Quantum chemistry study of proteins in bacterial photosynthetic reaction center

Quantum chemistry study on proteins L and M in the Rhodopseudomononas virid is (Rh. viridis) photosynthetic reaction center (PRC) are presented. The ca lculations were performed at ab initio level with Clementi minimal basis se t by means of the overlapping-dimer approximation (ODA)-extended negative f actor counting (ENFC) method. Additional point charges were added to indivi dual residues to simulated ionized aqueous environment of the proteins in t he calculations. Meanwhile, the electronic structure of protein complex MH (protein M plus the alpha -helix segment of protein H) was studied as well to examine the weak interaction between proteins M and H. As the first case of global quantum chemistry calculation for proteins in PRC, details of th e electronic structure and the influence of proteins on the electron transf er process (ET) were studied. Moreover, new three-dimensional structure plo ts of subunit L and M were given based on the distribution of the component s of frontier orbitals in order to more clearly understand the structure-fu nction relationship of the proteins in electron transfer reactions. Calcula tion results indicated that the components of frontier orbitals are extreme ly localized at individual residues. Amino acid residues, having contribute d to the frontier orbitals of protein L, are located at the flexible random area of L, while those having contributed to the frontier orbitals of prot ein M are located at the rigid alpha -helix area. This asymmetry of protein s L, and M provides new understanding the ET reaction that takes place main ly along branch L in the PRC of Rh. viridis. Meanwhile, there is frontier o rbital localized amino acid distribution around the V-shaped pocket areas o f protein L (M) that were expected to have an important interaction with Q( A) (Q(B)) All results indicate that protein provided a heterogeneous enviro nment for pigment molecules and some important interaction between protein residues and pigment molecules are worthy of further investigation. (C) 200 1 John Wiley & Sons, Inc.