Assignment of the Lowest QY-state and Spectral Dynamics of the CP29 Chlorophyll a/b Antenna Complex of Green Plants: A Hole-burning Study ‡

Abstract

Low‐temperature absorption, fluorescence and persistent nonphotochemical hole‐burned spectra are reported for the CP29 chlorophyll (Chl) a/b antenna complex of photosystem II of green plants. The absorption‐origin band of the lowest Qy‐state lies at 678.2 nm and carries a width of ∼130 cm−1 that is dominated by inhomogeneous broadening at low temperatures. Its absorption intensity is equivalent to that of one of the six Chl a molecules of CP29. The absence of a significant satellite hole structure produced by hole burning, within the absorption band of the lowest state, indicates that the associated Chl a molecule is weakly coupled to the other Chl and, therefore, that the lowest‐energy state is highly localized on a single Chl a molecule. The electron–phonon coupling of the 678.2 nm state is weak with a Huang–Rhys factor S of 0.5 and a peak phonon frequency (ωm) of ∼20 cm−1. These values give a Stokes shift (2Sωm) in good agreement with the measured positions of the absorption band at 678.2 nm and a fluorescence‐origin band at 679.1 nm. Zero‐phonon holes associated with the lowest state have a width of ∼0.05 cm−1 at 4.2 K, corresponding to a total effective dephasing time of ∼400 ps. The temperature dependence of the zero‐phonon holewidth indicates that this time constant is dominated at temperatures below 8 K by pure dephasing/spectral diffusion due to coupling of the optical transition to the glass‐like two‐level systems of the protein. Zero‐phonon holewidths obtained for the Chl b bands at 638.5 and 650.0 nm, at 4.2 K, lead to lower limits of 900 ± 150 fs and 4.2 ± 0.3 ps, respectively, for the Chl b→ Chl a energy‐transfer times. Downward energy transfer from the Chl a state(s) at 665.0 nm occurs in 5.3 ± 0.6 ps at 4.2 K.

Elucidation of structure–function relationships in plant major light-harvesting complex (LHC II) by nonlinear spectroscopy

“…A Chl b molecule absorbing at about 640 nm was found to be strongly coupled with Chl(s) a absorbing at about 670 nm in CP29 . These results were found to be consistent with other spectroscopic studies at RT as well as cryogenic temperatures (e.g., Pieper et al 2000;Leupold et al 2002;Leupold et al 2006).…”

“…The other Chl b (absorbing at about 650 nm) is not excitonically coupled (cf. Pieper et al 2000;Voigt et al 2002;Leupold et al 2002;Leupold et al 2006). …”

“…Another laser-spectroscopic technique-which may provide similar information as NLPF-is non-photochemical hole-burning spectroscopy (see e.g., Pieper et al 2000). However, this technique necessarily requires very low (unphysiological) temperatures at which also alterations of the structures of the pigment-protein complexes under investigation may occur (cf.…”

Water soluble chlorophyll binding protein of higher plants: A most suitable model system for basic analyses of pigment–pigment and pigment–protein interactions in chlorophyll protein complexes

“…4b) would require drastic band sharpening (thus reducing the overlap integral between donor and acceptor) and/or a significant lengthening of the distance at low temperatures which is not easily rationalized. On the other hand, these kinetics compare favourably to the effective dephasing times determined for the terminal (fluorescing) electronic states of trimeric LHC II (Pieper et al, 1999a;Tietz et al, 2001) and CP29 (Pieper et al, 2000). Therefore, we assigned the "100 ps component" to a protein relaxation process in the excited electronic state (Schmitt et al, 2008).…”

Elucidation of structure–function relationships in photosynthetic light-harvesting antenna complexes by non-linear polarization spectroscopy in the frequency domain (NLPF)

“…Non-photochemical hole burning spectroscopy (see e.g., Pieper et al, 2000) can -in principle -provide comparable information, however, it requires very low (unphysiological) temperatures at which also alterations of the structure of the pigment-protein complexes may occur (cf. e.g., Rogl et al, 2002).…”