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《Acta Physico-Chimica Sinica》 2010-11
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Effect of Water Ligation on the Redox Potential and Infrared Spectra of Chlorophyll-a

HU Qiong WANG Guo-Ying OU Jia-Ming WANG Rui-Li (College of Physics and Electronic Information, Yunnan Normal University, Kunming 650092, P. R. China)  
In the reaction center of photosystem I the accessory electron transfer cofactors are two monomeric chlorophyll-a molecules that are ligated to two water molecules. To study the effect of water ligation on the redox potential and vibrational properties of chlorophyll-a, we built three molecular models of water ligation of chlorophyll-a based on the X-ray crystal structure of photosystem I. Then, we systematically calculated the geometries, vibrational frequencies, bond dissociation energies, and redox potentials of these models using density functional theory. The calculations were conducted in the gas phase, water, and a simulated protein environment. In addition, three different basis sets were employed to investigate the influence of the basis set on the calculation results. 15N, 2H, and 13C labeled spectra of the models in the gas phase were also calculated. Our results show that the water ligand causes the Mg ion of chlorophyll-a to move away from the center of the porphyrin ring so that the Mg—N bond lengths increase and the Mg centered angles decrease. When a nearby amino acid, asparagine (ASNB591), provides a hydrogen bond to the water that is axial ligand to the chlorophyll-a, these changes increase further. Additionally, the Mg—O bond distance decreases, the dissociation energy increases, and the redox potential also decreases. Furthermore, the redox potentials of the molecules and their bond dissociation energies decrease as the relative dielectric constant of the media and the basis sets increase. However, differences in the frequencies of the corresponding carbonyl groups and the C=C vibrations of the porphyrin ring in the three models are less than 7 cm-1, and the differences in frequency shift upon isotope labeling between the models are less than 3 cm-1. These results provide useful information for further studies of the structural and functional properties of chlorophyll-a in the photosynthetic reaction center.
【Fund】: 国家自然科学基金(10764006)~~
【CateGory Index】: O621.1
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