K. Uehara et al., THE CHLOROPHYLL-A AGGREGATE ABSORBING NEAR 685 NM IS SELECTIVELY FORMED IN AQUEOUS TETRAHYDROFURAN, Photochemistry and photobiology, 58(1), 1993, pp. 127-132
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.