ENERGY TRANSFER STUDIES ON CRYPTOMONAD BILIPROTEINS

Abstract

Abstract. Fluorescence techniques of various types have been used to study the light‐gathering and energy transfer modes for various cryptomonad biliproteins (phycocyanin or phycoerythrins). Analysis of fluorescence polarization and absorption data demonstrates that each cryptomonad biliprotein is composed of at least two distinct types of absorbing chromophore, each attached to the protein through covalent linkages to different polypeptide chains. Examination of the fluorescence emission spectra as a function of excitation at several wavelengths demonstrates that only one of these absorbing chromo‐phores is responsible for the fluorescence. This behavior is consistent with a known phenomenon whereby photons are gathered by more than one chromophore and then after radiationless energy transfer are emitted by only one chromophore.Application of Förster dipole‐dipole energy transfer theory is made to the study of the mode by which energy absorbed by biliproteins migrates to Chl a. The spectral overlap integral between phycocyanin (Chroomonas sp.) and Chl a is 7.13 ± 10‐10cm6mol‐1and between phycocyanin and Chl c2 0.25 ± 10‐10cm6mol‐1. This large difference in overlap suggests, although does not prove, that phycocyanin might transfer energy directly to Chl a without a Chl c2 intermediary. The cryptomonad phycoerythrins may also use this method but a Chl c2 intermediate could not be ruled out for them. Radiationless energy transfer among homogeneous biliproteins is shown to be feasible. All these calculations are based on in vitro spectra and the interpretations extrapolated to the cellular situation, and these tentative conclusions are reached without knowledge of other factors, such as chromophore‐chro‐mophore orientation and distance, which could greatly influence the energy transfer scheme.