Effects of Nonpolar and Polar Solvents on the QX and QY Energies of Bacteriochlorophyll a and Bacteriopheophytin a

Abstract

The effects of nonpolar and polar solvents on the Qx and Qy energies of bacteriochlorophyll (BChl) a and bacteriopheophytin (BPhe) a were examined by electronic absorption spectroscopy. All of the four different energies exhibited a linear dependence on R(n) = (n2 - 1)/(n2 + 2), where n is the refractive index of the solvent, in both nonpolar and polar solvents. The energy of each state of both pigments could be expressed as v = -dR(n) + e (in cm-1) where coefficient d was related to the dispersive interaction between the solute and the solvent molecules. A theory developed by Nagae showed that coefficient d originates from the quantum-mechanical fluctuation of the multipole moments of the solute, in terms of which the following characteristics of the observed d values were explained: (1) In all of the four cases of the Qx and Qy energies of both BChl a and BPhe a, the d values for the polar solvents were smaller than those for the nonpolar solvents. (2) In both nonpolar and polar solvents, the d value of BChl a was larger than that of BPhe a in the Qy energy, whereas the d value of BPhe a was larger than that of BChl a in the Qx energy. (3) The d value of the Qx energy was larger than that of the Qy energy for either case of BChl a or BPhe a.