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11.
Xilin Ren Chingkuang Tu Deepa Bhatt J. Jefferson P. Perry John A. Tainer Diane E. Cabelli David N. Silverman 《Journal of Molecular Structure》2006,790(1-3):168-173
Incorporation of 3-fluorotyrosine and site-specific mutagenesis have been used with stopped-flow spectrophotometry and pulse radiolysis to investigate the catalytic properties of human manganese superoxide dismutase (MnSOD). All of the nine tyrosine residues in each of the four subunits of the homotetramer of human MnSOD were replaced with 3-fluorotyrosine. Previous studies showed that the crystal structures of the unfluorinated and fluorinated human MnSOD are nearly superimposable with the root-mean-square deviation for 198 -carbon atoms at 0.3 Å. However, the catalytic activity kcat/Km of the fluorinated MnSOD at 30 μM−1 s−1 was less than unfluorinated wild type at 800 μM−1 s−1. Comparison of the values of kcat/Km for fluorinated and unfluorinated wild-type andY34F MnSOD showed that this decrease for the fluorinated enzyme was in significant part due to 3-fluorotyrosine residues distant (>7 Å) from the active-site metal, not to 3-fluorotyrosine at position 34 close (5 Å) to the metal. Although many rate constants for the catalysis are decreased by this fluorination, the rate of dissociation of the product-inhibited complex appears unchanged by the presence of fluorinated tyrosines. These results suggest that Tyr34 is not a proton donor in the release of the product-inhibited complex, which involves protonation of a peroxo complex of the metal with release of hydrogen peroxide. 相似文献
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Dr. David Lindsay Professor Egon Matijević Professor Josip P. Kratohvil 《Colloid and polymer science》1976,254(4):421-428
Summary The effects of dextran sulfates of different molecular weights and charge densities on the stability of positively and negatively charged silver bromide solsin statu nascendi have been studied by means of light scattering.The plots of scattering intensities of a positively charged sol against the concentration of the added dextran sulfates show one pronounced maximum, the position which is independent on the molecular weight of the polyelectrolyte used. The maximum shifts to lower polyion concentrations with dilution of the sol.The effect of polyanions on the stability of silver bromide sols of the same sign of charge were investigated as a function of molecular weight of the polymers, the sol concentration, the charge and the concentration of counterions, and the addition of methanol. Destabilization only occurred if sufficient amount of indifferent electrolyte was present in the system. However, the concentration of counterions necessary to produce a flocculation maximum was lower in the presence of the polyelectrolytes than in their absence. The results obtained were compared with the data on sols containing nonionic dextrans reported earlier.
5 figures and 1 table
Supported by the NSF Grant GP 42331 X.
A part of the Ph.D. Thesis by David Lindsay. 相似文献
Zusammenfassung Der Einfluß der Dextransulfate von verschiedenen Molekulargewichten und Ladungsdichten auf die Stabilität der positiv und negativ geladenen Silberbromid-Solein statu nascendi wurden durch Lichtstreuung untersucht. Wenn die Intensitäten der Lichtstreuung eines positiv geladenen Silberbromid-Sols gegen die Konzentration der zugesetzten DextranSulfate aufgetragen werden, tritt ein ausgeprägtes Maximum auf, dessen Lage von dem Molekulargewicht des angewandten Polyelektrolyten unabhängig ist. Bei Verdünnung des Sols wird das Maximum zur niedrigeren Konzentration des Polyanions verschoben.Der Einfluß der Polyanionen auf die Stabilität der Silberbromid-Sole mit gleichem Ladungsvorzeichen wurde in Abhängigkeit von dem Molekulargewicht des zugesetzten Dextransulfats, der Solkonzentration, der Ladung und Konzentration der Gegenionen und dem Zusatz von Methylalkohol untersucht. Destabilisierung wird nur in Anwesenheit einer genügenden Menge von Neutralelektrolyten in den untersuchten Systemen beobachtet. Die Konzentration der Gegenionen, die ein Flockungsmaximum verursachen, ist immer niedriger als die kritische Koagulationsmenge derselben Ionen für das gleiche Sol in Abwesenheit von Makroionen. Die Resultate wurden mit den Wechselwirkungen der Silberbromid-Sole mit nichtionogenen Dextranen verglichen, und der Mechanismus der Flockung durch Polyelektrolyte wurde diskutiert.
5 figures and 1 table
Supported by the NSF Grant GP 42331 X.
A part of the Ph.D. Thesis by David Lindsay. 相似文献
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Sanders L Halder M Xiao TL Ding J Armstrong DW Petrich JW 《Photochemistry and photobiology》2005,81(1):183-186
We report the first separation of the enantiomers of hypericin. Their steady-state optical spectra and ultrafast primary photoprocesses are investigated in chiral environments. Within experimental error, there is no difference between the two enantiomers in any of the systems considered. This is consistent with the emerging picture that the rich and extended absorption spectrum of hypericin is not a result of ground-state heterogeneity. It is also consistent with the observation that the spectra and photophysics of hypericin are generally insensitive to environments in which it does not aggregate. 相似文献
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Rueda D Boyarkin OV Rizzo TR Chirokolava A Perry DS 《The Journal of chemical physics》2005,122(4):44314
Spectra of jet-cooled methanol in the overtone and combination region from 5000 to 14 000 cm(-1) have been obtained by means of infrared laser-assisted photofragment spectroscopy. Many of the observed features are assigned to combination bands of the type nnu(1)+nu(6), nnu(1)+nu(8), and nnu(1)+nu(6)+nu(8) (n=1,2,3), where nu(1) is the OH stretch, nu(6) is the OH bend, and nu(8) is the CO stretch. These bands show sharp torsion-rotation structure with features as narrow as 0.1 cm(-1). We also observe CH stretch overtones that are weaker than the OH containing combination bands and lack distinct torsion-rotation structure above v(CH)=2. The extent of observed structure on these bands allows us to place limits on the intramolecular vibrational energy redistribution decay rates in the upper vibrational states. We report a global fit of the observed band centers to a simple expression involving low-order anharmonicity constants. 相似文献
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