Förster Excitation Energy Transfer in Peridinin-Chlorophyll-a-Protein

Abstract

Time-resolved fluorescence anisotropy spectroscopy has been used to study the chlorophyll a (Chl a) to Chl 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 Chl a. The trimeric structure of PCP, resolved at 2 Å (Hofmann et al., 1996, Science. 272:1788 1791), allows accurate calculations of energy transfer times by use of the Förster equation. The anisotropy decay time constants of 6.8 ± 0.8 ps (τ1) and 350 ± 15 ps (τ2) are respectively assigned to intra- and intermonomeric excitation equilibration times. Using the ratio τ1/τ2 and the amplitude of the anisotropy, the best fit of the experimental data is achieved when the Qy transition dipole moment is rotated by 2 7° with respect to the y axis in the plane of the Chl a molecule. In contrast to the conclusion of Moog et al. (1984, Biochemistry. 23:1564 1571) that the refractive index (n) in the Förster 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.