Resonance Raman studies of genetically modified reaction centers from Rhodobacter capsulatus

Abstract

Resonance Raman (RR) spectra are reported for the photosynthetic reaction center (RC) proteins from Rhodobacter capsulatus wild type and the genetically modified systems GluL104----Leu and HisM200----Leu. The spectra were obtained with a variety of excitation wavelengths, spanning the UV, violet, and yellow-green regions of the absorption spectrum, and at temperatures of 30 and 200 K. The RR data indicate that the structures of the bacteriochlorin pigments in RCs from Rb. capsulatus wild type are similar to those in RCs from Rhodobacter sphaeroides wild type. The data also show that the amino acid modifications near the primary electron acceptor (GluL104----Leu) and special pair (HisM200----Leu) perturb only those bacteriochlorin pigments near the site of the mutation and do not influence the structures of the other pigments in the RC. In the case of the GluL104----Leu mutant, elimination of the hydrogen bond to the C9 keto group of BPhL results in frequency shifts of RR bands of certain skeletal modes of the macrocycle. This allows the assignment of bands to the individual BPhL and BPhM pigments. In the case of the HisM200----Leu mutant, in which the special pair is comprised of a bacteriochlorophyll (BChl)-bacteriopheophytin (BPh) heterodimer rather than the BChl2 unit bound in the wild type, certain skeletal vibrations due to the additional BPh unit are identified. The frequencies of these modes are similar to those of the analogous vibrations BPhL and BPhM, which indicates that the structure of the BPh in the heterodimer is not unusual in any discernible way.(ABSTRACT TRUNCATED AT 250 WORDS)