Structural and Spectroscopic Properties of Mg−Bacteriochlorin and Methyl Bacteriochlorophyllidesa,b,g, andhStudied by Semiempirical, ab Initio, and Density Functional Molecular Orbital Methods

Abstract

Ab initio HF/6-31G* and density functional B3LYP/6-31G* methods have been used to calculate fully optimized structures of methyl bacteriochlorophyllides a, b, g, and h and magnesium-bacteriochlorin. Semiempirical ZINDO/S CIS and ab initio CIS/6-31G* and CIS/6-311G** configuration interaction methods and time dependent HF/6-31G*, HF/6-311G**, B3LYP/6-31G*, and B3LYP/6-311G** methods were used to estimate corresponding spectroscopic transition energies of the chromophores. The effects of solvent coordination were also studied by optimizing structures of 1:1 complexes of the methyl bacteriochlorophyllides and acetone. The self-consistent reaction field model was used to estimate bulk solvent effects. Differences in B3LYP and HF bond lengths of the bacteriochlorin had a strong influence on the calculated transition energies. Large variations of calculated transition energies were also observed when coordinates from different X-ray structure determinations were used for the same pigment. In the five coordinated solvent complex, the Mg atom is shifted from the bacteriochlorin plane, inducing red shifts of the Qx and Soret transitions. Linear correlations of the calculated and experimental solution transition energies were obtained with characteristic slopes and intercepts for each method used, reflecting inadequacy of the methods to describe transition energies in bacteriochlorins. Such correlations were shown to be useful in prediction of site transition energies for a pigment (pigment group) in solution or in protein under a given computational approach. Best correlations and the best calculated transition energies were obtained by using the ZINDO/S CIS method with B3LYP/6-31G* optimized structures. Calculations suggested the existence of a number of dark electronic states of bacteriochlorophylls below the main Soret transition. The density of these states was dependent on pigment surroundings. Dark states may have an important role in carotenoid to chlorophyll or to bacteriochlorophyll energy transfer in photosynthetic light harvesting complexes. It was also shown that conformation of an acetyl group of methyl bacteriochlorophyll a has an effect on calculated transition energies.