Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P38: Focus Session: Quantum Coherence in Biology III |
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Sponsoring Units: DCP DBP Chair: Seth Lloyd, Massachusetts Institute of Technology Room: A130/131 |
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P38.00001: Barrier compression and tunneling in enzyme catalysed reactions Invited Speaker: Nuclear quantum mechanical tunneling is important in enzyme-catalysed H-transfer reactions. This viewpoint has arisen after a number of experimental studies have described enzymatic reactions with kinetic isotope effects that are significantly larger than the semi-classical limit. Other experimental evidence for tunneling, and the potential role of promoting vibrations that transiently compress the reaction barrier, is more indirect, being derived from the interpretation of e.g. mutational analyses of enzyme systems and temperature perturbation studies of reaction rates/kinetic isotope effects. Computational simulations have, in some cases, determined exalted kinetic isotope effects and tunneling contributions, and identified putative promoting vibrations. In this presentation, we present the available evidence -- both experimental and computational -- for environmentally-coupled H-tunneling in several enzyme systems, from our recent work on redox enzyme systems. We then consider the relative importance of tunneling contributions to these reactions. We find that the tunneling contribution to these reactions confers a rate enhancement of approx. 1000-fold. Without tunneling, a 1000-fold reduction in activity would seriously impair cellular metabolism. We infer that tunneling is crucial to host organism viability thereby emphasising the general importance of tunneling in biology. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P38.00002: The control of electron transfer pathways in biomolecular systems: the role of fluctuations Invited Speaker: Electron transfer reactions are ubiquitous in biology. They are observed in both protein and DNA systems. Biological electron transfer mechanisms range from tunnelling to thermally activated hopping. Due to the floppiness of biomolecules, molecular motion is an important determinant of the electron transfer rate. The electronic couplings that enable electron transfer in biomolecular systems can be understood in terms of competing and interfering electron transfer pathways that are controlled by structure, dynamics, and initial state preparation. We review recent theoretical progress on the effects of conformational distributions, excited-state polarization, and electron-nuclear dynamics on tunneling electron transfer reactions in different biomolecular systems. We discuss how electron-transfer-rate control can be achieved in the presence of a highly fluctuating environment. \\[4pt] [1] S. S. Skourtis, D. H. Waldeck, and D. N. Beratan. Fluctuations in biological and bioinspired electron-transfer reactions. Ann. Rev. Phys. Chem. Vol. 61 461-485 (2010). \\[0pt] [2] I. A. Balabin, D.N. Beratan, and S. S. Skourtis. The persistence of structure over fluctuations in biological electron transfer reactions. Phys. Rev. Lett. 101, 158102 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P38.00003: A molecular breakwater enhances electron transfer between proteins Nathan S. Babcock, Aur\'elien de la Lande, Jan \v{R}ez\'a\v{c}, Barry C. Sanders, Dennis R. Salahub Does natural selection optimize molecular biomachinery at the quantum level? We present statistical characterizations of molecular dynamics at an interprotein electron transfer (ET) interface. In simulations of the wild-type protein complex, we find that the most frequently occurring molecular configurations afford superior electronic coupling due to the consistent presence of a single water molecule hydrogen-bonded between the donor and acceptor sites. We attribute the persistence of this water bridge to a ``molecular breakwater'' composed of several hydrophobic residues surrounding the acceptor site. The breakwater supports the function of solvent-organizing residues by limiting the exchange of water molecules between the sterically constrained ET region and the surrounding bulk. When the breakwater is affected by a mutation, bulk solvent molecules disrupt the water bridge, resulting in reduced electronic coupling. These results suggest that protein surface residues may stabilize interprotein solvent dynamics to enable coherent ET along a single molecular pathway. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P38.00004: Mixed quantum classical simulations of vibrational energy transport in alpha-helices Anne Goj, Eric Bittner We use mixed classical/quantum simulations to study the time dependence of an excitation of a C=O vibration on a 3-10 helix of $\alpha$-Aminoisobutyric acid (AIB), a system which represents a test case for the formation of self-trapped vibrational excitation states on protein helices. Due to the inherent disorder in the system caused by the finite temperature and fluctuations in hydrogen bonding, the excitation tunnels randomly among C=O sites along the helix. Quantum forces are insufficient to establish a coherent relationship between the location of the excitation and the contraction of hydrogen bonds around this site. Our simulations indicate that the excitation frequently becomes localized on the end of the helix due to the defect in helical structure caused by unwinding. Our results generally do not support the existance of Davydov type solitons in biological helix systems under physiological conditions. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 10:12AM |
P38.00005: Quantum processes as a mechanism in olfaction for smell recognition? Invited Speaker: The physics of smell is not well understood. The biological processes that occur following a signalling event are well understood (Buck 1991). However, the reasons how and why a signalling event occurs when a particular smell molecule and receptor combination is made, remains un-established. Luca Turin proposes a signalling mechanism which determines smell molecules by quantum mechanics (Turin 1996). Investigation of this mechanism shows it to be physically robust (Brookes,et al, 2007), and consequences of the theory provides quantitative measurements of smell and interesting potential experiments that may determine whether the recognition of smell is a quantum event. Brookes, J.C, Hartoutsiou, F, Horsfield, A.P and Stoneham, A.M. (2007). Physical Review Letters 98, no. 3 038101 Buck, L. ( 1991) Cell, 65, no.1 (4): 175-187. Turin, L. (1996) Chemical Sences 21, no 6. 773-791 [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P38.00006: Recent developments in the physics of your sense of smell Andrew Horsfield, Luca Turin, Yeong-Ah Soh, Marion Sourribes, Marshall Stoneham, Lianheng Tong, Paul Warburton A radical proposal in 1996 [L. Turin, Chem. Senses 21, 773 (1996)] was that human olfactory receptors use phonon assisted electron tunnelling to probe the vibrational spectrum of odorants in order to determine their chemical identity. A development of this model [J. C. Brookes et al., Phys. Rev. Lett., 98, 038101 (2007)] showed that this Turin mechanism is indeed physically possible, even robust, but left a number of questions open. One such question is: between which sites does the tunnelling electron pass? Our recent calculations support a particular pair of likely sites. Because of the complexity of biological environments, probing the receptor is difficult. Thus we have begun to investigate the properties of a semiconductor nanowire device that mimics the key processes [A. P. Horsfield et al., J. Appl. Phys., 108, 014511 (2010)]. We will present the latest findings of this study. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P38.00007: A Quantum of Solace: molecular electronics of benzodiazepines Luca Turin, Andrew Horsfield, Marshall Stoneham Benzodiazepines and related drugs modulate the activity of GABA-A receptors, the main inhibitory receptor of the central nervous system. The prevailing view is that these drugs bind at the interface between two receptor subunits and allosterically modulate the response to GABA. In this talk I shall present evidence that benzodiazepines work instead by facilitating electron transport from the cytoplasm to a crucial redox-sensitive group in the gamma subunit. If this idea is correct, benzodiazepines should not only be regarded as keys fitting into a lock, but also as one-electron chemical field-effect transistors fitting into an electronic circuit. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P38.00008: Theoretical investigation of coherent exciton flow dynamics in light harvesting complex 2 (LH2) Seogjoo Jang The light harvesting complex 2 (LH2) is a peripheral antenna complex found in photosynthetic unit of purple bacteria. Numerous spectroscopic and computational studies demonstrated that quantum coherence plays an important role in the energetics and the dynamics of excitons created in LH2, but detailed and quantitative understanding is still missing regarding how the quantum coherence influences spectroscopic observables and how it boosts efficient energy transfer despite disorder and soft nature of the system. The present talk reports recent progress in the analysis of the single molecule spectroscopy (SMS) and application of resonance energy transfer theories, which account for multichromophoric and quantum coherence effects. These suggest that spectroscopic modeling allows development of reliable coarse-grained model for LH2 that can capture the major features of the excitons and that LH2 is a highly optimized natural system where the interplay between quantum coherence and disorder/fluctuation is maximally utilized. [Preview Abstract] |
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