Electron−Phonon Coupling in Solubilized LHC II Complexes of Green Plants Investigated by Line-Narrowing and Temperature-Dependent Fluorescence Spectroscopy

Abstract

Line-narrowed and temperature-dependent fluorescence spectra are reported for the solubilized trimeric lightharvesting complex of Photosystem II (LHC II). Special attention has been paid to eliminate effects owing to reabsorption and to ensure that the line-narrowed fluorescence spectra are virtually unaffected by hole burning or scattering artifacts. Analysis of line-narrowed fluorescence spectra at 4.2 K indicates that the lowest Q y -state of LHC II is characterized by weak electron-phonon coupling with a Huang-Rhys factor of ∼ 0.9 and a broad and strongly asymmetric one-phonon profile with a peak frequency ω m of 15 cm -1 and a width of Γ ) 105 cm -1 . The 4.2 K fluorescence data are further consistent with the assignment of the lowest Q y -state at ∼ 680.0 nm and an inhomogeneous width of ∼80 cm -1 gathered from a recent hole-burning study (Pieper et al. J. Phys. Chem. A 1999, 103, 2412. The temperature dependence of the fluorescence spectra of LHC II is simulated using the low-energy Q y -level structure reported in the latter study as well as the parameters of electron-phonon coupling determined in the present study. Up to a temperature of 120 K, the calculations reveal that this model satisfactorily describes the basic features of the fluorescence spectra such as thermal broadening and, especially, the blue-shift of the fluorescence peak with increasing temperature. An unexpected red-shift of the fluorescence peak above 150 K is attributed to conformational changes of the protein environment. The shape of the temperature-dependent fluorescence spectra indicates that the low-energy Q ystates are populated according to a Boltzmann distribution representing the thermal equilibrium of excitation energy.

Fluorescence hole-burning and site-selective studies of LHCII

“…The higher value of S for the shorter burning wavelengths may originate from the more efficient build-up of negative non-specific fluorescence changes (see above). If one assumes the same contributions of PSB and PPSB in A PSB/PPSB at λ B = 681 nm, and neglects the contribution from PPSB in the above formula for S, then S drops from 1.5 to 1.0 (following this assumption S is calculated from the formula: S = ln[(0.5*A PSB/PPSB +A ZPH )/A ZPH ]) which is in a good agreement with the value 0.9 reported for LHCII and derived from the FLN experiment (Pieper et al 2001). The S value of 1.0 was also estimated on the basis of pioneering FHB experiment on LHCII (Hala et al 1992).…”

“…2D for the excitation at 680 nm. Here, the spectra are much narrower than at the shorter excitation wavelengths, like in the classical fluorescence line narrowing (FLN) experiment (Pieper et al 2001, Peterman et al 1997. The maximum of the SSF spectrum before burning, at 680.8 nm, is 0.5-nm redshifted relative to that obtained at the shorter excitation wavelengths and that of non-selective spectra, and undergoes even a further red-shift, to 681.0 nm, after burning (~ 7×10 4 J m -2…”

“…A technique complementary to AHB, also wellestablished, is the fluorescence line narrowing (FLN; Peterman 1998, Pieper et al 2001, Timpmann et al 2004, Gibasiewicz et al 2005, Rätsep et al 2005. In photosynthetic complexes, low temperature emission originates from a distribution of the lowest-energy states.…”

“…Further, both AHB and FLN techniques were applied to investigate the electron-phonon coupling, which was concluded to be weak, and characterized by a mean phonon frequency ω m = 15-18 cm -1 and Huang-Rhys factor, S = 0.8-0.9 (Pieper et al 1999(Pieper et al , 2001). On the basis of the FLN studies, the 105-cm -1 width of the onephonon profile of individual emitters was found to be strongly asymmetric with a much steeper shorter than longer-wavelength slope (Pieper et al 2001).…”

Comments on the optical lineshape function: Application to transient hole-burned spectra of bacterial reaction centers

“…19, along with many later works [7][8][9][10][11][12][13][14][15][16][17][18] ). Although such a profile does provide a reasonable approximation to the true form of l R (ω) at low temperatures when the Gaussian and Lorentzian components have similar widths, the approximation breaks down quite seriously when Γ G is much different from √ RΓ L (see below).…”