COMPARISON OF THE EXCITED-STATE DYNAMICS OF 5-CHLOROPHYLL AND 6-CHLOROPHYLL PHOTOSYSTEM-II REACTION-CENTER COMPLEXES

Spectral hole-burning experiments have been performed at liquid He tem perature on the Q(y)-band of isolated reaction center complexes of pho tosystem II (PS II RC) containing five (RC-5) and six (RC-6) chlorophy ll a (Chl a) molecules. The aim was to investigate the nature of the r edmost shoulder in the absorption spectrum of RC-5 and to identify dis tributions of ''trap'' pigments. The ''effective'' homogeneous line wi dth Gamma(hom)' was measured at 682 nm as a function of temperature be tween 1.2 and 4.2 K. It follows a T1.3+/-0.1 power law in both complex es and extrapolates to the fluorescence lifetime-limited value, tau(n) = (4 +/- 1) ns, for T --> 0. These results indicate that the redmost absorbing pigments act as ''traps'' for the excitation energy. The spe ctral distribution of these traps was reconstructed from the hole dept h measured as a function of excitation wavelength lambda(exc) and comp ared to that of RC-6 previously obtained by us (Groot, M. L.; et al. J . Phys. Chem. 1996, 100, 11488). The maximum of the RC-5 trap distribu tion lies at (682.9 +/- 0.2) nm. We discovered a second distribution o f fluorescing pigments centered at (673.4 +/- 0.5) nm in both RC-5 and RC-6. The dependence of Gamma(hom)' on the delay time t(d) between bu rning and probing, for the red- and blue-absorbing pigments, is consta nt for t(d) less than or equal to 1 s and increases linearly with log t(d) for longer delay times. The molecules absorbing at similar to 674 nm, which can be chemically removed, are not free Chi a but are bound to a protein with the same mass as that of the RC complex.