ENERGY-TRANSFER IN LHCII MONOMERS AT 77K STUDIED BY SUBPICOSECOND TRANSIENT ABSORPTION-SPECTROSCOPY

Energy transfer from chlorophyll b (Chl b) to chlorophyll a (Chl a) in monomeric preparations of light-harvesting complex II (LHCII) from sp inach was studied at 77 K using pump-probe experiments, Sub-picosecond excitation pulses centered at 650 nm were used to excite preferential ly Chl b and difference absorption spectra were detected from 630 to 7 00 nm. Two distinct Chl I? to Chl a transfer times, similar to 200 fs and 3 ps, were found, A clearly distinguishable energy transfer proces s between Chl a molecules occurred with a time constant of IS ps. Tile LHCII monomer data are compared to previously obtained LHCII trimer d ata, and both data sets are fitted simultaneously using a global analy sis fitting routine. Both sets could be described with the following t ime constants: 140 fs, 600 fs, 8 ps, 20 ps, and 2.9 ns, In both monome rs and trimers 50% of the Chl b to Chl a transfer is ultrafast (<200 f s). However, for monomers this transfer oocurs to Chl rt molecules tha t absorb significantly more toward shorter wavelengths than for trimer s. Part of the transfer from Chl b to Chl a that occurs with a time co nstant of 600 fs in trimers is slowed down to several picoseconds in m onomers. However, it is argued that observed differences between monom ers and trimers should be ascribed to the loss of some Chl a upon mono merization or a shift of the absorption maximum of one or several Chl a molecules. It is concluded that Chl b to Chl a transfer occurs only within monomeric subunits of the trimers and not between different sub units.