Comparison of Calculated and Experimentally Resolved Rate Constants for Excitation Energy Transfer in C-Phycocyanin. 1. Monomers

Abstract

Rate constants for excitation energy transfer in the light-harvesting protein, C-phycocyanin (PC), in the monomeric aggregation state, isolated from the cyanobacterium Synechococcus sp. PCC 7002, are calculated using Forster theory and compared with the results of time-resolved fluorescence measurements. In addition to the relative distances and orientations between chromophores, obtained from the crystal structure of PC, the Forster calculations require spectroscopic resolution of several properties of the chromophore types ( $1 5 5 , aS4, p 8 4 ) found in PC monomers, including absorption and fluorescence spectra, molar absorptivities, fluorescence quantum yields, and fluorescence lifetimes. The resolution of the first two properties, the chromophore absorption and fluorescence spectra, was described in a previous paper [Debreczeny et al. J. Phys. Chem. 1993, 97, 9852-98621. The assignments of the energy-transfer rate constants in PC monomers are confirmed here by time-resolved fluorescence anisotropy measurements of the PC monomers isolated from both the wild-type and a mutant strain (cpcBKZ55S) whose PC is missing the P I 5 5 chromophore. These fluorescence anisotropy measurements also allow one to extract the angles between the transition dipoles of the chromophores within the p l 5 5 -p 8 4 (34') and a 8 4 -p S 4 (27') chromophore pairs. The values of the inverse of the sum of the forward and back calculated Forster rate constants for energy transfer within the p l 5 5 -p 8 4 , ~8~-/ & 4 , and p 1 5 5 -a 8 4 chromophore pairs are 49, 158, and 890 ps, respectively, and are in excellent agreement with the experimentally measured values. It is concluded that the Forster model of resonant energy transfer in the weak coupling limit successfully describes the dominant energy-transfer processes in this protein in the monomeric state.

In silico model of an antenna of a phycobilisome and energy transfer rates determination by theoretical Förster approach

“…In this model, it is not possible to detect two clear pathways but a participation of most of the chromophores in the ET along the antenna toward PCBb82. Experimental and theoretical studies 12,26,31 on C-PC always point to the energy being funneled toward PCBb82 from where the energy should be transferred to APC. Also, in PE, the energy is channeled toward PEBb82; in both cases, these chromophores are facing the center of the hexamer.…”

“…10 A good approach used to calculate ET constants for this system has been the F€ orster approach, which has been used since the first structural studies performed on Phycocyanin. [11][12][13][14] Theoretical calculations have been performed for Phycocyanin using their three-dimensional structures, [15][16][17] and Ref. 13 reported a comparison between theoretical and experimental results for the ET in monomers and trimers of Phycocyanin from Synechococcus sp.…”

Biogenesis of Phycobiliproteins

“…The PCB chromophore bound to Cys-82 plays a critical role in ␣␤ monomer stability, and in PC trimers, hexamers, and rods, it is also the acceptor (fluorescing) bilin (54). This could provide an explanation for why we detected small amounts of chromophorylated Cys-82 ␤-PC in the cpcS-I and cpcU mutant strains (this report), but in the cpcT mutant, no chromophorylated Cys-153 ␤-PC was detected (24).…”

The structure at 2 Å resolution of Phycocyanin from Gracilaria chilensis and the energy transfer network in a PC–PC complex

“…Förster theory has been confirmed to be a good approximation to study the light transfer processes occurring in phycobilisomes [19,20]. Förster defines [18] a transfer constant k DA as a measure of the frequency of events of energy transfer between a Donor (D) and an Acceptor (A).…”

“…Theoretical and experimental studies have been reported regarding the light transfer among chromophores inside one hexameric ring [8,[14][15][16][17], including models that represent the energy transfer between hexamers [8,14,16]. In these studies, preferential energy transfer pathways have been proposed from data calculated using the Förster resonance approach [18], considering only the chromophore-chromophore distances and a Förster radius of 50 Ǻ as approximation [8] or the orientation among the transition dipole moments in one trimer of phycocyanin [19,20]. This paper reports the three dimensional structure of phycocyanin from G. chilensis, the building of a docking model of two PC hexamers refined by molecular dynamics and the determination of the constants for the energy transfer in resonance between pairs of chromophores.…”