Diastereoselective Self-Assemblies of Chlorophylls a and a‘

Abstract

The aggregation behaviors of mixtures of chlorophylls (Chl) a and a‘, the C132-(R) and -(S) epimers, were examined in 2-propanol/water = 26/74 (v/v). In the aggregated states, the epimeric mixtures of Chl a/a‘ at molar ratios ranging from 70:30 to 30:70 gave two distinct Qy absorption bands (at ca. 700 and 750 nm) characteristic of the aggregate colloid of each epimer. On the other hand, a single 750 nm (700 nm) band was observed in the range of the epimeric composition ranges outside the one mentioned above. The time course of the aggregation, disaggregation behavior, filtrations of the colloidal solutions combined with dynamic light-scattering measurements, and fluorescence, resonance Raman, and circular dichroism spectroscopies demonstrated the coexistence of two types of aggregate structures in each colloidal particle. This is the first report suggesting that an epimer pair of chlorophyllous pigments self-assembles simultaneously through limited diastereoselective separation. The phenomenon was rationalized in terms of the phase separation of a regular solution. The diastereocontrol in the self-assembly of other type Chl diastereomers, bacteriochlorophylls c, d, and e possessing chirality at the C31-position, was also discussed in relation to supramolecular structures of the self-aggregates in antenna of green photosynthetic bacteria.