Molecular Assembly of Bachteriochlorophylls by Light-Harvesting Model Polypeptides

Abstract

It is interesting to note that the light-harvesting (LH) polypeptides of photosynthetic bacteria organize bachteriochlorophyll a (BChla) complex according to cooperative interactions between the polypeptide and BChla so that an efficient energy-transfer and electron-transfer processes of porphyrins may occur. Work from several laboratories has demonstrated self-assemblies of porphyrins by using synthetic polypeptides to organize an artificial hemoprotein model. However, there has been little study of molecular assemblies of BChla and its analogue by using synthetic polypeptides. In this paper, we now demonstrate molecular assemblies of BChla and its analogue (Scheme 1) by using hydrophobic a-helix polypeptides and 2 α-helix polypeptide with disulfide-linkage (Scheme 2) in n-octyl-β-D-glucopyranoside (OG) micelle to provide an insight into the effect of amino acid residues of the LH polypeptide on formation of the LH complex of photosynthetic bacteria. The key to the molecular assembly is ready availability of formation of the light-harvesting complex by using hydrophobic α-helix polypeptides and 2α-helix polypeptide with disulfide-linkage (Scheme 2) which are similar to the amino acid sequences of cut LH-βof R. sphaeroides.1 We selected 1 α-helix polypeptides and 2α-helix polypeptide with disulfide-linkage. The reasons are as follows. (1) the core light-havesting β-polypeptide from photosynthetic bacteria, R. sphaeroides easily induced B820 complex in OG micelle.1 (2) At the end of the α-helix, the disulfide linkage was placed as a formation of stable subunit-complex even in the lipid bilayers. Synthetic disulfid-linked α-helix hydrophobic polypeptides (the 60-peptide) (type 3) were prepared by Fmoc-method using peptide synthesizer and purified by Sephadex LH-60 gel chromatography, and then HPLC purification. These polypeptides were analyzed by TOF-MS to give expected molecular mass. BChla was obtained as described previously.1