Q(y)-level structure and dynamics of solubilized light-harvesting complex II of green plants: Pressure and hole burning studies

Nonphotochemical hole burning and pressure-dependent absorption and hole-bu rning results are presented for the isolated (disaggregated) chlorophyll al b light-harvesting II trimer antenna complex of green plants. Analysis of t he 4.2 K burn-fluence dependent hole spectra and zero-phonon hole action sp ectra indicates that the three lowest energy states (Q(y)) lie at 677.1, 67 8.4 and 679.8 nm, Their combined absorption intensity is equivalent to that of three Chi 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 thr lowest energy state of the trimer subunit with th e 30 cm(-1) separations due to the indigenous structural heterogeneity of p rotein complexes. The linear electron-phonon coupling of the 679.8 nm state is weak and characterized, in part, by a mean phonon frequency of omega(m) = 18 cm(-1) and Huang-Rhys factor of S-m = 0.8, values which yield the com et Stokes shift for fluorescence from the 679.8 nm state at 4.2 Ii. The tem perature dependence of the zerophonon hole (ZPH) width for that state is co nsistent with optical dynamics due to coupling with glasslike two-level sys tems of the protein. The ZPH width at 1.9 K( is 0.037 cm(-1). Satellite hol e 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 th e Chi a molecule of the subunit associated with them is weakly coupled to o ther Chi molecules. The linear pressure shift rates for the main Q(y)-absor ption bands are also low. The shift rates appear to be dictated by protein- Chi interactions rather than excitonic couplings. Holes burned into the 650 nm absorption band reveal energy transfer times of 1 ps and similar to 100 fs which are discussed in terms of time domain measurements of the Chi b - -> Chi a transfer rates (Connelly et al. J. Phys. Chem. B 1997, 101, 1902). The holewidths associated with burning into the 676 nm absorption band lea d 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).