Probing the many energy-transfer processes in the photosynthetic light-harvesting complex II at 77 K using energy-selective sub-picosecond transient absorption spectroscopy

The dynamics of energy equilibration in the main plant light-harvesting complex, LHCII, at a temperature of 77 K was probed using sub-picosecond excitation pulses at 649, 661, 672 and 682 nm and detection of the resulting difference absorption spectra from 630 to 700 nm. We find three distinct chlorophyll b to chlorophyll a (Chi a) transfer times, of < 0.3, 0.6 and 4-9 ps, respectively. From a comparison of the amplitudes of the bleaching signal, a plausible scheme for the Chi b to Chi a transfer in the LHCII complex is proposed. Two Chi b molecules transfer energy to Chi a in less than 0.3 ps, two Chi b molecules transfer with 0.6 ps and one Chi b has a transfer time of 4-9 ps. In the Chi a absorption region, a 2.4 ps energy-transfer process from a pigment absorbing around 661 nm, and a 0.4 ps process from a pigment absorbing around 672 nm is found. Furthermore, evidence is found for slow, 10-20 ps energy-transfer processes between some of the Chi a molecules. The data are compared to model calculations using the 3.4 Angstrom LHCII monomer structure (containing 5 Chi b and 7 Chi a molecules) and Forster energy transfer. We conclude that the observed energy-transfer rates are consistent with both the preliminary assignment of the Chi identities (a or b) of Kuhlbrandt et al. and a recent proposal for the arrangement of some of the transition dipole moments (Gulen et al.). Singlet-singlet and singlet-triplet annihilation processes are observed in two different experiments, and both these processes occur with time constants of 2-3 and 12-20 ps, suggesting that both annihilation pathways are at least partly limited by slow energy transfer. The wide range of observed time constants in the equilibration, from < 0.3 to similar to 20 ps, most likely reflects the irregular arrangement of the pigments in the complex, which shows much less symmetry than the recently obtained structure of the peripheral antenna complex of purple bacteria, LH-II (McDermott et al.).