Relationship between altered structure and photochemistry in mutant reaction centers in which bacteriochlorophyll replaces the photoactive bacteriopheophytin

Q(y)-excitation resonance Raman (RR) spectra are reported for two mutant re action centers (RCs) from Rhodobacter capsulatus in which the photoactive b acteriopheophytin (BPhL) is replaced by a bacteriochlorophyll (BChl) molecu le, designated beta. The pigment change in both mutants is induced via intr oduction of a histidine residue near the photoactive cofactor. In one mutan t, L(M212)H, the histidine is positioned over the core of the cofactor and serves as an axial ligand to the Mg+2 ion. In the other mutant, F(L121)H/F( L97)V, the histidine is positioned over ring V of the cofactor, which is no minally too distant to permit bonding to the Mg+2 ion. The salient observat ions are as follows: (1) The beta cofactor in F(L121)H/F(L97)V RCs is a fiv e-coordinate BChl molecule. However, there is no evidence for the formation of a Mg-His bond. This bond is either much weaker than in the L(M212)H RCs or completely absent, the latter implying coordination by an alternative L igand. The different axial ligation for beta in the F(L121)H/F(L97)V versus L(M212)H RCs in turn leads to different conformations of the BChl macrocyc les. (2) The C-9-keto group of beta in F(L121)H/F(L97)V RCs is free of hydr ogen bonding interactions, unlike the L(M212)H RCs in which the C-9-keto of beta is hydrogen bonded to Glu L104. The interactions between other periph eral substituents of beta and the protein are also different in the F(L121) H/F(L97)V RCs versus L(M212)H RCs. Accordingly, the position and orientatio n of beta in the protein is different in the two beta-containing RCs. Nonet heless, previous studies have shown that the primacy electron transfer reac tions are very similar in the two mutants but differ in significant respect s compared to wild-type RCs. Collectively, these observations indicate that changes in the conformation of a photoactive tetrapyrrole macrocycle or it s interactions with the protein do not necessarily lead to significantly pe rturbed photochemistry and do not underlie the alter-ed primary events in b eta-type RCs. (C) 1999 John Wiley & Sons, Inc.