SPECIFIC SOLVATION OF CHLOROPHYLL a: SOLVENT NUCLEOPHILITY, HYDROGEN BONDING AND STERIC EFFECTS ON ABSORPTION SPECTRA

Abstract— The characteristics of chlorophyll a visible absorption spectra have been analyzed in terms of empirical solvent scales of Lewis basicity (B) and acidity (E) as well as steric constants (E_s) (after Koppel and Palm, 1972; Palm, 1977). The presence or absence of the specific four-band structure in the long-wavelength region depends only on the solvent electron-pair donating capability. In protic solvents the Soret band intensity is suppressed, but, in contrast, the extinction coefficient of its first satellite remains practically constant. This allows us to group spectra into four types according to the Soret-band intensity and the structure of the red-band system and assign them to chlorophyll-solvent complexes formed via 1 or 2 donor-acceptor bonds to central Mg and with and without hydrogen bond(s) to some nucleophilic center of the solute molecule.