Comparison of the excited-state dynamics of five- and six-chlorophyll photosystem II reaction center complexes

Spectral hole-burning experiments have been performed at liquid He temperature on the Q(y)-band of isolated reaction center complexes of photosystem II (PS II RC) containing five (RC-5) and six (RC-6) chlorophyll a (Chl a) molecules. The aim was to investigate the nature of the redmost shoulder in the absorption spectrum of RC-5 and to identify distributions of "trap" pigments. The "effective" homogeneous line width Gamma(hom)' was measured at 682 nm as a function of temperature between 1.2 and 4.2 K. It follows a T1.3+/-0.1 power law in both complexes 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 spectral distribution of these traps was reconstructed from the hole depth measured as a function of excitation wavelength lambda(exc) and compared 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 distribution lies at (682.9 +/- 0.2) nm. We discovered a second distribution of 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 burning and probing, for the red- and blue-absorbing pigments, is constant 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.