Fluorescence polarization studies on four biliproteins and a bilin model for phycoerythrin 545

Fluorescence (excitation) polarization spectroscopy in the wavelength regio n of the bilin chromophores was applied to phycoerythrocyanin (CV-phycocyan in), phycocyanins 645 and 612, and phycoerythrin 545. The cryptomonad bilip roteins phycoerythrin 545 and phycocyanins 612 and 645 - were studied as bo th protein dimers having an alpha(2)beta(2) polypeptide structure and as al pha beta monomers, The cyanobacterial phycoerythrocyanin (CV-phycocyanin) w as a trimeric oligomer. The changes in polarization across the spectrum wer e attributed to transfers of energy between bilins. Cryptomonad biliprotein s are isolated as dimers. The similarities between their steady-state fluor escence polarization spectra and those of the corresponding monomers sugges ted that the monomers' conformations were analogous to the dimers. This sup ports the use of monomers in the study of dimer bilin organization. The unu sual polarization spectrum of phycoerythrin 545 was explained using a model for the topography of its bilins. Obtaining the emission spectra of phycoe rythrin 545 at several temperatures and a deconvolution of the dimer circul ar dichroism spectrum also successfully tested the bilin model. Circular di chroism spectroscopy was used to determine which polarization changes are f ormed by Forster resonance energy transfers and which may be produced by in ternal conversions between high- and low-energy states of pairs of exciton- coupled bilins. Attempts were made to assign energy transfer events to the corresponding changes in fluorescence polarization for each of the four bil iproteins. (C) 1999 Elsevier Science B.V. All rights reserved.