Excitation energy transfer and vibronic coherence in intact phycobilisomes

Phycobilisome’s Exciton Transfer Efficiency Relies on an Energetic Funnel Driven by Chromophore–Linker Protein Interactions

“…(Supporting Figure 1). This decay is in excellent agreement with the known fluorescence lifetime of the phycobilisome core. ,, Our observed timescale of ∼8 ps is closer to the 13 ps timescale suggested by the global analysis of 2DES data by Beck and co-workers but much faster than timescales obtained from other similar compartmental models. ,,, Moreover, Moran and co-workers do not see a signal redder than λ det = ∼645 nm along the detection axis in photon echo peak shift spectra of C-phycocyanin . The phycocyanin emission maximum is also at 645 nm .…”

“…This assumption is reasonable because phycocyanobilin is an approximately linear molecule. It has been recently shown by Beck and co-workers that excitations in phycocyanin hexamers remain delocalized over the α 84 –β 84 chromophore pair. The canceling of the ESA feature in our data suggests that in these nearly linear molecules, transition dipoles from singly excited states to higher-lying states are nearly parallel to the transition dipoles from the ground to singly excited states.…”

“…Despite seemingly deleterious slow exciton hopping over the sparse arrangement of highly fluorescent chromophores, the complex maintains near-unity exciton transfer efficiency. , As detailed in numerous studies, the mechanisms yielding this high transfer efficiency are not well known. ,− Electron microscopy (EM) advances in recent years have elucidated the detailed structure of these megacomplexes and provided many clues about the underlying mechanisms. ,,− However, progress in resolving energy transfer dynamics using time-resolved spectroscopy has been slower owing to massive spectral congestion from hundreds of phycobilisome pigments. − Small energetic separations in the donor and acceptor chromophores in the network confound isolation of signal kinetics from individual pigments or complexes, hiding the principles driving high transfer efficiencies that operate on the supercomplex or quaternary level. An alternative to using spectral resolution to reveal excitonic pathways is to use the different dipole directions of the linear phycocyanobilin chromophores in the complex through polarization-dependent spectroscopy.…”

“…In coherent time-resolved spectra, the phycobilisome shows a broad photoinduced absorption (PIA) feature suggested to arise from an electrochromic shift coupling ,, of excited- and ground-state chromophores, which masks the red-most absorption and complete emission profile. ,, Previous time-resolved studies have suggested that excitations created in the rods travel downhill to the higher-energy ApcA and ApcB (hereafter called APC 660 for their emission maxima) core proteins on the 30–50 ps timescale and from APC 660 to the core terminal emitters bound to ApcD, ApcE, and ApcF (hereafter called APC 680 ) proteins, on the 100 ps timescale before fluorescing with ca. 1–2 ns lifetime in detached complexes. ,,,, APC 680 terminal emitters transfer excitations to the photosystems that lie beneath the phycobilisomes. , A new study by Beck and co-workers uses global analysis of 2DES data and suggests that excitons travel to the core in 13 ps . The exact timescale of rod-to-core transfer remains a topic of debate and differs in different global analyses.…”

From a one-mode to a multi-mode understanding of conical intersection mediated ultrafast organic photochemical reactions

“…[249][250][251][252] For instance, it has been suggested that VC enhances the efficiency of energy transfer in a photosynthetic dimer model system 253 and charge separation at a donor-acceptor interface. 254 Moreover, the intimate coupling between vibrational modes and energy transfer in photosynthetic functions is gaining more attention over the years, driven both by (two-dimensional electronic) spectroscopic achievements [255][256][257][258] as well as mixed quantum-classical simulations, which clearly show a cooperative effect of multiple modes. 259,260 Despite this, often a single mode is assumed to be relevant in the description of vibronic coupling.…”