Low-Frequency Vibrational Spectra of Chlorophylls a and b in Solution: Effects ofAxial Coordination

The far-infrared spectra (500--100 cm-I) of chlorophyll a were observed in three solvent systems (benzene-pyridine, benzene-tetrahydrofuran, and benzene-acetone). The observed spectra could be classified into three types corresponding to the aggregate state, the five-coordinate monomeric state, and the six-coordinate state. The far-infrared spectra of I5N_and 26lV[g-substitutedspecies and the polarized resonance Raman spectra of chlorophyll a and pheophytin a in the region of 500~100 cm-I were studied. These experimental results indicate that bands characteristic of the three states are due to modes to which deformations of the macrocycle make major contributions and vibrations around the Mg atom make some contributions, and that no band observed in the far-infrared and Raman spectra is attributable to a vibrational mode associated predominantly with the Mg atom and the coordinating atoms. The spectral behavior of chlorophyll b in the benzene-pyridine system was similar to that of chlorophyll a.