Identifying the Pathways of Energy Transfer between Carotenoids and Chlorophylls in LHCII and CP29. A Multicolor, Femtosecond Pump−Probe Study

Abstract

Spectral and kinetic information on energy transfer from carotenoids (Cars) to chlorophylls (Chls) within light-harvesting complex II (LHCII) and CP29 was obtained from femtosecond transient absorption study by using selective Car excitation (489 and 506 nm) and detecting the induced changes over a wide spectral interval (460−720 nm). By examining the evolution of entire spectral bands rather than looking at a few single traces, we were able to identify the species (pigments and/or electronic states) which participate in the energy flow, as well as the lifetimes and quantum yields of individual processes. Hence, it was found that the initially excited Car S2 state decays very fast, with lifetimes of 70−90 fs in CP29 and 100 ± 20 fs in LHCII, via two competing channels: energy transfer to Chls (60−65%) and internal conversion to the lower, optically forbidden S1 state (35−40%). In CP29, the energy acceptors are exclusively Chls a, while in LHCII, this is only valid for lutein and violaxanthin. In the latter case, neoxanthin transfers energy mostly to Chls b. In both complexes, ca. 15−20% of the initial Car excitations are transferred to Chls a via the S1 level, with a time constant of around 1 ps, thus bringing the total Car−Chl transfer efficiency to ca. 80%. Given the yield of this process and the large difference between the transfer time and the intrinsic S1 lifetime (∼20 ps), it seems that lutein is the only species active on this pathway. From the measured transfer rates, we estimated that a coupling of 280−330 cm-1 drives the transfer via the S2 route, while a coupling value of around 100 cm-1 was estimated for the S1 transfer. The Car S2 state is coupled to both Qx and Qy states of the Chl through a Coulombic mechanism; from the available structural information, we estimated the dipole−dipole contribution to be 450−500 cm-1. The S1 state is coupled to the Chl a Qy transition via an exchange and/or a Coulombic mechanism.