A RESONANCE RAMAN INVESTIGATION OF THE CATION RADICAL OF CHLOROPHYLL-A AND SEVERAL DERIVATIVES

Soret excitation resonance Raman (RR) spectra of the electrochemically generated cation radical of chlorophyll a and several of its derivatives, including copper- and zinc-substituted pheophytin a, pyrochlorophyll a, and 10-hydroxychlorophyll a, have been obtained. An examination of the RR spectra of the ring five derivatives clearly shows that the band at 1717 cm-1 in the chlorophyll a cation radical spectrum is due to the C9 keto stretching mode. This band experiences the largest shift (from 1685 cm-1 in the neutral to 1717 cm-1 in the cation radical) following one-electron oxidation. The 32-cm-1 upshift is attributed to a decrease in the conjugation between the C9 keto group and the macrocycle as a result of the one-electron oxidation. Thus, the C9 keto group may play a role in the chlorophyll redox reactions in vivo. Differences in the RR spectra of the cation radicals of the metal-substituted pheophytins suggest that chlorophyll a may undergo significant conformational changes with loss of an electron. © 1990 American Chemical Society.