Energy transfer in the plant antenna complexes LHCII and CP29 of spinach studied by three pulse echo peakshift

Abstract

The light harvesting antennae LHCII and CP29 both contain a mixture of chlorophylls (Chls) a and b. Energy transfer between these pigments has been studied extensively, mostly with wavelength resolving techniques (both steady state and time dependent) which show transfer between pigments of different transition frequency. This transfer takes place on many timescales, which range from a few 100 fs for transfer between close lying pigments to several tens off ps for transfer between clusters of pigments. A method for studying energy transfer between pigments with the same transition frequency is the Three pulse Echo Peakshift (3PEPS) technique. Agarwal et al. have applied it to LHCII trimers and found a 200 fs decay process when exciting in the Chl b Qy region (650 nm) which they suggest to be Chl b <-> Chl b transfer between two of the 5 Chls in LHCII. We repeated their experiment and compared LHCII with CP29. CP29 has only two Chls b, at a large distance from each other, so no fast Chl b<-> Chl b energy transfer could occur. We measured the peakshift decay at 650 nm and 670 nm (Chl a Qy band) on both complexes. At 650 nm, we find a 200 fs decay for LHCII which is absent in CP29. Hence, Chl b<-> Chl b transfer must indeed occur in LHCII. For both complexes, Chl b -> Chl a energy transfer times known from other studies are also seen. A second difference between LHCII and CP29 is found on a 5-10 ps timescale at both wavelengths. Due to the larger amount of Chls a in LHCII compared to CP29 (21, vs 6), the amplitude of this decay process is also larger in LHCII