PARENT SESSION
Posters P3A Bacteriochlorophyll based antenna systems. Abstracts (219-238)

Self-aggregation of zinc oxo-bacteriochlorins as models of chlorosomal chlorophylls. Michio Kunieda^*,1, Hitoshi Tamiaki¹, ¹ Department of Bioscience and Biotechnology, Faculty and Science and Engineering, Kusatsu, Shiga, Japan

ABSTRACT- Extramembranous antenna systems of green photosynthetic bacteria, so-called chlorosomes are constructed by self-aggregation of bacteriochlorophyll(BChl)s-c/d/e. The chlorosomal self-aggregation is driven by their characteristic molecular structures, 3¹-hydroxy and 13-keto-carbonyl groups and central magnesium atom in along the Qy axes. Such sef-aggregates show red-shifted Qy bands in the region of 690-750 nm, compared with 640-670 nm of the monomeric Qy peaks. Recently, we reported self-aggregation of bacteriochlorin molecules possessing the above requisite substituents, showing a 860-nm peak as the red-shifted Qy maximum (M. Kunieda, T. Mizoguchi, H. Tamiaki, Photochem. Photobiol., 2004, 79, 55-61). A variety of optical properties of chlorosomal self-aggregates are of great advantage to utilize those for photoactive nano-devices. The previous bacteriochlorin had two hydrogen atoms at the 7,8-positions, and then the dehydrogenation to chlorin -conjugate readily occurred. Here we present chlorosomal self-aggregation of two isomeric zinc 8-ethyl-8-methyl-7-oxo- and 7-ethyl-7-methyl-8-oxo-3¹-hydroxy-13¹-oxo-bacteriochlorins 1 and 2, which have neither hydrogen atoms at the 7,8-positions. Self-aggregation of 1 and 2 gave their red-shifted Qy maxima at 880 and 793 nm, compared with their monomeric Qy maxima at 724 and 683 nm. The red-shifted values by self-aggregation were 2450 and 1970 cm^-1 for 1 and 2, respectively. The different properties between monomeric and oligomeric 1/2 were explained from the molecular model calculations. In a monomeric state, electronic absorption properties of 7/8-oxo-1/2 were controlled by the situation of an electron-withdrawing oxygen atom at the 7/8-positions, whereas their oligomeric properties were mainly dependent upon the steric bulkiness of dialkyl groups at the 8/7-positions in a supramolecule.

KEY WORDS: self-aggregation, chlorosome, bacteriochlorin, molecular model caluclation