Session W41: Focus Session: Quantum Coherence in Biological Systems

Quantum Coherence in Biology

Discussion of quantum mechanical effects in biology is generally restricted to molecular energetics, stability, and kinetics as determined by potential energy barriers. In recent years however, an increasing number of experiments have shown evidence for the existence of dynamical phenomena in biological systems that involve coherent quantum motion. One prominent instance is electronic quantum coherence in photosynthesis. I shall present theoretical studies that analyze the nature of this coherence, its relation to the non-local quantum correlations characteristic of entanglement, implications for relevance of quantum information processing in natural systems and address the question of whether and how such quantum coherence might result in a biological advantage.   

Absence of quantum oscillations in electronic excitation transfer in the Fenna-Matthews-Olson complex

Energy transfer in the photosynthetic Fenna-Matthews-Olson (FMO) complex of the Green Sulfur Bacteria is studied theoretically taking all three subunits (monomers) of the FMO trimer and the recently found eighth bacteriochlorophyll (BChl) molecule into account. For the calculations we use the efficient Non-Markovian Quantum State diffusion approach. Since it is believed that the eighth BChl is located near the main light harvesting antenna we look at the differences in transfer between the situation when BChl 8 is initially excited and the usually considered case when BChl 1 or 6 is initially excited. We find strong differences in the transfer dynamics, both qualitatively and quantitatively. When the excited state dynamics is initialized at site eight of the FMO complex, we see a slow exponential-like decay of the excitation. This is in contrast to the oscillations and a relatively fast transfer that occurs when only seven sites or initialization at sites 1 and 6 is considered. Additionally we show that differences in the values of the electronic transition energies found in the literature lead to a large difference in the transfer dynamics.   

Progress Towards Room-Temperature Electron Spin Detection in Biological Systems

We report on recent progress of room-temperature electron spin sensing for biological applications using nitrogen-vacancy (NV) centers in diamond. Our approach involves room-temperature detection of a small number of electron spins, situated outside the measurement substrate. Potential applications will be discussed, including detection of magnetic resonance signals from individual electron or nuclear spins of complex biological molecules, measurement of concentrations of radicals in living cells, and monitoring the ion channel function across cell membranes (important for exploring drug delivery mechanisms).   

Coherent transfer in biological systems: a study of the role of environmental noise

We study the effects of environmental noise on quantum mechanical systems, we focus mainly on biological systems in which noise is believed to assist coherent transport of energy. We use error correction methods aimed at avoiding decoherence and dissipation due to coupling to a reservoir, with the purpose of investigating if modifying the coupling to a noisy bath can provide further insight on its effects on transport mechanisms, and propose that this work is carried out experimentally.   

Noise induced quantum effects in photosynthetic complexes

Recent progress in coherent multidimensional optical spectroscopy revealed effects of quantum coherence coupled to population leading to population oscillations as evidence of quantum transport. Their description requires reevaluation of the currently used methods and approximations. We identify couplings between coherences and populations as the noise-induced cross-terms in the master equation generated via Agarwal-Fano interference that have been shown earlier to enhance the quantum yield in a photocell. We investigated a broad range of typical parameter regimes, which may be applied to a variety of photosynthetic complexes. We demonstrate that quantum coherence may be induced in photosynthetic complexes under natural conditions of incoherent light from the sun. This demonstrates that a photosynthetic reaction center may be viewed as a biological quantum heat engine that transforms high-energy thermal photon radiation into low entropy electron flux.   

FMO complex: exciton transfer and interaction with vibronic modes

We examine transport of excitons trough Fenna-Matthews-Olson (FMO) complex from a receiving antenna to a reaction center, using methods of condensed matter and statistical physics. Writing equation of motion for creation/annihilation operators, we are able to describe the exciton dynamics in the regime when the reorganization energy is of the order of the intra-system couplings. Well-known quantum oscillations of the site populations are obtained, in particular. We determine the exciton transfer efficiency and its dependencies of the system parameters. While the majority of vibronic modes are treated as a heat bath, we address the situation when specific modes are strongly coupled to excitons and examine the effects of these modes on the quantum oscillations and the energy transfer efficiency.   

Non-Markovian harmonic bath model for molecular systems: Influence of the bath spectral density

In quantum mechanical simulations of the electronic excitation dynamics in molecular complexes, like natural or artificial light-harvesting complexes, often open quantum system descriptions are applied, to treat a large number of degrees of freedom involved. A popular approach is then, to include only the electronic degrees of freedom into the system part and to couple them to a non-Markovian bath of harmonic vibrational modes. The coupling to the bath, representing intra-molecular as well as external vibrations, is usually described via the bath spectral density, which therefore is an important ingredient in this approach. Here, we discuss different aspects of the influence of the bath spectral density on dynamics and optical spectra. In particular, we consider structured spectral densities, consisting of multiple broadened peaks. It is often assumed that the strong coupling to an intra-molecular vibrational mode, which is damped by coupling to other vibrational modes, is described reasonably by such a broadened peak in the spectral density. Here we demonstrate that this interpretation should be used with caution, because the damping of the mode differs from the model for an intra-molecular mode that one would usually apply when including the mode directly in the system part.   

Curvature, torsion and temperature in energy transfer

Curvature and torsion can play a role in energy transfer in alpha-helical proteins [New J. Phys. 12, 055003 (2010)]. The phenomenon is specific to alpha-helices and not to beta-sheets in proteins due to the three strands of hydrogen bonds constituting the alpha-helical backbone. In this work we analyse and discuss how temperature can affect the role of geometry in the energy transfer.   

Quantum Process Tomography for Energy Transfer Systems via Ultrafast Spectroscopy

The description of excited state dynamics in energy transfer systems constitutes a theoretical and experimental challenge in modern chemical physics. A spectroscopic protocol that systematically characterizes both coherent and dissipative processes of the probed chromophores is desired [1,2]. In this talk, I show that a set of two-color photon-echo experiments performs quantum state tomography (QST) of the one-exciton manifold of a dimer by reconstructing its density matrix in real time. This possibility in turn allows for a complete description of excited state dynamics via quantum process tomography (QPT). Simulations of a noisy QPT experiment for an inhomogeneously broadened ensemble of model excitonic dimers show that the protocol distills rich information about dissipative excitonic dynamics, which appears nontrivially hidden in the signal monitored in single realizations of four-wave mixing experiments Progress on the experimental side will be discussed, as well as new insights that QPT has offered on the understanding of 2D electronic and vibrational spectroscopy. [1] J. Yuen-Zhou, J. J. Krich, A. Aspuru-Guzik, Quantum state and process tomography of energy transfer systems via ultrafast spectroscopy~Joel Yuen-Zhou, Jacob J. Krich, Masoud Mohseni and Al\'{a}n Aspuru-Guzik Proc. Nat. Acad. Sci. USA, Early Edition (2011).~ [2] J. Yuen-Zhou, A. Aspuru-Guzik, Quantum process tomography of molecular dimers from two-dimensional electronic spectroscopy I: General theory and application to homodimers~Joel Yuen-Zhou and Al\'{a}n Aspuru-Guzik . Chem. Phys. 134, 134505 (2011).   

Coherent vs. dissipative nonequilibrium dynamics in spectroscopy of molecular aggregates

Molecular aggregates embedded in a protein environment are the core elements in photosynthetic antennae units. Photoexcitations in these systems experience multistep relaxation, which could be traced using various time-resolved spectroscopy techniques. Initiated coherent processes turn into dissipative. Understanding of these processes is still a major theoretical task. We study theoretically spectroscopic properties of simple molecular aggregates coupled to a bath, which contains main ingredients of protein environemnt: high-energy vibrations, long-range correlations, and smooth spectrum of frequencies. At short times after the optical excitation high-energy coherent vibrational resonanses can be observed in two-dimensional rephasing spectroscopy. Their beats overlap with electronic quantum coherences, responsible for the quantum transport. We show the way to discriminate between them. At the long times we find that the conventional excitonic picture of eigenstates is valid only in the Markovian regime. In the non-Markovian regime the exciton concept breaks down and renormalized system parameters must be introduced: effective intermolecular coupling, widely used in polaron theories, can be used to account for the effects of the bath.   

New quantum state of protons and electrons in nano-confined water

Neutron Compton Scattering provides a means of directly and accurately measuring the momentum distribution of protons in water, which is determined primarily by the protons ground state wavefunction. We find that in water confined on scales of $\sim$20{\AA}, this wave function responds to the details of the confinement, corresponds to a strongly anharmonic local potential, shows evidence in some cases of coherent delocalization in double wells, and involves differences in zero point kinetic energy of the protons from that of bulk water at room temperature of -40 to +120 meV. This behavior is a generic feature of nanoscale confinement, and in particular, this state should be that which is present in water confined in biological cells. It is exhibited here in 16 {\AA} inner diameter carbon nanotubes, two different hydrated proton exchange membranes (PEMs), Nafion 1120 and Dow 858, and has been seen earlier in xerogel and 14 {\AA} diameter carbon nanotubes. The existence of this state is confirmed by xray Compton scattering measurements of the electron momentum distribution.   

Spectroscopic Characterization of the Water Oxidation Intermediates in the Blue Dimer Ru-Based Catalyst for Artificial Photosynthesis

Utilization of sunlight requires solar capture, light-to-energy conversion and storage. One effective way to store energy is to convert it into chemical energy by fuel-forming reactions, such as water splitting into hydrogen and oxygen. Ruthenium complexes are among few molecular-defined catalysts capable of water splitting. Mechanistic insights about such catalysts can be acquired by spectroscopic analysis of short-lived intermediates of catalytic water oxidation. Use of techniques such as EPR and X-ray absorption spectroscopy (XAS) are used to determine electronic requirements of catalytic water oxidation. About 30 years ago Meyer and coworkers reported first ruthenium-based catalyst for water oxidation, the ``blue dimer''. We performed EPR studies and characterized structures and electronic configurations of intermediates of water oxidation by the ``blue dimer''. Intermediates were prepared chemically by oxidation of Ru-complexes with defined number of Ce (IV) equivalents and freeze-quenched at controlled times. Changes in oxidation state of Ru atom were detected by XANES at Ru K-edges. K-edges are sensitive to changes in Ru oxidation state for Blue Dimer [3,3]$^{4+}$, [3,4]$^{4+}$, [3,4]'$^{4+}$ and [4,5]$^{3+}$ allowing a clear assignment of Ru oxidation state in intermediates. EXAFS demonstrated structural changes.