Electron Donor-Acceptor Properties of 10(S)-Chlorophyll a

Evidence for the self-aggregation of 10(S)-chlorophyll a (= chlorophyll a') in nonpolar solvents has been obtained by means of absorption spectroscopy in the visible and infrared regions. The visible absorption spectrum of chlorophyll a' exhibits a shoulder at 680 nm in aliphatic hydrocar­ bon solvents and the infrared spectrum measured in carbon tetrachloride contains an “aggrega­ tion peak” at 1650 cm "1. IR spectroscopy shows, in addition, that the C-10 epimers o f chlorophyll a are both present as monomers in tetrahydrofuran. H owever, in the presence of a small amount of aliphatic alcohol or water, the IR spectrum in tetrahydrofuran at lowered temperatures exhibits an intense, wide absorption band at ~ 1650 cm -1 which was interpreted as arising from the hydrogen bonding o f the hydroxyl group to the C-9 keto carbonyl of the chlorophyll.