Picosecond excitation energy transfer of allophycocyanin studied in solution and in crystals

Cyanobacteria perform photosynthesis with the use of large light-harvesting antennae called phycobilisomes (PBSs). These hemispherical PBSs contain hundreds of open-chain tetrapyrrole chromophores bound to different peptides, providing an arrangement in which excitation energy is funnelled towards the PBS core from where it can be transferred to photosystem I and/or photosystem II. In the PBS core, many allophycocyanin (APC) trimers are present, red-light-absorbing phycobiliproteins that covalently bind phycocyanobilin (PCB) chromophores. APC trimers were amongst the first light-harvesting complexes to be crystallized. APC trimers have two spectrally different PCBs per monomer, a high- and a low-energy pigment. The crystal structure of the APC trimer reveals the close distance (~21 Å) between those two chromophores (the distance within one monomer is ~51 Å) and this explains the ultrafast (~1 ps) excitation energy transfer (EET) between them. Both chromophores adopt a somewhat different structure, which is held responsible for their spectral difference. Here we used spectrally resolved picosecond fluorescence to study EET in these APC trimers both in crystallized and in solubilized form. We found that not all closely spaced pigment couples consist of a low- and a high-energy pigment. In ~10% of the cases, a couple consists of two high-energy pigments. EET to a low-energy pigment, which can spectrally be resolved, occurs on a time scale of tens of picoseconds. This transfer turns out to be three times faster in the crystal than in the solution. The spectral characteristics and the time scale of this transfer component are similar to what have been observed in the whole cells of Synechocystis sp. PCC 6803, for which it was ascribed to EET from C-phycocyanin to APC. The present results thus demonstrate that part of this transfer should probably also be ascribed to EET within APC trimers

Changes in aggregation states of light-harvesting complexes as a mechanism for modulating energy transfer in desert crust cyanobacteria

In this paper we propose an energy dissipation mechanism that iscompletely reliant on changes in the aggregation state of thephycobilisome light-harvesting antenna components. All photosyntheticorganisms regulate the efficiency of excitation energy transfer(EET) to fit light energy supply to biochemical demands. Not many dothis to the extent required of desert crust cyanobacteria. Followingpredawn dew deposition, they harvest light energy with maximumefficiency until desiccating in the early morning hours. In thedesiccated state, absorbed energy is completely quenched. Timeand spectrally resolved fluorescence emission measurements of thedesiccated desert crust Leptolyngbya ohadii strain identified (i) reducedEET between phycobilisome components, (ii) shorter fluorescencelifetimes, and (iii) red shift in the emission spectra, comparedwith the hydrated state. These changes coincide with a loss of theordered phycobilisome structure, evident from small-angle neutronand X-ray scattering and cryo-transmission electron microscopy data.Based on these observations we propose a model where in the hydratedstate the organized rod structure of the phycobilisome supportsdirectional EET to reaction centers with minimal losses due tothermal dissipation. In the desiccated state this structure is lost, givingway to more random aggregates. The resulting EET path will exhibitincreased coupling to the environment and enhanced quenching