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 S 2 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 S 1 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 S 1 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 S 1 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 S 2 route, while a coupling value of around 100 cm -1 was estimated for the S 1 transfer. The Car S 2 state is coupled to both Q x and Q y 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 S 1 state is coupled to the Chl a Q y transition via an exchange and/or a Coulombic mechanism.