FORSTER TRANSFER CALCULATIONS BASED ON CRYSTAL STRUCTURE DATA FROM Agmenellum quadruplicatum

Excitation energy transfer in C-phycocyanin is modeled using the Forster inductive resonance mechanism. Detailed calculations are carried out using coordinates and orientations of the chromophores derived from X-ray crystallographic studies of C-phycocyanin from two different species (Schirmer et al., J . Mol. Biol. 184, 257-277 (1985) and ibid., 188, 651-677 (1986)). Spectral overlap integrals are estimated from absorption and fluorescence spectra of C-phycocyanin of Mastigocladus laminosus and its separated subunits. Calculations are carried out for the p-subunit, a@-monomer, (ap),-trimer and (ap),-hexamer species with the following chromophore assignments: pl55 = 's' (sensitizer), p84 = 'f' (fluorescer) and a84 = 'm' (intermediate)$. The calculations show that excitation transfer relaxation occurs to 298% within 200 ps in nearly every case; however, the rates increase as much as 10-fold for the higher aggregates. Comparison with experimental data on fluorescence decay and depolarization kinetics from the literature shows qualitative agreement with these calculations. We conclude that Forster transfer is sufficient to account for all of the observed fluorescence properties of C-phycocyanin in aggregation states up to the hexamer and in the absence of linker polypeptides.