水分子配位对叶绿素a氧化还原势与红外光谱的影响

以光系统I反应中心电子传递链上的辅助叶绿素a为目标,采用密度泛函理论中的B3LYP方法,结合三种基组,系统计算了该叶绿素及其两种配位分子模型在气相、模拟蛋白质环境和水中的氧化还原势和离解能;同时计算了这三种模型在气相中的几何结构、红外光谱及其13C、15N和2H的同位素标记谱.计算及分析结果表明:水分子配位引起镁离子偏离叶绿素a的卟啉环平面中央,导致以镁原子为中心的键角减小,Mg—N键长增长;而天冬酰胺对配位的水分子施加氢键影响后,使得Mg—N键进一步增长,镁离子与水分子中氧原子的配位键Mg—O键长减小,离解能增加,合成分子的氧化还原势减少;另外,分子的氧化还原势和配位键离解能随着相对介电常数的增加以及计算基组的增大而减小;三种分子模型的羰基(C襒O)和卟啉环上C襒C键的特征振动频率差值小于7cm-1,而同位素标记引起其峰位变化量的差值小于3cm-1.该计算为研究光合反应中心电子传递链上叶绿素a的作用与功能提供理论参考依据.

Effect of Water Ligation on the Redox Potential and Infrared Spectra of Chlorophyll-a

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 fromthe 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.