AGGREGATION OF CHLOROPHYLL IN NONPOLAR SOLVENTS FROM MOLECULAR WEIGHT MEASUREMENTS

The state of aggregation of chlorophylls a and b, pyrochlorophyll a, bacteriochlorophyll, pheophytin a and pyropheophytin a in a variety of nonpolar solvents has been deduced from molecular weight measurements made by vapor-phase osmometry. In carbon tetrachloride and benzene solution, chlorophyll a and pyrochlorophyll a exist predominately as dimers over a wide concentration range, although there is evidence for oligomer formation in concentrated solutions. In aliphatic or cycloaliphatic hydrocarbon solvents, chlorophyll a can form oligomers with aggregation numbers as large as 20. In even the most dilute cyclohexane solutions studied, chlorophyll a exists as a tetramer. Chorophyll b and bacteriochlorophyll aggregate more strongly. The basic unit for these tow chlorophyll is the trimer which has a strong tendency to form hexamer and higher oligomers in aliphatic hydrocarbon solvents. Infrared spectra for solutions of chlorophyll of known states of aggregation are presented. © 1969.