Singlet-singlet annihilation kinetics in aggregates and trimers of LHCII

Singlet-singlet annihilation experiments have been performed on trimeric an d aggregated light-harvesting complex II (LHCII) using picosecond spectrosc opy to study spatial equilibration times in LHCII preparations, complementi ng the large amount of data on spectral equilibration available in literatu re. The annihilation kinetics for trimers can well be described by a statis tical approach, and an annihilation rate of (24 ps)(-1) is obtained. In con trast, the annihilation kinetics for aggregates can well be described by a kinetic approach over many hundreds of picoseconds, and it is shown that th ere is no clear distinction between inter- and intratrimer transfer of exci tation energy. With this approach, an annihilation rate of (16 ps)(-1) is o btained after normalization of the annihilation rate per trimer. It is show n that the spatial equilibration in trimeric LHCII between chlorophyll a mo lecules occurs on a time scale that is an order of magnitude longer than in Photosystem I-core, after correcting for the different number of chlorophy ll a molecules in both systems. The slow transfer in LHCII is possibly an i mportant factor in determining excitation trapping in Photosystem II, becau se it contributes significantly to the overall trapping time.