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Spectroscopic Studies on Self‐aggregation of Bacteriochlorophyll‐ e in Nonpolar Organic Solvents: Effects of Stereoisomeric Configuration at the 3 1 ‐Position and Alkyl Substituents at the 8 1 ‐Position ¶
Author(s) -
Saga Yoshitaka,
Matsuura Katsumi,
Tamiaki Hitoshi
Publication year - 2001
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2001)0740072ssosao2.0.co2
Subject(s) - bacteriochlorophyll , chemistry , alkyl , photochemistry , homologous series , circular dichroism , chlorosome , monomer , fluorescence , pigment , stereochemistry , crystallography , organic chemistry , polymer , physics , quantum mechanics
Self‐aggregation of naturally occurring bacteriochlorophyll (BChl)‐ e in nonpolar organic solvents was investigated by visible absorption, fluorescence emission and circular dichroism spectra. Cultured brown‐colored photosynthetic bacteria have several BChl‐ e as light‐harvesting antenna pigments. Three major BChl‐ e homologs were separated from the extracts of the culture by reverse‐phase high‐performance liquid chromatography (HPLC) and characterized by 1 H‐NMR and fast‐atom bombardment mass spectroscopy: 8‐ethyl‐12‐ethyl ([E,E])‐, 8‐propyl‐12‐ethyl‐ and 8‐isobutyl‐12‐ethyl‐BChl‐ e farnesyl esters. All the homologs consisted of a mixture of the 3 1 ‐epimers, and epimerically pure BChl‐ e were also given by HPLC separation. All the separated BChl‐ e epimers, the epimeric mixtures and the homologous mixtures formed self‐aggregates in 2% dichloromethane/hexane, giving visible absorption spectra similar to that of the whole cells, which showed two peaks (or shoulders) around 430–450 and 520 nm at the Soret region as well as a red‐shifted Q y band relative to the monomeric. The spectral properties of the Soret band were basically unchanged among the epimers or epimeric/homologous mixtures. In contrast, the Q y band of aggregates of epimeric mixtures (except [E,E]) and homologous mixtures red‐shifted and broadened compared with the epimerically pure. The red‐shift and broadening of the Q y band are advantageous for efficient energy transfer from BChl‐ e aggregates to BChl‐ a in a baseplate in chlorosomes because their spectral overlap increases.