FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDY OF THE STRUCTURE OF CHLOROPHYLL-A AND CHLOROPHYLL-B IN HIGHLY DILUTE WATER-SATURATED CARBON-TETRACHLORIDE SOLUTIONS

Fourier-transform (FT) infrared (IR) absorption spectra have been meas ured for chlorophyll a (Chl a), chlorophyll b (Chl b), pheophytin a (P heo a), and pheophytin b (Pheo b) in highly dilute (10(-5)-10(-6) M) w ater-saturated carbon tetrachloride solutions. Frequencies of IR bands due to C=O stretching modes of the 9-keto group indicate that Chl a a ssumes largely a dimeric structure in the concentrated (10(-2)-10(-3) M) water-saturated carbon tetrachloride solutions but it remains mostl y a monomer with one or two coordinated water molecules in dilute (10( -5)-10(-6) M) solutions. Although it seems that Chl b also assumes pre dominantly dimeric form in concentrated solutions and monomeric form i n dilute solutions, the relative intensity change of two C=O stretchin g bands ascribed to the free and coordinated 3-aldehyde groups with de creasing concentration suggests that the aldehyde group is also involv ed in formation of the dimer. The relative intensity of two C=O stretc hing bands due to the free and coordinated aldehyde groups changes sig nificantly for Pheo b in water-saturated carbon tetrachloride solution s. This observation suggests that some of Pheo b also assume dimeric f orm via the aldehyde group in concentrated solutions.