Altun Ahmet, Thiel Walter
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany.
J Phys Chem B. 2005 Jan 27;109(3):1268-80. doi: 10.1021/jp0459108.
The pentacoordinated ferric and ferrous cytochrome P450(cam) complexes have been investigated by combined quantum mechanical/molecular mechanical (QM/MM) calculations in the presence of a protein/solvent environment and by QM calculations on the isolated QM regions with use of density functional theory. The B3LYP functional has been found more reliable than the BLYP and BHLYP functionals for estimating the relative state energies. The B3LYP/CHARMM calculations with an all-electron basis set for iron give high-spin ground states for the title complexes, in agreement with experiment. The comparison of the B3LYP/CHARMM results of the entire protein system with the B3LYP calculations on the naked QM regions shows that the amount of stabilization by the protein environment is largest for the intermediate-spin states, followed by the high-spin states of the complexes. The calculation of Mössbauer parameters in the presence of the enzyme environment confirms the double occupation of the d(xz) orbital in the quintet spin state of the ferrous complex, consistent with the computed QM/MM energies in the enzyme environment, while the d(x)2(-)(y)2 orbital is doubly occupied in the gas-phase quintet state.
通过在蛋白质/溶剂环境中进行的组合量子力学/分子力学（QM/MM）计算以及使用密度泛函理论对孤立QM区域进行的QM计算，研究了五配位铁和亚铁细胞色素P450（cam）配合物。已发现B3LYP泛函在估计相对态能量方面比BLYP和BHLYP泛函更可靠。使用铁的全电子基组进行的B3LYP/CHARMM计算给出了标题配合物的高自旋基态，与实验结果一致。将整个蛋白质系统的B3LYP/CHARMM结果与裸QM区域上的B3LYP计算结果进行比较表明，蛋白质环境的稳定化程度对于中间自旋态最大，其次是配合物的高自旋态。在酶环境存在下对穆斯堡尔参数的计算证实了亚铁配合物五重态自旋态中d（xz）轨道的双重占据，这与酶环境中计算出的QM/MM能量一致，而在气相五重态中d（x）2（-）（y）2轨道被双重占据。
