Raman and Infrared Studies on the Effects of Axial Coordination to Chlorophyll a

Abstract

The effects of axial ligands (solvent molecules) on the vibrational spectra of chlorophyll a in solution were studied. When the Mg atom of chlorophyll a changes from the fivecoordinate state (one axial ligand) to the six-coordinate state (two axial ligands), four Raman bands (Figs.3-5) and two infrared bands (Fig.6) in the C=C stretching region shifted to lower frequencies by about.10 cm^-1, whereas a far-infrared band (Fig.7) showed an upshift of about 20 crn^-1. The latter band probably arises from a mode due to Mg-N stretching. These shifts were explained as a result of the elongation of C=C bonds and the shortening of Mg-N distances in a chlorin ring caused by the movement of the Mg atom from an out-of-plane position to the center of the ring.