Specific solvation of chlorophyll b: A site-selection study at 5 K

Abstract

The site-selection fluorescence excitation spectra of chlorophyll b were recorded in three different types of polar solvent host at 5 K. The influence of specific solvation of chlorophyll b on the S 1 state vibrational modes active in these spectra was studied in the high frequency vibrational region above 1000 cm −1. The nucleophilic solvation of chlorophylls ( i.e. one or two axial extra ligands at the central magnesium atom), unlike electrophilic interactions (hydrogen bonding), can be detected very successfully by the method used. The maximum down-shift of 20 – 30 cm −1 of the high characteristic vibrational frequencies in the CC stretching region estimated under electronic excitation of chlorophyll b is comparable with the effect of magnesium biligation and is explained in terms of an expansion of the tetrapyrrolic macrocycle core of the molecule. The broad-band fluorescence spectra of chlorophyll b in the same frozen solutions recorded at different laser excitation wavelengths are also presented and discussed.