Heating changed suprastructure of self-assembled zinc methyl bacteriopheophorbide-c

Abstract

Green photosynthetic bacteria have a unique light-harvesting antenna system so-called chlorosome. In a chlorosome, a number of bacteriochlorophyll (BChl)-c, -d and -e molecules self-aggregate to form rod-like oligomers which are surrounded with galactolipids. Interaction among BChl molecules is important to make chlorosomal aggregates. By the way, some green bacteria are thermophilic and cultured in heated media. For instance, optimum temperature for culturing Chlorobium tepidum and Chloroflexus aurantiacus is 45 and 55 °C, respectively. However, most of in vitro model studies on chlorosomal aggregates have been performed at room temperature. Here we report self-aggregate of zinc analog of BChls-c in a heated aqueous medium. We found that heating changed supramolecular structure of the artificial aggregate. A methanol solution of zinc methyl 31S-bacteriopheophorbide-c 1 and lecithin, a phospholipid, was diluted with large volume of water at room temperature. Absorption spectrum of the aqueous 1 showed Qy bands at 668 and 704 nm, which correspond to monomer and aggregates of 1, respectively. When the aqueous solution was heated at 45°C for 2 h, the Qy absorption bands changed to a single band at 738 nm. Absorption spectrum of the heated aggregate was unchanged when the solution gradually cooled to room temperature. These spectral changes suggested that the kinetic products of aggregate 1 converged to thermodynamically favorable structure in a heated medium.