ENERGY-TRANSFER AND EXCITON-STATE RELAXATION PROCESSES IN ALLOPHYCOCYANIN

We have employed picosecond spectroscopic techniques to characterize the photophysics of the phycocyanobilin chromophores in linker-free allophycocyanin isolated from the cyanobacterium Synechococcus PCC 6301 (AN112 mutant). In analogy with the known structure of the related phycobiliprotein C-phycocyanin, allophycocyanin is probably organized as a ringlike homotrimer; the monomeric units are composed of an alpha and a beta-subunit, each of which binds a phycocyanobilin chromophore via a thioether linkage to a cysteine residue at amino acid position 84. We observe bidirectional excitation transfer in the alpha-beta monomer between the alpha-84 and beta-84 chromophores with a 140-ps time constant. Absorption anisotropy measurements show that the transition dipole moments of the alpha-84 and beta-84 chromophores are tilted 13 +/- 9-degrees apart in alpha-beta monomers. In (alpha-beta)3 trimers, however, the alpha-84 and beta-84 chromophores in adjacent alpha-beta monomers are brought close together, forming strong chromophore-chromophore interactions across the intermonomer interface. We interpret the observed photophysics using a model consisting of exciton levels formed by mixing of the monomeric singlet-state levels of the alpha-84 and beta-84 chromophores in the (alpha-beta)3 trimers; a system of three symmetry-equivalent but well separated chromophore dimers is formed, which produces a pair of triply degenerate singlet exciton states. We assign an ultrafast (< 2-ps time constant) anisotropy and photobleaching transient observed only in (alpha-beta)3 trimers to an interexciton level transition; the transient occurs with a polarization change that is consistent with a transition between the orthogonal upper and lower exciton states. The upper exciton state also relaxes directly to the ground state through a decay process with a 45-ps time constant. We attribute the heterogeneous relaxation of the upper exciton state through these two paths to an inhomogeneous broadening due to site heterogeneity, which was previously observed in C-phycocyanin in hole-burning experiments at low temperature (Kohler et al. Chem. Phys. Lett. 1988, 143, 169). Excited-state absorption, originating from the lower exciton state, is assigned to a transition yielding a doubly-excited exciton state (van Amerongen; Struve, J. Phys. Chem. 1991, 95, 9020). Excitation transfer among the degenerate lower exciton states is detected in terms of a 70-ps anisotropy decay observed in the photobleaching and stimulated emission. The interexciton level transition rapidly concentrates excitation in the lower exciton state of allophycocyanin (alpha-beta)3 trimers; this kind of spectral relaxation process may be important in facilitating directional excitation transfer in reaction center/light-harvesting protein assemblies.