Excitation transfer in C-phycocyanin. Förster transfer rate and exciton calculations based on new crystal structure data for C-phycocyanins from Agmenellum quadruplicatum and Mastigocladus laminosus

Abstract

F~rs:er mechanlsm; ExcitonCalculations of excitation tra~er rates among the chr,mmphores of C-phycocyanin using the F6rster inductive resonance transfer mechanism have been carried out using the new coordinates for the pesition and ovlentation of the dn-omophores (Schirmer, T., Bode, W. and Hubor, R. (1967) J. Mol. Biol. 196, 677-69b'). Several of the rate constants are significantly altered from the results of oor calculations using the previously published emmlinates (Sauer, K., Scheer, H. and Sauer, P. (1987) Photochem. Photoblol. 46, 427-4d0). Ill partiolllar, for the (a/~)3-trimers of Mastigoclmtm/am/Iosm or for the (al0)3-trimels or the (~8)¢hexamers of Agmn~,llmn ~adr~icatm~ the new calculations predict excited state relaxation components with exponential time constants shorter than 1 ps. In fact, some of the interactions are so strong that exeiton coupling is probably the relevant mechanism of interaction. The largest exciton energy is calculated to be about 56 an -I, for the imeracfion betweea the adjacent a84 and ~84 ehromophores of neighboring monomer units within the (a~)3.triaters or (aB)¢hexamers. An energy transfer model invoking a combination of pairwlse exciton formation followed by slower Fc~rster transfer steps is described. that transfer excitation to it with an efficiency of 95~ or greater and in a time that is much shorter than the lifetime (a few nanoseconds) of the excited state in the absence of trapping [1]. A model, known as the Pebble Mosaic Model, was developed to describe this excitation transfer process [2]. It is based on the knowledge that most photosynthetic pigments (chromophores) occur in welldefined associations with proteins and that these pigment proteins occur in organized arrays with respect to the embedded reaction centers in photosynthetically active membranes [3]. In the Pebble Mosaic Model excitation is considered to appear in collective excited states (excitons) that encompass all of the chromophores that interact with 0005-2728/88/$03.50

Lyase Activities of CpcS- and CpcT-like Proteins from Nostoc PCC7120 and Sequential Reconstitution of Binding Sites of Phycoerythrocyanin and Phycocyanin β-Subunits

“…These values compare well to the respective band positions assigned to the ␤155-chromophores of PEC and CPC from M. laminosus (41)(42)(43). The absorption and fluorescence spectra were very similar when PecB or CpcB from Nostoc PCC7120 were used as acceptor proteins, and the extinction coefficients and fluorescence yields of all products ( Table 2) are typical of native phycobiliproteins (43). Two of the products, PCB-PecB(C84A) and PCB-CpcB(C84S), were purified and analyzed in more detail.…”

“…pho1-pcyA, pETDuet-cpcT1, and pCDF-cpcS1 were co-transformed into E. coli BL21(DE3) together with pCOLA-pecB; the purified phycobiliprotein formed, however, had an absorption maximum at 602 nm, which is redshifted from that of ␤-PEC isolated from M. laminosus ( max ϭ 598 nm) (42). The same reconstitution based on pCOLAcpcB resulted in a chromoprotein with an absorption maximum at 618 nm, which again is red-shifted compared with that of ␤-CPC isolated from M. laminosus ( max ϭ 609 nm) (41,43). By contrast, however, the fluorescence maxima were in both cases the same for the reconstituted chromoprotein and the respective isolated subunit (629 and 644 nm for ␤-PEC and ␤-CPC, respectively).…”

Photophysics of phycoerythrocyanins from the cyanobacterium Westiellopsis prolifica studied by time-resolved fluorescence and coherent anti-Stokes Raman scattering spectroscopy

“…Such an increase in transfer time compared with trimers is reasonable in view of the fact that, in trimers, the energy transfer between a-and /3-chromophores is enhanced by the close proximity of a84 and/384 chromophores in neighbouring monomers (see Fig. 1) [26]. Since, with detection at 590 nm, the contribution of the terminal emitter to the fluorescence decay curve is only about 5%, the two shorter lifetimes (0.24 and 0.8 ns) could be related to the emission of ot-chromophores which do not participate in fast energy transfer, either because of large distances or because of a modified protein--chromophore arrangement.…”

“…All other isotropic kinetic components were longer than 100 ps. As in other phycobiliproteins [26,27], the ultrafast component is discussed in terms of energy transfer from t~84 PVB to the/3-chromophores of the neighbouring subunit. The second component, which showed a scatter in decay time between 110 and 330 ps, was related to energy transfer between the /3-chromophores.…”

Dissociating effect of chromophore modifications on C-phycocyanin heterohexamers

“…Energy transfer in phycobiliproteins is still only partly understood . Some main features of excited state kinetics have been modelled by Fijrster transfer [13], but there is experimental 119, 201 and theoretical 113, 17, 181 evidence for exciton coupling, in particular among the proximate a-84 and 884 chromophores of different (Crp), heterodimer ("monomer") units within the heterohexamers or heterododecamers. There are also several reports on more complicated excited state kinetics which are unaccounted for by the simple model, and which have been taken as evidence for microheterogeneity [17,18,21,221.…”