Energy-transfer and exciton-state relaxation processes in allophycocyanin

Abstract

4658region of the phase diagram, implying also an invariant total surfactant headgroup area in this region. These results were supported by QELS and SAXS measurements, which demonstrated that the micellar radius changes linearly with watersurfactant ratio, w,,. The measured total surfactant headgroup area agreed well with values estimated from phase boundary titration results. Conductivity measurements indicated no onset of percolation over the composition range studied. Above wo values J. Phys. Chem. 1992, 96, 4658-4666 of 5-7, the water in the reversed micellar cores had the properties of bulk water.Acknowledgment. This work was supported by the National Science ~oundation through a Presidential Young Investigator's Award to T.A.H., with matching funds from Dow Chemical Co.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 (AN1 12 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 a and a ,9 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 a,9 monomer between the a84 and ,984 chromophores with a 140-ps time constant. Absorption anisotropy measurements show that the transition dipole moments of the a84 and ,984 chromophores are tilted 13 * 9' apart in a@ monomers. In trimers, however, the a84 and 884 chromophores in adjacent a@ 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 a84 and ,984 chromophores in the 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 ( q 2 -p~ time constant) anisotropy and photobleaching transient observed only in 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 a...

Femtosecond spectral and anisotropy study of excitation energy transfer between neighbouring α-80 and β-81 chromophores of allophycocyanin trimers

“…The results obtained with picosecond resolution [16] are also in agreement with this model. Almost complete recovery of the transient bleaching at 620-633 nm was observed during < 2 ps [16].…”

“…According to the first one (excitonic model), two excitonic states arise as a result of the splitting of the doubly degenerate S 1 electronic state of spectrally very similar a-80 and /3-81 chromophores. This idea was first proposed by MacColl and coworkers [17] and later further elaborated in fluorescence [18] and absorption [16] ultrafast measurements. In this model the trimer absorption spectrum as a whole belongs to both (a-80 and /3-81) spectrally nearly identical chromophores or, more correctly, to the excitonically coupled dimer formed upon aggregation.…”

“…The suggested excited state absorption peak arising at 615-640 nm after 440-fs relaxation process is not surprising for excitonic interaction. In the case of the a-80//3-81 dimer such an excited state absorption could originate from the excitation of the second molecule in the pair and as a result a doubly excited dimer state will be formed [16].…”

“…It will be close to the theoretical maximum value of 0.4 only if both transitions are parallel. However, the transition from the relaxed excitotonic state to a doubly excited state should be polarized perpendicularly to the transition initially excited at 615 nm [16]. Therefore, absorption recovery kinetics will be accompanied at 615 nm by anisotropy decay.…”

Energy migration in allophycocyanin‐B trimer with a linker polypeptide: Analysis by the principal multi‐component spectral estimation (PMSE) method

“…The three-dimensional structure of the APC trimer is considered to be similar to that of the PC trimer, including the Cs symmetry axis, since the primary structure of APC is similar to that of phycocyanin (PC) [16]. The a-84 chromophore in the PC trimer, which is covalently bound to the cysteine residue of the 84th amino acid of the a-subunit in one monomer, is located close to thep-84 chromophore in an adjacent monomer, and the center-to-center distance between the two chromophores is about 21 A [ 17-191. Since this geometry is analogous to the APC-B trimer, a strong interaction is expected between the a-84 and the P-84 chromophores, and the decay kinetics can be interpreted either by a simple transfer process or by coherent decay of the exciton state [20]. The degree of interaction can be discriminated by spectral resolution of the components and their kinetics.…”