THE CHLOROPHYLL-A AGGREGATE ABSORBING NEAR 685 NM IS SELECTIVELY FORMED IN AQUEOUS TETRAHYDROFURAN

An aqueous solution of 2-12% (vol/vol) tetrahydrofuran (THF) induced t he selective aggregation of chlorophyll a (Chl a) to form a novel spec ies, A-685, absorbing near 685 nm. The formation of A-685 was closely correlated with a decrease in water activity of the solution. A Raman spectrum of the Chl a species formed in the presence of 6% THF suggest s a unique interaction among Chl a, solvent THF and water molecules to give a stacked aggregate (Chl a.THF.H2O.THF.Chl a). The circular dich roic spectrum of the Chl a species formed in the 6% THF aqueous soluti on showed an intense signal that had negative and positive wings with about 100-fold larger molar ellipticity for the A-685 than for monomer . However, Chl a', the CIO epimer of Chl a, and chlorophyllide, with a phytyl chain replaced by an ethyl group, did not form A-685 in 6% THF . These clearly indicate that 10-methylcarboxylate and the phytyl chai n have a significant role in stabilizing A-685. A possible structure f or A-685 is proposed as a novel in vitro model for the P-680 Chl a dim er.