Diastereoselective Control of Bacteriochlorophyll e Aggregation. 3-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, H 2 O/monogalactosyldiglyceride, or H 2 O/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]BChl) 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-BChl e is essential for the formation of chlorosome-like aggregates. Finally, we describe a refined model for the supramolecular chlorosomal aggregate structure.