STRUCTURE-FUNCTION-RELATIONSHIPS IN THE 47-KDA ANTENNA PROTEIN AND ITS COMPLEX WITH THE PHOTOSYSTEM-II REACTION-CENTER CORE - INSIGHTS FROM PICOSECOND FLUORESCENCE DECAY KINETICS AND RESONANCE RAMAN-SPECTROSCOPY

We report the fluorescence decay kinetics and, the vibrational properties of Chlorophyll a bound to the 47-kDa antenna protein (CP47) of spinach photosystem II. The chlorophyll fluorescence of CP47 samples decays with four lifetimes (tau = 75.8 ps, 1.05 ns, 3.22 ns, and 5.41 ns). The 75.8-ps and 3.22-ns components are associated with chlorophyll a bound tb relatively intact centers, the 1.05-ns component corresponds to chlorophyll bound to centers that are slightly perturbed, and the the 5.41-ns phase probably originates from centers that are severely denatured. The resonance Raman spectrum of CP47 at 441.6 nm (this work) and at 406.7 nm [de Paula, J. C.; Ghanotakis, D. F., Bowlby, N. R., Dekker, J. P., Yocum, C. F., & Babcock, G. T. (1990) in Current Research in Photosynthesis (Baltscheffsky, M., Ed.), Vol. I, pp 643-646, Kluwer Academic Publishers, Dordrecht, The Netherlands] shows heterogeneity in the C=0 stretching region. This part of the spectrum monitors the environment of the keto group at position 9 of the chlorophyll a molecule. We show that several structurally distinct pools of chlorophyll a are bound to CP47. Four of these may be distinguished by their C-9=0 stretching frequencies (nu(C=O) = 1670, 1688, 1693, and 1701 cm(-1)). By analyzing the resonance enhancement pattern of these modes, we ascribe the 1693-cm(-1) vibration to denatured centers. Of the remaining populations, we propose that the 1670-cm(-1) vibration is consistent with a hydrogen bond between the C-9=0 group of chlorophyll a and the protein. We elaborate on the role of this chromophore-protein interaction in the mechanism of energy transfer within the 47-kDa antenna protein.