THE TRIPLET-STATE OF THE PRIMARY DONOR IN REACTION CENTERS OF THE HL(L173) AND HL(M202) HETERODIMER MUTANTS OF RHODOBACTER-SPHAEROIDES

The triplet state of reaction centers of the HL(L173) and HL(M202) mut ants of Rhodobacter sphaeroides, whose primary donor is a bacteriochlo rophyll-bacteriopheophytin (BChl-BPh) heterodimer instead of a BChl-BC hl homodimer, has been studied with absorbance-detected magnetic reson ance in zero-magnetic field, The zero-field splitting parameters of th e triplet state of the two heterodimers are close to those of monomeri c (3)BChl alpha, indicating that the triplet state is largely localize d on the BChl-half of the heterodimers, but not identical, demonstrati ng that the two BChls of the native primary donor are inequivalent. Th e microwave-induced triplet-minus-singlet absorbance-difference spectr a of the two mutant reaction centers differ in the Q(Y)-absorption reg ion of the BChls that are located close to the dimer. The latter diffe rence is attributed to differences in interaction between the dimer-BC hls and the two adjacent BChls for the two mutants.