Femtosecond fluorescence depolarization study of photosynthetic antenna proteins: observation of ultrafast energy transfer in trimeric C-phycocyanin and Allophycocyanin

C-phycocyanin (CPC) and Allophycocyanin (APC) are pigment-protein complexes isolated from antenna systems in cyanobactria. The crystal structure of CPC has been solved recently13 and APC has a similar structure. CPC and APC have a trimeric structure, monomeric subunits are composed of an a and f polypeptide chain, each has a tetrapyrrole chromophore chemically bound to position 84. In CPC and APC trimers, the a84 and 384 chromophores in adjacent monomers are in close proximity, forming relatively strong coupled pairs. Calculation of pairwise energy transfer rates using Förster theory has suggested an extremely fast transfer (> ips 1) between the a84 and 184 pair in CPC.4 We have constructed a femtosecond fluorescence up-conversion apparatus which achieves subhundred femtosecond time resolution. Using this technique, we have experimentally observed the fast energy transfer process between the a84 and 384 pair in both CPC and APC. We also observed a wavelength depençlence of the fluorescence depolarization kinetics which is inconsistent with Förster inductive resonance energy transfer theory.