IDENTIFICATION OF THE UPPER EXCITON COMPONENT OF THE B850 BACTERIOCHLOROPHYLLS OF THE LH2 ANTENNA COMPLEX, USING A B800-FREE MUTANT OF RHODOBACTER-SPHAEROIDES

In this paper, we report the circular dichroism (CD) spectra of two ty pes of LH2-only mutants of Rhodobacter sphaeroides. In the first, only the wild type LH2 is present, while in the second, the B800 binding s ite of LH2 has been either destabilized or removed. For the first time , we have identified a band in the CD spectrum of LH2, located at simi lar to 780 nm, that can be ascribed to the high exciton component of t he B850 band. The experimental spectra have been modeled by theoretica l calculations. On this basis, the average interaction strength betwee n monomers in the B850 ring can be estimated to be approximately 300 c m(-1). In addition, we suggest that in LH2 of Rb. sphaeroides the angl es made by the Q(y) transitions of the B850 BChls with respect to the plane of the ring are slightly different from those calculated from th e crystal structure of the Rhodopseudomonas acidophila LH2 complex.