Photosynthesis begins with light harvesting, where specialized pigmentprotein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.

Recent measurements using two-dimensional electronic spectroscopy (2D ES) have shown that the initial dynamic response of photosynthetic proteins can involve quantum coherence. We show how electronic coherence can be differentiated from vibrational coherence in 2D ES. On that basis we conclude that both electronic and vibrational coherences are observed in the phycobiliprotein light-harvesting complex PC645 from Chroomonas sp. CCMP270 at ambient temperature. These light-harvesting antenna proteins of the cryptophyte algae are suspended in the lumen, where the pH drops significantly under sustained illumination by sunlight. Here we measured 2D ES of PC645 at increasing levels of acidity to determine if the change in pH affects the quantum coherence; quantitative analysis reveals that the dynamics are insensitive to the pH change.

Observation of coherent oscillations in the 2D electronic spectra (2D ES) of photosynthetic proteins has led researchers to ask whether nontrivial quantum phenomena are biologically significant. Coherent oscillations have been reported for the soluble light-harvesting phycobiliprotein (PBP) antenna isolated from cryptophyte algae. To probe the link between spectral properties and protein structure, we determined crystal structures of three PBP light-harvesting complexes isolated from different species. Each PBP is a dimer of αβ subunits in which the structure of the αβ monomer is conserved. However, we discovered two dramatically distinct quaternary conformations, one of which is specific to the genus Hemiselmis. Because of steric effects emerging from the insertion of a single amino acid, the two αβ monomers are rotated by ∼73°to an "open" configuration in contrast to the "closed" configuration of other cryptophyte PBPs. This structural change is significant for the light-harvesting function because it disrupts the strong excitonic coupling between two central chromophores in the closed form. The 2D ES show marked cross-peak oscillations assigned to electronic and vibrational coherences in the closedform PC645. However, such features appear to be reduced, or perhaps absent, in the open structures. Thus cryptophytes have evolved a structural switch controlled by an amino acid insertion to modulate excitonic interactions and therefore the mechanisms used for light harvesting.X-ray crystallography | quantum coherence | protein evolution | excitonic switching L ight-harvesting complexes capture and funnel the energy from light using organic chromophore molecules that are bound to scaffolding proteins. The protein structure thereby sets the relative positions and orientations of the chromophores to control excitation transport. In other words, the protein plays a deciding role in building the "electronic Hamiltonian"-the electronic coupling between chromophores and the chromophoric energy landscape that directs energy flow. This strong connection between structural biology and physics means that ultrafast light-harvesting functions are under genetic and evolutionary control. Cryptophytes, a group of marine and freshwater single-celled algae, are an intriguing example, because one of their light-harvesting antenna complexes was completely reengineered by combining a unique bilin-binding polypeptide with a single subunit from the ancestral red algal phycobilisome (1, 2). Here we report a further example of biological manipulation of this phycobiliprotein (PBP) light-harvesting system. We have discovered an elegant but powerful genetic switch that converts the common form of this PBP into a distinct structural form in which the mechanism underpinning light harvesting is vastly different-in essence because strong excitonic interactions within the PBP are switched from on to off.The crystal structure of the cryptophyte PBP phycoerythrin PE545 from Rhodomonas CS24 showed that the protein is a dimer of two αβ monomers (3, ...

We demonstrate the ability of two-dimensional electronic spectroscopy (2DES) to map ultrafast energy transfer and dynamics in two systems: the pigment-protein complex photosystem I (PSI) and aggregates of the conjugated polymer poly(3-hexylthiophene) (P3HT). A detailed description of our experimental set-up and data processing procedure is also given.First, we will give a brief introduction to 2DES and a description of our experimental set-up, followed by a detailed description of our data processing procedure. Finally, we demonstrate how 2DES can be used to elucidate ultrafast (0-400 fs timescale) photophysical events in two systems: poly(3-hexylthiophene) (P3HT) and photosystem I (PSI). TWO-DIMENSIONAL ELECTRONIC SPECTROSCOPYA wealth of information on energy-transfer dynamics and electronic structure can be obtained by applying electronic spectroscopy to complex condensed-phase systems. Steady-state measurements such as linear absorption, fluorescence, excitation anisotropy, and circular dichroism can provide information on the electronic structure. Additional information can be obtained by conducting these measurements at cryogenic temperatures to reduce the effects of homogeneous line broadening; however, the distinction between homogeneous and inhomogeneous broadening is still ambiguous [12,13]. Time-resolved methods, such as transient absorption and pump-probe spectroscopies, can be used to gain further insight into electronic structure while also monitoring energy-transfer dynamics [14,15]. However, when using these techniques, one must choose between either high frequency or time resolution.2DES has emerged as an optical technique that can accomplish many of the objectives of conventional spectroscopies, while overcoming the limitations mentioned above. A 2D spectrum spreads information contained in a 1D pump-probe spectrum over two frequency axis resulting in a frequency-frequency correlation map, where each excitation frequency is correlated to each detection frequency [16,17]. To obtain a 2D spectrum, three femtosecond optical fields, E 1 , E 2 , and E 3 having wavevectors k 1 , k 2 , and k 3 interact with the sample leading to the emission of the signal, E s . In the most common implementation of 2D optical spectroscopy, the incoming fields are arranged in a box geometry so that the signal is emitted in the background-free direction, and the two relevant phase-matching conditions, referred to as "rephasing" (k signal = -k 1 + k 2 + k 3 ) and "nonrephasing" (k signal = +k 1k 2 + k 3 ), are collected independently. The beam geometry and pulse sequence are displayed in Fig. 1. The first pulse, E 1 , creates a coherent superposition between the ground state and a first excited state of the system effectively labeling the molecules. This superposition state evolves during the coherence time, t 1 . The arrival of the second pulse, E 2 , creates either a population or another coherence, marking the end of t 1 and the beginning of the waiting time, t 2 . The system is free to evolve during the waiting time, w...

Resonance energy transfer * is a spectroscopic process whose relevance in all major areas of science is reflected both by a wide prevalence of the effect, and through numerous technical applications. It is an optical near-field mechanism which effects a transportation of electronic excitation between physically distinct atomic or molecular components, based on transition dipole-dipole coupling. In this chapter a comprehensive survey of the process is presented, beginning with an outline of the history and highlighting the early contributions of Perrin and Förster. A review of the photophysics behind resonance energy transfer follows, and then a discussion of some prominent applications of resonance energy transfer. Particular emphasis is given to techniques used in molecular biology, ranging from the 'spectroscopic ruler' measurements of functional group separation, to fluorescence lifetime microscopy. Finally, applications to synthetic polymers and chemical sensors are examined.* RET is also known as Förster-or fluorescence-resonance energy transfer (FRET), or electronic energy transfer (EET)

Mass is the most important stellar parameter, but it is not directly observable for a single star. Spherical model stellar atmospheres are explicitly characterized by their luminosity ($L_\star$), mass ($M_\star$) and radius ($R_\star$), and observations can now determine directly $L_\star$ and $R_\star$. We computed spherical model atmospheres for red giants and for red supergiants holding $L_\star$ and $R_\star$ constant at characteristic values for each type of star but varying $M_\star$, and we searched the predicted flux spectra and surface-brightness distributions for features that changed with mass. For both stellar classes we found similar signatures of the star's mass in both the surface-brightness distribution and the flux spectrum. The spectral features have been use previously to determine $\log_{10} (g)$, and now that the luminosity and radius of a non-binary red giant or red supergiant can be observed, spherical model stellar atmospheres can be used to determine the star's mass from currently achievable spectroscopy. The surface-brightness variations with mass are slightly smaller than can be resolved by current stellar imaging, but they offer the advantage of being less sensitive to the detailed chemical composition of the atmosphere.Comment: 24 pages, 9 figure