Given that non-covalent molecular interactions dominate in all biological recognition processes, their application in molecular imprinting is in principle unlimited. The general applicability of the non-covalent approach for the imprinting of various substrate molecules is reflected in the increasing number of publications in recent years. Non-covalent molecularly imprinted polymers have been applied in many areas, for example as stationary phases in chiral separation, as affinity adsorbents for sample pre-treatment using solid-phase extraction, as antibody mimics for drug determination, etc. Summarised in this thesis are novel applications for molecularly imprinted polymers prepared via the non-covalent approach. Artificial antibodies against corticosteroids have been prepared that exhibit binding behaviour similar to their natural counterparts. The utility of imprinted polymers for the screening of combinatorial chemical libraries is demonstrated, where the compounds of interest can be specifically isolated from libraries composed of many similar structures. As specific adsorbents, molecularly imprinted polymers are used for product purification following chemical synthesis and for in situ product removal during a biotransformation process to enhance product yield. Finally, a new imprinting methodology is developed for the production of affinity microspheres that have various applications in analytical and medical sciences.
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| 34. |
- Ye, Lei, et al.
(författare)
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Molecularly imprinted monodisperse microspheres for competitive radioassay
- 1999
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Ingår i: Analytical Communications. - : Royal Society of Chemistry (RSC). - 1359-7337 .- 1364-5536. ; 36:2, s. 35-38
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, molecularly imprinted affinity sorbents against theophylline and 17β-estradiol are synthesised via precipitation polymerisation, a synthetic method that yields monodisperse, spherical polymer particles in the micronscale range, quickly, cleanly and in good yield. The specific binding sites that are created during the imprinting process are analysed via radioligand binding analysis. The molecularly imprinted microspheres are highly specific and have higher load capacities compared to the 'classical' particles obtained by grinding the imprinted monolith. The successful imprinting against model compounds with very different hydrophobicities demonstrates the generality of the current simple approach.
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| 35. |
- Ye, Lei, et al.
(författare)
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Molecularly Imprinted Polymeric Adsorbents for Byproduct Removal
- 1998
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Ingår i: Analytical Chemistry. - : American Chemical Society (ACS). - 0003-2700 .- 1520-6882. ; 70:14, s. 2789-2795
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Tidskriftsartikel (refereegranskat)abstract
- In this study, both diastereo- and enantioselective adsorbents for a dipeptide derivative were prepared using a molecular imprinting technique. The diastereo- and enantioisomers for the dipeptide derivative N-(benzyloxy-carbonyl)aspartylphenylalanine methyl ester (ZAPM), in addition to the α- and β-isomers, were chosen as test compounds for the investigation of the imprinting effect The close similarities between the structures of different isomers make it possible to interpret the roles of template structure on specific molecular recognition. A highly specific byproduct scavenger was prepared by simultaneously incorporating methacrylic acid and vinylpyridine as functional monomers. The binding selectivities of polymeric adsorbents for the α- and β-isomers are shown to be greatly enhanced by introducing enantiocomplementarities into the polymer matrixes. An anti-β-L,L-ZAPM polymer was applied in a solid-phase extraction protocol, for the purification of the product in the chemical synthesis of N-protected aspartame. Finally, polymer beads were also imprinted against β-L,L-ZAPM using suspension polymerization performed in perfluorocarbon fluid. The imprinted polymer beads displayed the same binding characteristics as the imprinted bulk polymer and can be envisaged for the use of product purification in chromatographic mode.
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| 36. |
- Ye, Lei, et al.
(författare)
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Use of molecularly imprinted polymers in a biotransformation process
- 1999
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Ingår i: Biotechnology and Bioengineering. - 0006-3592. ; 64:6, s. 650-655
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Tidskriftsartikel (refereegranskat)abstract
- Molecularly imprinted polymers are highly stable and can be sterilised, making them ideal for use in biotransformation process. In this communication, we describe a novel application of molecularly imprinted polymers in an enzymatic reaction. The enzymatic condensation of Z-L-aspartic acid with L-phenylalanine methyl ester to give Z-L-Asp-L-Phe-OMe (Z- aspartame) was chosen as a model system to evaluate the applicability of using molecularly imprinted polymers to facilitate product formation. When the product-imprinted polymer is present, a considerable increase (40%) in product yield is obtained. Besides their use to enhance product yields, as demonstrated here, we suggest that imprinted polymers may also find use in the continuous removal of toxic compounds during biochemical reactions.
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