| 1. |
- Zhang, Fan, 1983-
(författare)
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The Mussel Adhesive Protein (Mefp-1) : A GREEN Corrosion Inhibitor
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Corrosion of metallic materials is a natural process, and our study shows that even in an alkaline environment severe corrosion may occur on a carbon steel surface. While corrosion cannot be stopped it can be retarded. Many of the traditional anti-corrosion approaches such as the chromate process are effective but hazardous to the environment and human health.Mefp-1, a protein derived from blue mussel byssus, is well known for its extraordinary adhesion and film forming properties. Moreover, it has been reported that Mefp-1 confers a certain corrosion protection for stainless steel. All these facts indicate that this protein may be developed into corrosion inhibitors with ‘green’, ‘effective’ and ‘smart’ properties.In this study, a range of surface-sensitive techniques have been used to investigate adsorption kinetics, film forming and film compaction mechanisms of Mefp-1. In situ atomic force microscopy (AFM) enables the protein adsorption on substrates to be visualized, whereas the ex situ AFM facilitates the characterization of micro- and nano-structures of the protein films. In situ Peak Force AFM can be used to determine nano-mechanical properties of the surface layers. The quartz crystal microbalance with dissipation monitoring (QCM-D) was used to reveal the build-up of the Mefp-1 film on substrates and measure the viscoelastic properties of the adsorbed film. Analytical techniques and theoretical calculations were applied to gain insights into the formation and compaction processes such as oxidation and complexation of pre-formed Mefp-1 films. The electron probe micro analyzer (EPMA) and X-ray photoelectron spectroscopy (XPS) were utilized to obtain the chemical composition of the surface layer. Electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the corrosion inhibition efficiency of different forms of Mefp-1 on carbon steel substrates.The results demonstrate that Mefp-1 adsorbs on carbon steel surfaces across a broad pH interval, and it forms a continuous film covering the substrate providing a certain extent of corrosion protection. At a higher pH, the adsorption is faster and the formed film is more compact. At neutral pH, results on the iron substrate suggest an initially fast adsorption, with the molecules oriented preferentially parallel to the surface, followed by a structural change within the film leading to molecules extending towards solution. Both oxidation and complexation of the Mefp-1 can lead to the compaction of the protein films. Addition of Fe3+ induces a transition from an extended and soft protein layer to a denser and stiffer one by enhancing the formation of tri-Fe3+/catechol complexes in the surface film, leading to water removal and film compaction. Exposure to a NaIO4 solution results in the cross-linking of Mefp-1, which also results in a significant compaction of the pre-formed protein film. Mefp-1 is an effective corrosion inhibitor for carbon steel when added to an acidic solution, and the inhibition efficiency increases with time. As a film-forming corrosion inhibitor, the pre-formed Mefp-1 film provides a certain level of corrosion protection for short term applications, and the protection efficiency can be significantly enhanced by the film compaction processes.For the long term applications, a thin film composed of Mefp-1 and ceria nanoparticles was developed. The deposited Mefp-1/ceria composite film contains micro-sized aggregates of Mefp-1/Fe3+ complexes and CeO2 particles. The Mefp-1/ceria film may promote the further oxidation of ferrous oxides, and the corrosion resistance increases with time. Moreover, phosphate ions react with Fe ions released from the surface and form deposits preferentially at the surface defect sites. The deposits incorporate into the Mefp-1/ceria composite film and heal the surface defects, which result in a significantly improved corrosion inhibition effect for the Mefp-1/ceria composite film in both initial and prolonged exposure situations
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| 2. |
- Zhang, Jingji, 1983-
(författare)
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Accuracy of mRNA Translation in Bacterial Protein Synthesis
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Reading of messenger RNA (mRNA) by aminoacyl-tRNAs (aa-tRNAs) on the ribosomes in the bacterial cell occurs with high accuracy. It follows from the physical chemistry of enzymatic reactions that there must be a trade-off between rate and accuracy of initial tRNA selection in protein synthesis: when the current accuracy, the A-value, approaches its maximal possible value, the d-value, the kinetic efficiency of the reaction approaches zero. We have used an in vitro system for mRNA translation with purified E. coli components to estimate the d- and A-values by which aa-tRNAs discriminate between their cognate and near cognate codons displayed in the ribosomal A site. In the case of tRNALys, we verified the prediction of a linear trade-off between kinetic efficiency of cognate codon reading and the accuracy of codon selection. These experiments have been extended to a larger set of tRNAs, including tRNAPhe, tRNAGlu, tRNAHis, tRNACys, tRNAAsp and tRNATyr, and linear efficiency-accuracy trade-off was observed in all cases. Similar to tRNALys, tRNAPhe discriminated with higher accuracy against a particular mismatch in the second than in the first codon position. Remarkably high d-values were observed for tRNAGlu discrimination against a C-C mismatch in the first codon position (70 000) and for tRNAPhe discrimination against an A-G mismatch in the second codon position (79 000). At the same time, we have found a remarkably small d-value (200) for tRNAGlu misreading G in the middle position of the codon (U-G mismatch).Aminoglycoside antibiotics induce large codon reading errors by tRNAs. We have studied the mechanism of aminoglycoside action and found that the drug stabilized aminoacyl-tRNA in a codon selective in relation to a codon non-selective state. This greatly enhanced the probability of near cognate aminoacyl-tRNAs to successfully transcend the initial selection step of the translating ribosome. We showed that Mg2+ ions, in contrast, favour codon non-selective states and thus induce errors in a principally different way than aminoglycosides. We also designed experiments to estimate the overall accuracy of peptide bond formation with, including initial selection accuracy and proofreading of tRNAs after GTP hydrolysis on EF-Tu. Our experiments have now made it possible to calibrate the accuracy of tRNA selection in the test tube to that in the living cells. We will now also be able to investigate the degree to which the accuracy of tRNA selection has been optimized for maximal fitness.
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| 3. |
- Zhang, Shi-Jin, 1957-
(författare)
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Regulation of Intracellular Calcium in Brown Adipocytes
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Intracellular Ca2+ is considered a primary regulator of cell function. In the present study, the control and the effects of intracellular Ca2+ in brown adipocytes have been investigated. Cytosolic Ca2+ levels ([Ca2+]i) are the resultant of the activity of Ca2+ transport systems. Results concerning Ca2+transport systems in the plasma membrane, endoplasmic reticulum and mitochondria are presented.[Ca2+]i, monitored with Fura-2/AM, is increased when brown adipocytes are stimulated with norepinephrine (NE). The NE effect is mediated via a1-adrenoceptors and involves both release from intracellular Ca2+ stores and influx of extracellular Ca2+. The NE-induced [Ca2+]i response could be desensitized by pretreatment with NE. The desensitization is also mediated by a1-receptors and intracellularly by increased [Ca2+]i and calmodulin but not by protein kinase C. The kinetics of the desensitization are similar to those of inhibition of protein synthesis or transcription and the desensitization is associated with a comparable decrease in the number of a1-receptors.Mitochondrial Ca2+ levels ([Ca2+]m) were monitored within brown adipocytes with mitochondrially targeted aequorin. [Ca2+]m was not a simple reflection of [Ca2+]i; rather, evidence is presented for the existence of a b-adrenergic, cAMP-mediated signal that augments the [Ca2+]m/[Ca2+]i ratio. This signal causes the mitochondria to sequester Ca2+ even in the absence of increased cytosolic levels. Inhibition of mitochondrial Ca2+ uptake augments the cytosolic responses. Mitochondria may thus play an important role even in cytosolic Ca2+homeostasis in brown adipocytes.Chronic treatment of brown adipocytes with NE resulted in marked alterations of cytosolic Ca2+ handling, but the mitochondria retained their ability to sequester Ca2+during adrenergic stimulation, i.e. under conditions when UCP1 should be active.The effects of an increase in [Ca2+]i involve activation of a cAMP phosphodiesterase, and the presence of this component explains the unusual kinetic characteristics of norepinephrine-induced cAMP accumulation. [Ca2+]i is also involved in the regulation of gene expression: increased [Ca2+]i interacts synergistically with cAMP in the control of c-fos expression which may be of significance for regulation of cell proliferation and differentiation.It was concluded that Ca2+ is a primary regulator of physiological functions in brown adipocytes. The Ca2+ transport systems in brown adipocytes are involved in the regulation of intracellular and intraorganellar Ca2+. Changes of the free cytosolic Ca2+ concentration by hormone stimulation induces the activation of many physiological processes.
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