Diastereoselective self-aggregation of synthetic 3-(1-hydroxyethyl)-bacteriopyrochlophyll-a as a novel photosynthetic antenna model absorbing near the infrared regions.

ABSTRACT

3-Deacetyl-3-(1-hydroxyethyl)bacteriopyrochlorophyll-a (1), 7,8-dihydrobacteriochlorophyll-d possessing 8-ethyl, 12-methyl and 17(4)-phytyl groups, was prepared by modifying naturally occurring bacteriochlorophyll-a. The synthetic 3(1)-epimers were separated by high-performance liquid chromotagraphy, and the absolute configuration at the 3(1)-position was determined by derivatization of 1 to a structurally determined chlorin. A dichloromethane solution of 3(1)R-1 or 3(1)S-1 was diluted with a 1000-fold volume of cyclohexane to give self-aggregation species absorbing light at a near-infrared (NIR) region (<910 nm). The resulting Qy maximum in 3(1)R-1 was 860 nm and redshifted by 2170 cm(-1) from the monomeric one, whereas epimeric 3(1)S-1 showed a less redshifted peak at ca 800 nm, with a small dimeric band around 740 nm. Such visible spectra indicated that 3(1)R/S-1 formed different supramolecular structures in the self-aggregates. In contrast, self-aggregation of the 7,8-dehydro-compound 2, bacteriochlorophyll-dP, found in natural antennas of photosynthetic green bacteria showed much smaller diastereomeric control. The self-aggregates of 3(1)R-1 absorbing light in the NIR region would be models of intrinsic membraneous light-harvesting complexes 1 in photosynthetic purple bacteria as well as extramembranous antennas in green bacteria.