Forster excitation energy transfer in peridinin-chlorophyll-a-protein

Time-resolved fluorescence anisotropy spectroscopy has been used to study the chlorophyll a (Chl a) to Chi a excitation energy transfer in the water-soluble peridinin-chlorophyll a-protein (PCP) of the dinoflagellate Amphidinium carterae. Monomeric PCP binds eight peridinins and two Chi a. The trimeric structure of PCP, resolved at 2 Angstrom (Hofmann et at., 1996, Science. 272:1788-1791), allows accurate calculations of energy transfer limes by use of the Forster equation, The anisotropy decay time constants of 6.8 +/- 0.8 ps (tau(1)) and 350 +/- 15 ps (tau(2)) are respectively assigned to intra- and intermonomeric excitation equilibration times. Using the ratio tau(1)/tau(2) and the amplitude of the anisotropy, the best fit of the experimental data is achieved when the Q(y) transition dipole moment is rotated by 2-7 degrees with respect to the y axis in the plane of the Chi a molecule. In contrast to the conclusion of Moog et al, (1984, Biochemistry. 23:1564-1571) that the refractive index (n) in the Forster equation should be equal to that of the solvent, n can be estimated to be 1.6 +/- 0.1, which is larger than that of the solvent (water), Based on our observations we predict that the relatively slow intermonomeric energy transfer in vivo is overruled by faster energy transfer from a PCP monomer to, e.g., the light-harvesting a/c complex.