Changes in Carbon-Carbon and Carbon-Nitrogen Stretching Force Constants in the Macrocycles of Bacteriochlorophyll a and Bacteriopheophytin a upon Triplet and Singlet Excitation: Resonance-Raman Spectroscopy and Normal-Coordinate Analysis of the Unlab

摘要

The Raman spectra of bacteriochlorophyll a and bacteriopheophytin a in the S_0, T_1 states were recorded for the unlabeled species and the totally-labeled species with the ~(15)N, ~(13)C, and ~2H isotopes, and an empirical normal-coordinate analysis of the Raman data was performed to establish the assignments of Raman lines in the 1650-1200 cm~(-1) region and to determine the stretching force constants of the carbon-carbon and the carbon-nitrogen bonds within the macrocycle. By the use of the stretching force constants as a scale, changes in bond order upon triplet and singlet excitation were characterized in both BChl a and BPhe a as follows. (a) For BChl a, upon triplet excitation, the carbon-carbon bonds in the methine bridge (C_a-C_m) and those in the pyrrole rings I and III (C_a-C_b and C_B-C_b) decrease in bond order, whereas the carbon-nitrogen bonds in all the pyrrole rings (C_a-N) and the carbon-carbon bonds in rings II and IV (C_(a')-C_(b')) increase in bond order. Upon singlet excitation, the C_b-C_b bonds substantially increase in bond order at the expense of decrease in the bond orders of the C_a-C_m and C_a-C_b bonds; the C_(a')-C_(b') and the C_a-N bonds also increase in bond order as in the case of triplet excitation. (b) For BPhe a, upon triplet excitation, changes in bond order similar to those in the case of BChl a take place. Upon singlet excitation, the C_b-C_b bonds drastically increase in bond order at the expense of decrease in the bond orders of the C_a-C_m, C_a-C_b, and C_a-N bonds. Change in the stretching force constant of each bond, upon singlet or triplet excitation, varies in magnitude depending on its location in the macrocycle, a fact which suggests that the HOMO and the LUMO are not completely delocalized. Possible relevance of this result to the pigment arrangements in the bacterial photoreaction center is discussed.