Effects of Axial Coordination, Electronic Excitation and Oxidation on Bond Orders in the Bacteriochlorin Macrocycle, and Generation of Radical Cation on Photo- Excitation of in vitro and in vivo Bacteriochlorophyll a Aggregates: Resonance Raman Studies

Abstract

Solutions of bacteriochlorophyll (BChl) a in both monomeric and aggregated forms, and also in solutions of pigment-protein complexes, were examined by resonance-Raman spectroscopy. The results permit us to report and discuss: (1) The effects of axial coordination forming the 5- and 6-coordinated states as well as those of triplet excitation and one-electron oxidation forming the T1 (lowest triplet) and the D0 (radical-cation) states on bond orders in the macrocycle of BChl a, as probed by the ring-breathing frequency. A core-expansion mechanism is proposed by which the ring-breathing frequency reflects the coordination states in the S0, T1 and D0 electronic states. Solvent parameters determining the relative stabilities of the 5- and 6-coordinated states are also discussed. (2) Changes in bond order in the macrocycles of BChl a and bacteriopheophytin a upon photo-excitation to the T1 and S1 (the first-excited singlet) states as determined by changes in stretching force constants that were obtained by normal-coordinate analysis of the Raman spectra of the unlabeled and totally 15N-, 13C- and 2H-labeled species. Based on the results, the implications of the pigment arrangement in the reaction center, facilitating electron transfer and triplet energy-transfer reactions, are discussed in terms of the partial localization of the HOMO and LUMO (the highest occupied and the lowest unoccupied molecular orbitals) that were identified as large changes in bond order. (3) The generation of the T1 state and subsequent transformation into the D0 state on photo-excitation of BChl a aggregates, both in solution and in the carotenoid-less light harvesting complexes, LH1 and LH2. These have been identified by the use of the key ring-breathing Raman lines in the T1 and D0 states, and then, confirmed by electronic-absorption and EPR spectroscopy.