Qy-Level Structure and Dynamics of Solubilized Light-Harvesting Complex II of Green Plants: Pressure and Hole Burning Studies

Abstract

Nonphotochemical hole burning and pressure-dependent absorption and hole-burning results are presented for the isolated (disaggregated) chlorophyll a/b light-harvesting II trimer antenna complex of green plants. Analysis of the 4.2 K burn-fluence dependent hole spectra and zero-phonon hole action spectra indicates that the three lowest energy states (Qy) lie at 677.1, 678.4 and 679.8 nm. Their combined absorption intensity is equivalent to that of three Chl a molecules. The inhomogeneous broadening of their absorption bands is 70 cm-1. It is argued that these states, separated by 30 cm-1, are associated with the lowest energy state of the trimer subunit with the 30 cm-1 separations due to the indigenous structural heterogeneity of protein complexes. The linear electron−phonon coupling of the 679.8 nm state is weak and characterized, in part, by a mean phonon frequency of ωm = 18 cm-1 and Huang−Rhys factor of Sm = 0.8, values which yield the correct Stokes shift for fluorescence from the 679.8 nm state at 4.2 K. The temperature dependence of the zero-phonon hole (ZPH) width for that state is consistent with optical dynamics due to coupling with glasslike two-level systems of the protein. The ZPH width at 1.9 K is 0.037 cm-1. Satellite hole structure produced by burning in the above three states as well as their low linear pressures shift rates (about − 0.08 cm-1/MPa) indicate that the Chl a molecule of the subunit associated with them is weakly coupled to other Chl molecules. The linear pressure shift rates for the main Qy-absorption bands are also low. The shift rates appear to be dictated by protein−Chl interactions rather than excitonic couplings. Holes burned into the 650 nm absorption band reveal energy transfer times of 1 ps and ∼100 fs which are discussed in terms of time domain measurements of the Chl b → Chl a transfer rates (Connelly et al. J. Phys. Chem. B 1997, 101, 1902). The holewidths associated with burning into the 676 nm absorption band lead to Chl a → Chl a transfer times in the 6−10 ps range, in good agreement with the time domain values (Savikhin et al. Biophys. J. 1994, 66, 1597).