PHYCOERYTHRIN-545 - MONOMERS, ENERGY MIGRATION, BILIN TOPOGRAPHY, ANDMONOMER DIMER EQUILIBRIUM/

Phycoerythrin 545 was isolated having an alpha(2) beta(2) (dimer) prot ein structure at pH 6.0 and 2 g/L protein concentration with eight bil in chromophores. Monomers (alpha beta) were produced by lowering the p rotein concentration to 0.15 g/L and the pH to 4.5. Dimer dissociation was monitored by dynamic light scattering and gel-filtration column c hromatography. Monomers were stable and had bilin optical spectra diff erent from the alpha(2) beta(2) dimers, although they have very simila r protein secondary structures. The optical spectra of phycoerythrin 5 45 showed four types of behavior with temperature: 10-20 degrees C, di mers; 40-50 degrees C, dimers/monomers; 60 degrees C, nearly fully dis ordered; 70 degrees C, disordered alpha and beta polypeptides. At 40 d egrees C, the protein dissociated partially to monomer, which could be totally reversed to dimers at 20-25 degrees C. The visible circular d ichroism difference spectrum for the protein dimers minus monomers exh ibited positive and negative bands-such spectra may indicate exciton s plitting between closely-spaced bilins. Circular dichroism also reveal ed a spectrum suggesting exciton coupling for the second excited state of the bilins. Ultrafast fluorescence using a two-photon method showe d the fastest time for protein dimers to be 2.4 ps and monomers had a 39-ps lifetime. Phycocyanin 645 was found to have a 550-fs lifetime.