The first step in photosynthesis is the absorption of light by the light-harvesting (LH) apparatus. Transfer of energy from the LH to the reaction center (RC) leads to a stabilized charge-separated state across the membrane, which drives chemical transduction. The unique structure of the pigment-protein complexes (PPCs) that compose the LH apparatus of bacterial photosynthesis has motivated extensive theoretical and experimental investigations in an attempt to understand both the biological and physical significance of the structure of these complexes and their organization within the membrane. The photosynthetic bacterium Rhodobacter sphaeroides serves as a model system to understand the functional role of this organization. The PSU of Rh. Sphaeroides is composed of two types of LH complexes: LH2, whose role is to efficiently collect solar flux in variable amounts and LH1, which accepts energy from LH2 and transfers it to the tightly associated reaction center (RC) complex. The energy transfer dynamics in a wide variety of isolated LH2 and LH1/RC complexes has been studied for over a decade. In LH2 numerous time-resolved experiments have measured intracomplex energy transfer in under a picosecond at room temperature. However, even for this isolated complex, the mechanism of transfer is poorly understood although the consensus is that the pigments do not act in isolation from their protein host. Among the big remaining open questions is whether the protein helps direct the flow of energy through the pigments or, rather, if its primary role is simply structural, i.e. to hold the pigments in optimal alignment. Theoretical work has shown that the dynamical properties of the bath are a major consideration in correctly predicting energy transfer rates. Recent observations of long-lived coherent oscillations, known as quantum coherences, in LH2 and other photosynthetic PPCs has suggested that the role of the protein may be far more complex than previously realized. For instance, the protein may protect against the rapid destruction of coherence (i.e. to thwart decoherence) by allowing for long-range spatial correlations. Fluctuations in the protein on the timescale of transfer results in correlated exciton energy fluctuations. Because the timescale of coherences may no longer be separated from that of population transfer, the secular approximation, which forms the basis of most transport theories, no longer applies. Recent theoretical calculations by numerous groups have hinted that such environment-assisted quantum transport may be the missing link to revealing the origin of highly efficient energy transfer under physiological conditions.
While current experimental work in our lab and others is examining the details of coherent energy transfer in isolated PPCs, the case of networks of PPCs such as those that exist in the photosynthetic unit (PSU) remains almost entirely unexplored owing to a lack of experimental tools. In the case of purple bacteria, for instance, intercomplex transfer30, such as between LH2 complexes or between LH2 and LH1, cannot be tracked easily by spectroscopy because of a lack of sufficient spectral discrimination between complexes. Such limitations have left a gaping hole in our understanding of the basic mechanism responsible for long-range transport in either the PSU or biomimetic analogues. In the latter, recent steady-state fluorescence experiments on PPC crystals and hybrid nanostructures have revealed long-range transport well beyond what is possible in natural organisms. These observations cannot be readily explained by classical transport theories that ignore coherent effects. Theoretical work on these systems have shown that coherent oscillations supported by the protein scaffold of LH2 extend the transport lengths by almost an order of magnitude, supporting the idea that quantum effects are not only present in these open quantum systems, but that they may dramatically aid in transport. Yet, there is no experimental verification of these quantum mechanisms because linear techniques, such as time-resolved and stead-state fluorescence, cannot measure the quantum states that encode these coherences. We are left to infer, based solely on decay rates, the mechanism of transport, thereby leading to theories devoid of direct experimental probes. Our group is currently developing new nanoscale imaging methods to directly probe long range energy transfer in networks of PPCs and synthetic analogs. Stay tuned for exciting experimental results!
Team Members: Shawn Zhao