Chlorophyll a self-assembly in polar solvent-water mixtures

The conversion of chlorophyll a (Chl a) monomers into large aggregates in s ix polar solvents upon addition of water has been studied by means of absor ption, fluorescence spectroscopy and fluorescence lifetime measurements for the purpose of elucidating the various environmental factors promoting Chi a self-assembly and determining the type of its organization. Two empirica l solvent parameter scales were used for quantitative characterization of t he different solvation properties of the solvents and their mixtures with w ater. The mole fractions of water f(1/2) giving rise to the midpoint values of the relative fluorescence quantum yield were determined for each solven t, and then various solvent-water mixture parameters for the f(1/2) values were compared. On the basis of their comparison, it is concluded that the h ydrogen-bonding ability and the dipole-dipole interactions (function of the dielectric constant) of the solvent-water mixtures are those that promote Chi a self-assembly. The influence of the different nature of the nonaqueou s solvents on the Chi aggregation is manifested by both the different water contents required to induce Chi monomer --> aggregate transition and the f ormation of two types of aggregates at the completion of the transition: sp ecies absorbing at 740-760 nm tin methanol, ethanol, acetonitrile, acetone) and at 667-670 nm (in pyridine and tetrahydrofuran), It is concluded that the type of Chi organization depends on the coordination ability and the po larizability (function of the index of refraction) of the organic solvent, The ordering of the solvents with respect to the f(1/2) values-methanol < e thanol < acetonitrile < acetone ( pyridine < tetrahydrofuran-yielded a typi cal lyotropic (Hofmeister) series. On the basis of this solvent ordering an d the disparate effects of the two groups of solvents on the Chi a aggregat e organization, it is pointed out that the mechanism of Chi a self-assembly in aqueous media can be considered a manifestation of the Hofmeister effec t, as displayed in the lipid-phase behavior (Koynova et at, Ear. J. Biophys . 25, 261-274, 1997), It relates to the solvent ability to modify the bulk structure and to distribute unevenly between the Chl-water interface and bu lk liquid.