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 that 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 scattering (SAXS) measurements were performed on lyophilized precipitates of the aggregate colloids. The analyses on the Chl a colloidal precipitate obtained from a 26/74 2-propanol/H 2 O solution demonstrated that the supramolecular structure was similar to those of the aggregates of hydrated Chl a . On the other hand, the vibrational spectra of the Chl a ‘ colloidal precipitate formed in 40/60 MeOH/H 2 O were quite similar to those of anhydrous Chl a aggregates. A trace hydroxyl stretching IR absorption (at around 3350 cm -1 ) of the Chl a ‘ aggregate precipitate was as small as that which the anhydrous Chl a aggregates could show. A SAXS profile of the Chl a ‘ colloidal precipitate demonstrated a lamellar structure with a 52-Å bilayer spacing, 8 Å greater 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 swollen. A possible model of the supramolecular structure of the Chl a ‘ aggregate precipitate is presented. It is concluded that the difference between the supramolecular structures of the Chl a and a ‘ aggregates does not depend simply on the steric hindrance between the bulky substituents at the C13 2 and C17 positions but also on the possibility to form rigid intermolecular hydrogen-bonding networks. This is the origin of the diastereoselective separation of Chl a and a ‘, and it is also closely correlated to a structural degradation of the Chl a ‘ aggregate that occurs during the preparation of the colloidal precipitate.