SUPRAMOLECULAR STRUCTURES OF THE CHLOROPHYLL A' AGGREGATE AND THE ORIGIN OF THE DIASTEREOSELECTIVE SEPARATION OF CHLOROPHYLL-A AND A'

The supramolecular structure of the aggregate of chlorophyll a' (Chl a ', the C13(2)-(S)-epimer of Chl a) was examined in comparison with tha t of the Chl a aggregate. The Chl a/a' aggregate colloids were formed in aqueous alcohols, and IR, resonance Raman, and small-angle X-ray sc attering (SAXS) measurements were performed on lyophilized precipitate s of the aggregate colloids. The analyses on the Chl a colloidal preci pitate obtained from a 26/74 2-propanol/H2O solution demonstrated that the supramolecular structure was similar to those of the aggregates o f hydrated Chl a. On the other hand, the vibrational spectra of the Ch l a' colloidal precipitate formed in 40/60 MeOH/H2O were quite similar to those of anhydrous Chl a aggregates. A trace hydroxyl stretching I R absorption (at around 3350 cm(-1)) of the Chl a' aggregate precipita te was as small as that which the anhydrous Chl a aggregates could sho w. A SAXS profile of the Chl a' colloidal precipitate demonstrated a l amellar structure with a 52-Angstrom bilayer spacing, 8 Angstrom great er than that of the Chl a aggregate formed in the 2-propanol/water. It was supposed that the Chl a' aggregate was essentially anhydrous even in aqueous alcohols, while the Chl a aggregate was hydrated or swolle n. A possible model of the supramolecular structure of the Chl a' aggr egate precipitate is presented. It is concluded that the difference be tween the supramolecular structures of the Chl a and a' aggregates doe s not depend simply on the steric hindrance between the bulky substitu ents at the C13(2) and C17 positions but also on the possibility to fo rm rigid intermolecular hydrogen-bonding networks. This is the origin of the diastereoselective separation of Chl a and a', and it is also c losely correlated to a structural degradation of the Chl a' aggregate that occurs during the preparation of the colloidal precipitate.