THE CHLOROPHYLL a AGGREGATE ABSORBING NEAR 685 nm IS SELECTIVELY FORMED IN AQUEOUS TETRAHYDROFURAN*

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Abstract
An aqueous solution of 2–12% (vol/vol) tetrahydrofuran (THF) induced the selective aggregation of chlorophyll a (Chl a) to form a novel species, 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 suggests 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 dichroic spectrum of the Chl a species formed in the 6% THF aqueous solution 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 C10 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 chain have a significant role in stabilizing A‐685. A possible structure for A‐685 is proposed as a novel in vitro model for the P‐680 Chl a dimer.

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