Chlorophyll a Self-assembly in Polar Solvent–Water Mixtures †

The conversion of chlorophyll a (Chl a) monomers into large aggregates in six polar solvents upon addition of water has been studied by means of absorption, fluorescence spectroscopy and fluorescence lifetime measurements for the purpose of elucidating the various environmental factors promoting Chl a self-assembly and determining the type of its organization. Two empirical solvent parameter scales were used for quantitative characterization of the different solvation properties of the solvents and their mixtures with water. The mole fractions of water f_1/2 giving rise to the midpoint values of the relative fluorescence quantum yield were determined for each solvent, 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 hydrogen-bonding ability and the dipole–dipole interactions (function of the dielectric constant) of the solvent–water mixtures are those that promote Chl a self-assembly. The influence of the different nature of the nonaqueous solvents on the Chl aggregation is manifested by both the different water contents required to induce Chl monomer → aggregate transition and the formation of two types of aggregates at the completion of the transition: species absorbing at 740–760 nm (in methanol, ethanol, acetonitrile, acetone) and at 667–670 nm (in pyridine and tetrahydrofuran). It is concluded that the type of Chl organization depends on the coordination ability and the polarizability (function of the index of refraction) of the organic solvent. The ordering of the solvents with respect to the f_1/2 values—methanol < ethanol < acetonitrile < acetone < pyridine < tetrahydrofuran—yielded a typical lyotropic (Hofmeister) series. On the basis of this solvent ordering and the disparate effects of the two groups of solvents on the Chl a aggregate organization, it is pointed out that the mechanism of Chl a self-assembly in aqueous media can be considered a manifestation of the Hofmeister effect, as displayed in the lipid-phase behavior (Koynova et al., Eur. 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 bulk liquid.