title: 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.
abstract: 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 1H-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 Qy 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 Qy 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 Qy band are advantageous for efficient energy transfer from BChl-e aggregates to BChl-a in a baseplate in chlorosomes because their spectral overlap increases.
authors.affiliation: Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Saga
authors.forename: K
authors.initials: K
authors.lastname: Matsuura
authors.forename: H
authors.initials: H
authors.lastname: Tamiaki
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry*; D002942:Circular Dichroism; D046911:Macromolecular Substances; D010777:Photochemistry; D012997:Solvents; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry; D013237:Stereoisomerism
chemical_list: D001429:Bacteriochlorophylls; D046911:Macromolecular Substances; D012997:Solvents; C120930:bacteriochlorophyll e