ANION AND CATION RADICALS OF BACTERIOCHLOROPHYLL AND BACTERIOPHEOPHYTIN-B - THEIR ROLE IN THE PRIMARY CHARGE SEPARATION OF RHODOPSEUDOMONAS-VIRIDIS

Optical, ESR, ENDOR, and redox characteristics of anion and cation radicals of bacteriochlorophyll b (BChl) and bacteriopheophytin b (BPh) have been obtained in nonaqueous solvents. The radicals exhibit properties similar to those of BChl a derivatives, as expected from extended Hückel and Pariser-Parr-Pople MO calculations. The electronic configurations of the radicals have been assigned on the basis of the MO calculations and by analogy with BChl a and BPh a results. Oxidized reaction centers of Rhodopseudomonas viridis do not display the √2 narrowing of the ESR line width nor the 50% decrease in ENDOR splittings expected for a symmetric cation dimer, when compared to BChl b+. Nonetheless, computer simulations indicate that a dimeric model for P960, the primary donor of R. viridis, can be rationalized by imposing a torsional angle on ring IV different from that found in crystals of Chl a, i.e., P960 may be a dimer with a twist. Comparison of the resolved ESR spectra and ENDOR splittings of the primary acceptor (I-) of R. viridis with in-vitro results indicates that I- is a monomeric anion. Optical and redox data favor BPh over BChl as the primary acceptor. These results allow a description of the electronic profile of the species which evolve within a picosecond time domain in the primary conversion of light into chemical energy at ∼1000 nm. © 1979 American Chemical Society.