ESR AND ENDOR STUDIES OF PI-CATION RADICAL OF BACTERIOCHLOROPHYLL

Chemical oxidation of bacteriochlorophyll (BChl) [from Chromatium vinosum and Rhodospirillum rubrum] in nonaqueous solvents, under rigorously anaerobic and anhydrous conditions, yields a stable cation radical (BChl.+). This species, investigated by computer-assisted magnetic resonance techniques, displays, at 0.degree. C, partially resolved ESR spectra and reveals, through high power ENDOR [electron double nuclear resonance], at least 7 proton hyperfine splittings. The latter are assigned on the basis of reported molecular orbital calculations, selective deuterations and model compound studies, and the assignments are supported by reconstruction of the experimental ESR spectra of protonated and deuterated BChl.+. Predictions based on these same assignments for the overall line widths of the Gaussian singlet ESR spectra to be expected for protonated and deuterated P870.+ (the oxidized form of BChl in vivo) are also in good agreement with reported experimental values. The new data provide further support for the dimer model of BChl as the primary electron donor in bacterial photosynthesis and help define the electronic configuration of the P870 radical.