Diastereoselective control of bacteriochlorophyll e aggregation.: 31-S-BChl e is essential for the formation of chlorosome-like aggregates

We have investigated supramolecular bacteriochlorophyll (BChl) e aggregates in four different solvent systems. Aggregates of HPLC-purified BChl e species, 3(1)-R-8-ethyl-12-ethyl BChl e, 3(1)-R-8-propyl-12 ethyl BChl e, 3(1)-S-8-propyl-12-ethyl BChl e, and 3(1)-S-8-isobutyl-12-ethyl BChl e, in cyclohexane, n-hexane, H2O/ monogalactosyldiglyceride, or H2O/lecithin, indicate two different aggregate structures, as judged from absorption, circular dichroism, infrared, and resonance Raman spectroscopies. Aggregates of 3(1)-R-BChl (R-BChl) e were characterized by a 706 nm absorption band and broad C-13(1) carbonyl stretching bands at 1650-1680 cm(-1). In contrast, aggregates of 3(1)-S-BChl (S-BChl) e displayed a Q(y) band maximum at 717 nm and a carbonyl stretching band at 1650 cm(-1) only. All measurements indicated that the aggregates of S-8-isobutyl-12-ethyl BChl (S[IE]BChl) e mimicked the supramolecular aggregates in intact chlorosomes of Chlorobium phaeobacteroides much better than did the R-8-ethyl-12-ethyl BChl (R[EE]BCl) e-type aggregate. However, when mixtures of R[EE]BChl e and S[IE]BChl e were used for aggregate formation, even small amounts of S[IE]BChl e were sufficient to cause the formation of chlorosome-like aggregates. We conclude that S-BCl e is essential for the formation of chlorosome-like aggregates. Finally, we describe a refined model for the supramolecular chlorosomal aggregate structure.