LINEAR DICHROISM OF MEMBRANE-BOUND REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES - ALTERATIONS OF THE B-BAND INDUCED BY SITE-SPECIFIC MUTATIONS

Low temperature absorption and linear dichroism (LD) measurements were performed on oriented membranes containing wild type Rhodobacter spha eroides reaction centers, a mutant reaction center with the change Phe M197 to Arg (FM197R), and a double mutant reaction center where, in a ddition, Gly M203 was replaced by Asp (FM197R/GM203D). The monomeric b acteriochlorophyll band (B), which is highly congested in the wild typ e reaction center, was separated into two bands in the mutant reaction centers peaking 10 nm (single mutant) or 15 nm (double mutant) apart. This separation arose principally from changes in the interaction of the protein with the L-side monomer bacteriochlorophyll B-L. The abili ty to separate the B bands is extremely useful in spectroscopic studie s. The orientations of the two monomer-type transitions contributing t o the B band were similar in all three reaction centres studied, and w ere asymmetric with respect to the orientation axis, with the transiti on mostly associated with B-L making a smaller angle with the C-2 axis . Differences in the LD observed in wild type membrane-bound or isolat ed reaction centers can be ascribed either to differences in shifts of the B transitions or to differences in the orientation axis.