Characterization of Orientation and Photochemical Function of Chlorophyll a Molecules in Self-Assembled Membranes

The long chained DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) and the short chained DHPC (1,2-dihexanoyl-sn-glycero-3-phosphocholine) mixtures provided bicelle (disk like membrane) structures at high phospholipid concentration [1]. The morphology of DMPC/DHPC self-assembly changed to vesicle in diluted conditions. Self-assemblies of phospholipids, the bicelle and the vesicle, were employed to organize photosynthesis pigments, chlorophyll a (Chla) molecules, in aqueous solution. In nature, Chla act as reaction center (RC) in light harvesting (LH), wherein they are lined up around apoprotein rings. These organized pigments transfer the absorbed light energy efficiently to the RC. Inspired from the orientation of Chla molecules in natural LH systems; we here investigated the behavior of inserted Chla molecules in phospholipid membranes. The behaviors of a Chla-incorporated phospholipid membranes were evaluated by optical techniques. In this study, we revealed the phase diagram of DMPC/DHPC mixture in dilution process based on membrane properties such as turbidity, membrane fluidity and membrane polarity of that. The designed assembly grounded the phase diagram (DMPC/DHPC = 3:2 including 1.2mol% Chla) resulted J-aggregate, and the Chla-Chla interaction was also maintained at diluted condition. In addition, the photo-function of the vesicle with a Chla was examined the influence of the membrane properties on the photoreduction with methyl viologen (MV) [2]. The J-aggregate of Chla on DMPC/DHPC = 3:2 efficiently catalyzed photoreduction of methyl viologen, suggesting the importance of controlled orientation of pigments by used self-assembly membrane. Preparing highly efficient LH membrane with Chla can contribute to develop the photochemical energy conversion system.

[1] L. van Dam, G. Karlsson, K. Edwards, Biochim. Biophys. Acta 2004, 1664, 241. [2] M. Kobayashi, S. Ohashi, K. Iwamoto, Y. Shiraiwa, Y. Kato, and T. Watanabe, Biochim. Biophys. Acta 2007, 1767, 596.