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- Panas, Itai, 1959, et al.
(författare)
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The mechanism for the O2 dissociation on Ni(100)
- 1989
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 90, s. 6791-6801
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Tidskriftsartikel (refereegranskat)abstract
- The dissociation of O2 on Ni(100) has been studied using a cluster model approach. The three principally different reaction pathways, over an on‐top position, over a bridge position, and over a fourfold hollow position, were considered. The dissociation mechanisms were found to be very similar for these pathways. In the entrance channel a chemisorbed, peroxo‐form, of molecular O2 is first formed, which is strongly bound to the Ni(100) surface by two polar covalent bonds. The binding energy at the fourfold hollow site is found to be 78 kcal/mol, which is about 20 kcal/mol larger than for the other two sites, and much larger than the chemisorption energies for the experimentally observed O2 on Pt(111) and Ag(110). The reason for this difference is discussed. In a simplified valence‐bond picture the wave function of this molecularly bound O2 has a large component of a πu to πg excited state of O2. The dissociation of O2 then proceeds by two stepwise electron transfers from the surface over to the O2 3σu orbital, which completes the breaking of the O–O bond. In this latter process the energy passes over a local barrier, which is still far below the long distance asymptote, however. The local barrier height is much higher for the fourfold hollow dissociation, 35 kcal/mol over the local molecular minimum, than for the other two pathways, where the barrier height is only 6–8 kcal/mol. The 3d orbitals on nickel remain passive for all the three dissociation pathways, which is in line with the fact that also nontransition metals dissociate O2. This behavior is in contrast to the dissociation of H2 on Ni(100), where the 3d orbitals play a key role for the on‐top dissociation.
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| 2. |
- Pettersson, Gösta, et al.
(författare)
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A rapid‐equilibrium model for the control of the Calvin photosynthesis cycle by cytosolic orthophosphate
- 1987
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Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956 .- 1432-1033. ; 169:2, s. 423-429
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Tidskriftsartikel (refereegranskat)abstract
- A simple model based on rapid‐equilibrium assumptions is derived which relates the steady‐state activity of the Calvin cycle for photosynthetic carbohydrate formation in C3 plants to the kinetic properties of a single cycle enzyme (fructose bisphosphatase) and of the phosphate translocator which accounts for the export of photosynthate from the chloroplast. Depending on the kinetic interplay of these two catalysts, the model system may exhibit a single or two distinct modes of steady‐state operation, or may be unable to reach a steady state. The predictions of the model are analysed with regard to the effect of external orthophosphate on the steady‐state rate of photosynthesis in isolated chloroplasts under conditions of saturating light and CO2. Due to the possible existence of two distinct steady states, the model may account for the stimulatory as well as the inhibitory effects of external phosphate observed in experiments with intact chloroplasts. Stability arguments indicate, however, that only the steady‐state case corresponding to phosphate inhibition of the rate of photosynthesis could be of physiological interest. It is concluded that chloroplasts under physiological conditions most likely operate in a high‐velocity steady state characterized by a negative Calvin cycle flux control coefficient for the phosphate translocator. This means that any factor enhancing the export capacity of the phosphate translocator can be anticipated to decrease the actual steady‐state rate of photosynthate export due to a decreased steady‐state rate of cyelic photosynthate production.
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| 3. |
- Reslow, Mats, et al.
(författare)
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The influence of water on protease‐catalyzed peptide synthesis in acetonitrile/water mixtures
- 1988
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Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956. ; 177:2, s. 313-318
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Tidskriftsartikel (refereegranskat)abstract
- Protease‐catalyzed peptide synthesis in acetonitrile water mixtures, containing 0–90% water, was investigated. α‐Chymotrypsin, as well as thermolysin, were deposited on solid supports, prior to exposure to the reaction media. Peptide syntheses were performed using both a kinetically controlled process (chymotrypsin) and an equilibrium‐controlled synthesis (thermolysin). The activity of chymotrypsin decreased at low water contents. However, at low water contents (1–10%) hydrolytic side reactions were suppressed and high yields of dipeptides were obtained. Optimal water content for the thermolysin‐catalyzed reaction was 4–8%. The dipeptides produced were fully soluble in the reaction mixtures. High operational stability for α‐chymotrypsin was obtained during 216 h of reaction.
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