Diastereoselective Self‐aggregation of Synthetic 3‐(1‐Hydroxyethyl)‐bacteriopyrochlophyll‐a as a Novel Photosynthetic Antenna Model Absorbing Near the Infrared Region¶

Abstract

ABSTRACT3‐Deacetyl‐3‐(l‐hydroxyethyl) bacteriopyrochlorophyll‐a (1), 7, 8‐dihydrobacteriochlorophyll‐d possessing 8‐ethyl, 12‐methyl and 174‐phytyl groups, was prepared by modifying naturally occurring bacteriochlorophyll‐a. The synthetic 31‐epimers were separated by high‐performance liquid chromotagraphy, and the absolute configuration at the 31‐position was determined by derivatization of 1 to a structurally determined chlorin. A dichloromethane solution of 31‐R‐1 or 31S‐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 31R‐1 was 860 nm and redshifted by 2170 cm−1 from the monomeric one, whereas epimeric 31S‐1 showed a less redshifted peak at ca 800 nm, with a small dimeric band around 740 nm. Such visible spectra indicated that 31R/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 31R‐1 absorbing light in the NIR region would be models of intrinsic membranous light‐harvesting complexes 1 in photosynthetic purple bacteria as well as extramembranous antennas in green bacteria.