1,212 research outputs found
Vibronic coupling explains the ultrafast carotenoid-to-bacteriochlorophyll energy transfer in natural and artificial light harvesters
The initial energy transfer in photosynthesis occurs between the
light-harvesting pigments and on ultrafast timescales. We analyze the
carotenoid to bacteriochlorophyll energy transfer in LH2 Marichromatium
purpuratum as well as in an artificial light-harvesting dyad system by using
transient grating and two-dimensional electronic spectroscopy with 10 fs time
resolution. We find that F\"orster-type models reproduce the experimentally
observed 60 fs transfer times, but overestimate coupling constants, which leads
to a disagreement with both linear absorption and electronic 2D-spectra. We
show that a vibronic model, which treats carotenoid vibrations on both
electronic ground and excited state as part of the system's Hamiltonian,
reproduces all measured quantities. Importantly, the vibronic model presented
here can explain the fast energy transfer rates with only moderate coupling
constants, which are in agreement with structure based calculations.
Counterintuitively, the vibrational levels on the carotenoid electronic ground
state play a central role in the excited state population transfer to
bacteriochlorophyll as the resonance between the donor-acceptor energy gap and
vibrational ground state energies is the physical basis of the ultrafast energy
transfer rates in these systems
God\u27s Church Is Just: A Specific Discussion Of Some Cases of Church Discipline
https://digitalcommons.acu.edu/crs_books/1364/thumbnail.jp
Non-equilibrium relaxation of hot states in organic semiconductors: Impact of mode-selective excitation on charge transfer.
The theoretical study of open quantum systems strongly coupled to a vibrational environment remains computationally challenging due to the strongly non-Markovian characteristics of the dynamics. We study this problem in the case of a molecular dimer of the organic semiconductor tetracene, the exciton states of which are strongly coupled to a few hundreds of molecular vibrations. To do so, we employ a previously developed tensor network approach, based on the formalism of matrix product states. By analyzing the entanglement structure of the system wavefunction, we can expand it in a tree tensor network state, which allows us to perform a fully quantum mechanical time evolution of the exciton-vibrational system, including the effect of 156 molecular vibrations. We simulate the dynamics of hot states, i.e., states resulting from excess energy photoexcitation, by constructing various initial bath states, and show that the exciton system indeed has a memory of those initial configurations. In particular, the specific pathway of vibrational relaxation is shown to strongly affect the quantum coherence between exciton states in time scales relevant for the ultrafast dynamics of application-relevant processes such as charge transfer. The preferential excitation of low-frequency modes leads to a limited number of relaxation pathways, thus "protecting" quantum coherence and leading to a significant increase in the charge transfer yield in the dimer structure.A.M.A. acknowledges the support of the Engineering and Physical Sciences Research Council (EPSRC) for funding under Grant No. EP/L015552/1
Quantum entanglement in photosynthetic light harvesting complexes
Light harvesting components of photosynthetic organisms are complex, coupled,
many-body quantum systems, in which electronic coherence has recently been
shown to survive for relatively long time scales despite the decohering effects
of their environments. Within this context, we analyze entanglement in
multi-chromophoric light harvesting complexes, and establish methods for
quantification of entanglement by presenting necessary and sufficient
conditions for entanglement and by deriving a measure of global entanglement.
These methods are then applied to the Fenna-Matthews-Olson (FMO) protein to
extract the initial state and temperature dependencies of entanglement. We show
that while FMO in natural conditions largely contains bipartite entanglement
between dimerized chromophores, a small amount of long-range and multipartite
entanglement exists even at physiological temperatures. This constitutes the
first rigorous quantification of entanglement in a biological system. Finally,
we discuss the practical utilization of entanglement in densely packed
molecular aggregates such as light harvesting complexes.Comment: 14 pages, 7 figures. Improved presentation, published versio
Non-Markovian stochastic description of quantum transport in photosynthetic systems
We analyze several aspects of the transport dynamics in the LH1-RC core of
purple bacteria, which consists basically in a ring of antenna molecules that
transport the energy into a target molecule, the reaction center, placed in the
center of the ring. We show that the periodicity of the system plays an
important role to explain the relevance of the initial state in the transport
efficiency. This picture is modified, and the transport enhanced for any
initial state, when considering that molecules have different energies, and
when including their interaction with the environment. We study this last
situation by using stochastic Schr{\"o}dinger equations, both for Markovian and
non-Markovian type of interactions.Comment: 21 pages, 5 figure
Community-based prevention leads to an increase in condom use and a reduction in sexually transmitted infections (STIs) among men who have sex with men (MSM) and female sex workers (FSW): the Frontiers Prevention Project (FPP) evaluation results
<p>Abstract</p> <p>Background</p> <p>India has an estimated 2.0 million to 3.1 million people living with HIV; it has the highest number of HIV-positive people in Asia and ranks third in the world. The Frontiers Prevention Project (FPP) was implemented in 2002 to conduct targeted prevention intervention geared towards female sex workers (FSW) and men who have sex with men (MSM) in the state of Andhra Pradesh (AP). This paper reports the overall changes in behaviour and STI outcomes between 2003/4 and 2007 and also describes the changes attributed to the FPP.</p> <p>Methods</p> <p>The evaluation used two cross-sectional surveys among MSM and FSW at 24 sites in AP. Surveys were implemented using a similar methodology. Univariate analyses were conducted by comparing means: baseline vs. four-year follow-up and FPP vs. non-FPP. For both MSM and FSW, random and fixed-effects logit regression models at the site level were estimated for <it>condom use with last partner</it>, <it>syphilis sero-positivity </it>and <it>HSV 2 sero-positivity</it>. In addition, for FSW we estimated models for <it>condom use with regular partner</it>, and for MSM we estimated models for <it>condom use with last female partner</it>.</p> <p>Results</p> <p>Among MSM, fixed-effects analysis revealed that FPP was positively correlated with the probability of <it>condom use with last female sexual partner </it>and negatively correlated with the individual probability of <it>sero-positivity to syphilis and HSV 2</it>. Among FSW, the FPP intervention was significantly correlated with increased <it>condom use with regular partners </it>and with lower probability of <it>STI sero-positivity</it>.</p> <p>Discussion</p> <p>Important changes in behaviours related to an increase in prevention activities translated to reductions in STI sero-prevalence in AP, India. In contrast with non-FPP sites, the FPP sites experienced an intense community approach as part of the FPP intervention, and the general increase in condom use and its effect on STI sero-prevalence reflected the efficacy of these intense prevention activities focused on key populations in AP.</p
Quantum physics meets biology
Quantum physics and biology have long been regarded as unrelated disciplines,
describing nature at the inanimate microlevel on the one hand and living
species on the other hand. Over the last decades the life sciences have
succeeded in providing ever more and refined explanations of macroscopic
phenomena that were based on an improved understanding of molecular structures
and mechanisms. Simultaneously, quantum physics, originally rooted in a world
view of quantum coherences, entanglement and other non-classical effects, has
been heading towards systems of increasing complexity. The present perspective
article shall serve as a pedestrian guide to the growing interconnections
between the two fields. We recapitulate the generic and sometimes unintuitive
characteristics of quantum physics and point to a number of applications in the
life sciences. We discuss our criteria for a future quantum biology, its
current status, recent experimental progress and also the restrictions that
nature imposes on bold extrapolations of quantum theory to macroscopic
phenomena.Comment: 26 pages, 4 figures, Perspective article for the HFSP Journa
Resonance energy transfer: The unified theory via vector spherical harmonics
In this work, we derive the well-established expression for the quantum amplitude associated with the resonance energy transfer (RET) process between a pair of molecules that are beyond wavefunction overlap. The novelty of this work is that the field of the mediating photon is described in terms of a spherical wave rather than a plane wave. The angular components of the field are constructed in terms of vector spherical harmonics while Hankel functions are used to define the radial component. This approach alleviates the problem of having to select physically correct solution from non-physical solutions, which seems to be inherent in plane wave derivations. The spherical coordinate system allows one to easily decompose the photon’s fields into longitudinal and transverse components and offers a natural way to analyse near-, intermediate-, and far-zone RET within the context of the relative orientation of the transition dipole moments for the two molecules
Motional effects on the efficiency of excitation transfer
Energy transfer plays a vital role in many natural and technological
processes. In this work, we study the effects of mechanical motion on the
excitation transfer through a chain of interacting molecules with application
to biological scenarios of transfer processes. Our investigation demonstrates
that, for various types of mechanical oscillations, the transfer efficiency is
significantly enhanced over that of comparable static configurations. This
enhancement is a genuine quantum signature, and requires the collaborative
interplay between the quantum-coherent evolution of the excitation and the
mechanical motion of the molecules; it has no analogue in the classical
incoherent energy transfer. This effect may not only occur naturally, but it
could be exploited in artificially designed systems to optimize transport
processes. As an application, we discuss a simple and hence robust control
technique.Comment: 25 pages, 11 figures; completely revised; version accepted for
publicatio
Observation of electronic excitation transfer through light harvesting complex II using two-dimensional electronic–vibrational spectroscopy
Light Harvesting Complex II (LHCII) serves a central role in light harvesting for oxygenic photosynthesis, and is arguably the most important photosynthetic antenna complex. In this work, we present two-dimensional electronic-vibrational (2DEV) spectra of LHCII isolated from spinach, demonstrating the possibility of using this technique to track the transfer of electronic excitation energy between specific pigments within the complex. We assign the spectral bands via comparison with the 2DEV spectra of the isolated chromophores, chlorophyll a and b, and present evidence that excitation energy between the pigments of the complex are observed in these spectra. Finally, we analyze the essential components of the 2DEV spectra using singular value decomposition, which makes it possible to reveal the relaxation pathways within this complex
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