Spectral Characteristics and Colloidal Properties of Chlorophyll a‘ in Aqueous Methanol

The "phase behavior" of chlorophyll a' (Chl a', C132-epimer of Chl a) dissolved in aqueous methanol was examined in terms of the composition of the solvent. This study aimed at elucidating the property of Chl a', the exotic pigment found in a photosynthetic reaction center complex, as well as at clarifying the nature of the Chl aggregation in aqueous media. Visible absorption, circular dichroism (CD), fluorescence and resonance Raman spectroscopies, dynamic light-scattering measurements, and electron microscopy were utilized. Chl a' formed either of two types of colloids depending on the solvent composition. The one formed over a wide methanol volume percentage (ca. 73−30%) commonly possessed a single microscopic structural unit that yielded the double-peaked absorption (ca. 690 and 715 nm) accompanied by a symmetric dispersed-type CD spectrum. Increasing methanol concentration within this solvent composition range enhanced the size of the colloid and finally caused critical opalescence, which was reminiscent of the critical behavior of the aqueous solution of nonionic surfactants. These findings indicate that the microscopic structure of the Chl a' aggregate was independent of the size and shape of the colloid. This is in sharp contrast to the solvent composition dependence of the Chl a aggregation in the same medium: an increase in methanol concentration in going from 40% to 70% (vol/vol) shifted a broad red-most absorption band from ca. 700 to 750 nm, which was correlated to the enhancement of the aggregation number and the colloidal size. The difference between the aggregation behaviors of Chl a and a' suggests a narrower choice of possible molecular arrangements in the Chl a' aggregate as an inherent property of the pigment. The nature of the Chl aggregation in aqueous media is discussed in relation to the micellization of nonionic surfactant.