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

Time-resolved fluorescence anisotropy spectroscopy has been used to study t he chlorophyll a (Chl a) to Chi a excitation energy transfer in the water-s oluble peridinin-chlorophyll a-protein (PCP) of the dinoflagellate Amphidin ium carterae. Monomeric PCP binds eight peridinins and two Chi a. The trime ric 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 in tra- and intermonomeric excitation equilibration times. Using the ratio tau (1)/tau(2) and the amplitude of the anisotropy, the best fit of the experim ental 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 lar ger than that of the solvent (water), Based on our observations we predict that the relatively slow intermonomeric energy transfer in vivo is overrule d by faster energy transfer from a PCP monomer to, e.g., the light-harvesti ng a/c complex.