Effects of nonpolar and polar solvents on the Q(x) and Q(y) energies of bacteriochlorophyll a and bacteriopheophytin a

The effects of nonpolar and polar solvents on the Q(x) and Q(y) 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) = (n(2) - 1)/(n(2) + 2), where a 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 upsilon = -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 Q(x) and Q(y) 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 Q(y) energy, whereas the d value of BPhe a was larger than that of BChl a in the Q(x) energy, (3) The d value of the Q(x) energy was larger than that of the Q(y) energy for either case of BChl a or BPhe a.