EXCITATION-ENERGY TRANSFER FROM THE CHLOROPHYLL SPECTRAL FORMS TO PHOTOSYSTEM-II REACTION CENTERS - A FLUORESCENCE INDUCTION STUDY

Excitation energy transfer from Photosystem II antenna to reaction centres as a function of excitation wavelength, between 646-701 nm, was studied by analysis of the fluorescence quenching efficiency of RCs in both a BBY-grana preparation and thylakoids of the chlorina barley mutant lacking LHCPII. For BBY-grana, maximum transfer was observed with an excitation wavelength slightly greater than 680 nm, with significant decreases occurring for both longer and shorter wavelengths. The data are analysed in terms of the chlorophyll spectral forms as determined by asymmetric gaussian deconvolution of room temperature absorption spectra. For the short-wavelength side of the transfer maximum (646-682 nm) the following relative transfer efficiencies have been determined: 684 > 670 > 650; 678 > 661; 684 > 678 nm. Energy transfer between Photosystem II units is shown to increase with decreasing absorption wavelength below 683 nm. Thus, the above relative transfer efficiencies are associated with transfer to reaction centres within Photosystem II units and not between PS II units. The data are discussed in terms of alternative antenna models. It is argued that they are best accommodated by a model in which the antenna spectral forms have no particular macroscopic distribution with respect to RCs and in which the lower-wavelength-absorbing spectral forms may function as 'antitraps' for the longer-wavelength forms as determined by the transfer microparameters. Analysis of the gaussian band associated with Chl b in BBY-grana and LHCII suggest that this antenna component has a low-transition dipole strength within Photosystem II antenna complexes. It is suggested that this will enhance the role of Chl b as an 'antitrap' for excited states associated with the longer-wavelength-absorbing chlorophyll species. © 1990.