RESONANCE RAMAN-SPECTRA OF THE ANION AND CATION RADICALS OF BACTERIALPHOTOSYNTHETIC PIGMENTS

Resonance Raman (RR) spectra are reported for the radical ions of the bacterial photosynthetic pigments bacteriochlorophyll a (BCh) and its metal-free analog bacteriopheophytin a (BPh). The radical anions, BCh( -) and BPh(-) were the primary focus of the RR study; however, the rad ical cation, BCh(+), was also examined. The RR data for all the radica l species were acquired by using a variety of excitation wavelengths i n the UV-violet region. Data were also obtained for BCh(+) and BCh(-) in solvents in which the Mg(II) ion axially coordinates either one or two solvent molecules. The RR data for the radical ions suggest that o xidation/reduction results in characteristic frequency shifts for the carbonyl and ring skeletal modes of the bacteriochlorin macrocycles. T hese frequency shifts provide benchmarks for the characterization of t ransient radicals in vivo via time-resolved vibrational spectroscopy. The oxidation/reduction-induced frequency shifts observed for BPh and BCh are indicative of significant structural and/or electronic perturb ations of the bacteriochlorin macrocycle. The C-9=0 and the C-2a=O gro ups appear to be the key structural elements in these perturbations. T he RR data further suggest that oxidation/reduction alters the forms o f the vibrational eigenvectors of the carbonyl and ring skeletal modes . This in turn suggests that the relationship between the vibrational frequency shifts and structural and/or electronic perturbations is gov erned by the interplay of a number of subtle factors. Comparison of th e RR data for the radical species in solution with those obtained from reaction center proteins suggests that the pigment-protein interactio ns may be significantly affected by cation/anion formation. Collective ly, the RR data suggest that oxidation/reduction-induced changes in pi gment-protein interactions could provide a means of mediating the redo x properties of BCh and BPh in vivo.