Effects of excited state mixing on transient absorption spectra in dimers: Application to photosynthetic light-harvesting complex II

Abstract

The excited state mixing effect is taken into account considering the difference spectra of dimers. Both the degenerate ͑homo͒ dimer as well as the nondegenerate ͑hetero͒ dimer are considered. Due to the higher excited state mixing with the two-exciton states in the homodimer, the excited state absorption ͑or the difference spectrum͒ can be strongly affected in comparison with the results obtained in the Heitler-London approximation. The difference spectrum of the heterodimer is influenced by two resonance effects ͑i͒ mixing of the ground state optical transitions of both monomers in the dimer and ͑ii͒ mixing of the excited state absorption of the excited monomer with the ground state optical transition in the nonexcited monomer. These effects have been tested by simulating the difference absorption spectra of the light-harvesting complex of photosystem II ͑LHC II͒ experimentally obtained with the 60 fs excitation pulses at zero delay times and various excitation wavelengths. The pairs of coupled chlorophylls a and b for simulations have been taken from the best LHC II assignment model obtained by simulating the steady-state spectra and the transient absorption at various excitation wavelengths. Qualitatively the spectral peculiarities of the difference spectra are explained by means of the resonance interpigment interactions, which are responsible for the excited state mixing.

Two-Photon Excited Fluorescence from Higher Electronic States of Chlorophylls in Photosynthetic Antenna Complexes: A New Approach to Detect Strong Excitonic Chlorophyll a/b Coupling

“…Regarding LHC II at ambient temperature, there are few (more or less indirect) hints for the existence of Chl a/b heterodimers in the literature (Renger and May, 1997;Valkunas et al, 1999 and references therein), but to our knowledge there has been no unequivocal experimental proof. However, a recent determination of the dipole moment of the 680-nm transition of LHC II indicated coupling of at least two Chls at ambient temperature (Schubert et al, 2002;compare also Leupold et al, 1999).…”

A comparison of the three isoforms of the light-harvesting complex II using transient absorption and time-resolved fluorescence measurements

“…They reported for both carotenoid to chlorophyll and chlorophyll to chlorophyll similar transfer times and efficiencies to those of native LHC II complexes. Modeling of the energytransfer dynamics in LHC II has also been attempted in the past few years (Renger and May, 1997;Gradinaru et al 1998;Valkunas et al 1999;Iseri and Gu¨len 2001;Novoderezhkin et al 2003). An improvement in the description of the observed rate constants was achieved with a modified Redfield approach, including a strong electronphonon coupling.…”

Carotenoid-to-Chlorophyll Energy Transfer in Recombinant Major Light-Harvesting Complex (LHCII) of Higher Plants. I. Femtosecond Transient Absorption Measurements

“…Also, a great deal of steady-state and triplet spectroscopy has been performed (Barzda et al, 1998;Gülen et al, 1997;Peterman et al, 1995Peterman et al, , 1996Peterman et al, , 1997aNussberger et al, 1994). Several papers have been devoted primarily to elaborate modeling of the spectroscopic kinetic data on the basis of the known structure details (Gradinaru et al, 1998;Gülen et al, 1997;Visser et al, 1996;Trinkunas et al, 1997Trinkunas et al, -1999Valkunas et al, 1998Valkunas et al, , 1999. However, despite that substantial amount of work, no complete picture about the Chl organization, the pigment-pigment coupling, and the functional role of the carotenoids has been obtained.…”

“…The possibility of comparing the signals from these two samples turned out to be instrumental in arriving at a detailed kinetic model of the carotenoidto-Chl transfer and to propose a specific location and spectral properties of the Chls in general, and of Chls b located in the vicinity of the carotenoids in particular. This is part of a diversified approach in our group to better understand the pigment organization and function in this antenna complex, using inter alia dimer modeling (Valkunas et al, 1999), kinetic modeling (Trinkunas et al, 1997(Trinkunas et al, -1999, and annihilation experiments (Müller et al, unpublished results).…”