EVIDENCE FOR DELOCALIZATION OF THE TRIPLET-STATE P-3(680) IN THE D(1)D(2)CYTB(559)-COMPLEX OF PHOTOSYSTEM-II

Spin polarized transient ESR spectra of P-3(680) in D(1)D(2)cytb(559)- complexes of Photosystem II are studied as a function of temperature. The spin polarization as well as the rise time of the ESR signals are characteristic of triplet formation via recombination from the primary radical pair P(680)(+.)Pheo(-.). Below 100 K the well-known spectrum of P-3(680) is observed with zero-field splitting (zfs) parameters ass ociated with the tripler slate of monomeric chlorophyll-a (Chl). Betwe en 100 K and 200 K the spectral pattern changes drastically and above 200 K a considerably narrowed triplet spectrum (i.e. reduced zfs param eters) is observed which suggests a delocalization of the triplet exci tation. When the temperature-dependent spectra are normalized to the s ame triplet yield, isosbestic points are observed indicating that the spectra are weighted sums of the limiting case spectra at low and high temperature (10 K and 250 K, respectively). This is taken as evidence that a transition between two triplet states of different electronic origin occurs. The coefficients of this superposition follow a sigmoid al temperature dependence which suggests a distribution of activation energies for the triplet delocalization process in accordance with a g lass-like distribution of protein matrix conformations. The narrowing of the spectrum at high temperature can be interpreted as thermally ac tivated delocalization of the triplet excitation over at least two Chl units with different orientations with respect to the PS II reaction center, viz. one oriented with its plane lilted 30 degrees with respec t to the membrane plane and the other with its plane perpendicular. Th e similarities and differences in the cofactor arrangement of PS II an d purple bacteria reaction center are discussed.