AN INFRARED STUDY OF CHLOROPHYLL-CHLOROPHYLL AND CHLOROPHYLL-WATER INTERACTIONS

The infrared spectra of chlorophyll a, chlorophyll b, bacteriochlorophyll, and pyrochlorophyll a in nonpolar solvents in the 1600-1800- and 3000-3800-cm-1 regions have been studied in the presence and absence of water. The data in dry systems are interpreted in terms of chlorophyll-chlorophyll dimers and oligomers formed by coordination interaction between the C-9 ketone oxygen function of one chlorophyll molecule and the central magnesium atom of another: keto C=O⃛Mg. In systems containing water, the results of chlorophyll-water interaction are found to be solvent and concentration dependent. In CC14 or benzene at chlorophyll concentrations of 10-6 to 10-2 Mand in aliphatic hydrocarbon solvents at chlorophyll concentrations of 10-6 to 10-5 M, water acts as a nucleophile to form Chl2H2O and Q1IH2O species. In aliphatic or cycloaliphatic hydrocarbon solvents at chlorophyll concentrations in the range 10-4 to 10-2 M, water coordinated to the Mg of one chlorophyll molecule is hydrogen bonded to the ketone and carbomethoxy carbonyl functions of another chlorophyll molecule: keto C=O⃛HO(Mg)H⃛O=C ester. Repetition of this interaction results in the formation of chlorophyll-water micelles of colloidal dimensions. A set of equilibria is advanced to account for the effects of solvent, concentration, and temperature on the chlorophyll species present in solution. The ir spectra are used to suggest structures for the dimers, oligomers, and chlorophyll-water adducts. © 1969, American Chemical Society. All rights reserved.