Femtosecond fluorescence depolarization study of photosynthetic antenna proteins: observation of ultrafast energy transfer in trimeric C-phycocyanin and Allophycocyanin Xiaoliang Xie+, Mei Du, Laurens Mets° and Graham R. Fleming* Department of Chemistry and the James Frank Institute The University of Chicago, 5735 South Ellis Avenue, Chicago, IL60637 0 Department of Molecular Genetics and Cell Biology The University of Chicago, 1 103 East 57th Street, Chicago, 1L60637 + Current Address: Molecular Science Research Center, Battelle, Pacific Northwest Laboratory Battelle Boulevard, P.O. Box 999, Richiand, WA99352 ABSTRAO 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. 1. INTRODUCTION In the initial steps of photosynthesis, electronic excitation of pigments by light absorption in light harvesting antenna complexes is rapidly transferred among the pigments and trapped in photosynthetic reaction centers where electron transfer occurs with high efficiency. Without knowledge of the intermolecular distances and relative orientations of chromophores, it is often difficult to investigate the mechanism of energy transfer from a chemical physics point of view. Recently, a few structures of antenna proteins have become available at atomic resolution.13'58 This offers an opportunity to study the mechanism of energy transfer in a photosynthetic system in a detailed fashion. C-phycocyanin (CPC) and Allophycocyanin (APC) are isolated phycobiliproteins from phycobilisomes, an antenna protein complex in cyanobacteria.9'1° Both CPC and APC have chemically identical chromophores, the open chain tetra-pyrroles (phycocyanobilins). However the absorption 690 / SPIE Vol. 1640 Time-Resolved Laser Spectroscopy in Biochemistry II! (1992) 0-81 94-0786-0/92/$4.00 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 01/16/2015 Terms of Use: http://spiedl.org/terms