<title>Femtosecond fluorescence depolarization study of photosynthetic antenna proteins: observation of ultrafast energy transfer in trimeric C-phycocyanin and allophycocyanin</title>

Abstract

C-phycocyanin (CPC) and Allophycocyanin (APC) are pigment-protein complexes isolated from antenna systems in cyanobacteria. The crystal structure of CPC has recently been solved and APC has a similar structure. CPC and APC have a trimeric structure, monomeric subunits are composed of an (alpha) and (beta) polypeptide chain, each has a tetrapyrrole chromophore chemically bound to position 84. In CPC and APC trimers, the (alpha) 84 and (beta) 84 chromophores in adjacent monomers are in close proximity, forming relatively strong coupled pairs. Calculation of pairwise energy transfer rates using Foerster theory has suggested an extremely fast transfer (&gt; 1 ps<SUP>-1</SUP>) between the (alpha) 84 and (beta) 84 pair in CPC. A femtosecond fluorescence up-conversion apparatus was constructed which achieves subhundred femtosecond time resolution. This allows experimental observation of the fast energy transfer process between the (alpha) 84 and (beta) 84 pair in both CPC and APC. There was also a wavelength dependence of the fluorescence depolarization kinetics which is inconsistent with Foerster inductive resonance energy transfer theory.