1,729 research outputs found
Physiological Evidence for Isopotential Tunneling in the Electron Transport Chain of Methane-Producing Archaea
Many, but not all, organisms use quinones to conserve energy in their electron transport chains. Fermentative bacteria and methane-producing archaea (methanogens) do not produce quinones but have devised other ways to generate ATP. Methanophenazine (MPh) is a unique membrane electron carrier found in Methanosarcina species that plays the same role as quinones in the electron transport chain. To extend the analogy between quinones and MPh, we compared the MPh pool sizes between two well-studied Methanosarcina species, Methanosarcina acetivorans C2A and Methanosarcina barkeri Fusaro, to the quinone pool size in the bacterium Escherichia coli. We found the quantity of MPh per cell increases as cultures transition from exponential growth to stationary phase, and absolute quantities of MPh were 3-fold higher in M. acetivorans than in M. barkeri. The concentration of MPh suggests the cell membrane of M. acetivorans, but not of M. barkeri, is electrically quantized as if it were a single conductive metal sheet and near optimal for rate of electron transport. Similarly, stationary (but not exponentially growing) E. coli cells also have electrically quantized membranes on the basis of quinone content. Consistent with our hypothesis, we demonstrated that the exogenous addition of phenazine increases the growth rate of M. barkeri three times that of M. acetivorans. Our work suggests electron flux through MPh is naturally higher in M. acetivorans than in M. barkeri and that hydrogen cycling is less efficient at conserving energy than scalar proton translocation using MPh
Phase-field Crystals with Elastic Interactions
We report on a novel extension of the recent phase-field crystal (PFC) method
introduced in [Elder et al., Phys. Rev. Lett., Vol. 88, 245701:1-4 (2002)],
which incorporates elastic interactions as well as crystal plasticity and
diffusive dynamics. In our model, elastic interactions are mediated through
wave modes that propagate on time scales many orders of magnitude slower than
atomic vibrations but still much faster than diffusive times scales. This
allows us to preserve the quintessential advantage of the PFC model: the
ability to simulate atomic-scale interactions and dynamics on time scales many
orders of magnitude longer than characteristic vibrational time scales. We
demonstrate the two different modes of propagation in our model and show that
simulations of grain growth and elasto-plastic deformation are consistent with
the microstructural properties of nanocrystals.Comment: 4 pages, 6 figure
Phase field crystal dynamics for binary systems: Derivation from dynamical density functional theory, amplitude equation formalism, and applications to alloy heterostructures
The dynamics of phase field crystal (PFC) modeling is derived from dynamical
density functional theory (DDFT), for both single-component and binary systems.
The derivation is based on a truncation up to the three-point direct
correlation functions in DDFT, and the lowest order approximation using scale
analysis. The complete amplitude equation formalism for binary PFC is developed
to describe the coupled dynamics of slowly varying complex amplitudes of
structural profile, zeroth-mode average atomic density, and system
concentration field. Effects of noise (corresponding to stochastic amplitude
equations) and species-dependent atomic mobilities are also incorporated in
this formalism. Results of a sample application to the study of surface
segregation and interface intermixing in alloy heterostructures and strained
layer growth are presented, showing the effects of different atomic sizes and
mobilities of alloy components. A phenomenon of composition overshooting at the
interface is found, which can be connected to the surface segregation and
enrichment of one of the atomic components observed in recent experiments of
alloying heterostructures.Comment: 26 pages, 5 figures; submitted to Phys. Rev.
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Stein variational gradient descent: Many-particle and long-time asymptotics
Stein variational gradient descent (SVGD) refers to a class of methods for Bayesian inference based on interacting particle systems. In this paper, we consider the originally proposed deterministic dynamics as well as a stochastic variant, each of which represent one of the two main paradigms in Bayesian computational statistics: emphvariational inference and emphMarkov chain Monte Carlo. As it turns out, these are tightly linked through a correspondence between gradient flow structures and large-deviation principles rooted in statistical physics. To expose this relationship, we develop the cotangent space construction for the Stein geometry, prove its basic properties, and determine the large-deviation functional governing the many-particle limit for the empirical measure. Moreover, we identify the emphStein-Fisher information (or emphkernelised Stein discrepancy) as its leading order contribution in the long-time and many-particle regime in the sense of -convergence, shedding some light on the finite-particle properties of SVGD. Finally, we establish a comparison principle between the Stein-Fisher information and RKHS-norms that might be of independent interest
Relative entropy as a measure of inhomogeneity in general relativity
We introduce the notion of relative volume entropy for two spacetimes with
preferred compact spacelike foliations. This is accomplished by applying the
notion of Kullback-Leibler divergence to the volume elements induced on
spacelike slices. The resulting quantity gives a lower bound on the number of
bits which are necessary to describe one metric given the other. For
illustration, we study some examples, in particular gravitational waves, and
conclude that the relative volume entropy is a suitable device for quantitative
comparison of the inhomogeneity of two spacetimes.Comment: 15 pages, 7 figure
Mutual cross-talk between fibronectin integrins and the EGF receptor: Molecular basis and biological significance
Extension of the plasma membrane is one of the first steps in cell migration. Understanding how cells “choose” between various types of membrane protrusion enhances our knowledge of both normal and cancer cell physiology. The EGF receptor is a paradigm for understanding how transmembrane receptor tyrosine kinases regulate intracellular signaling following ligand stimulation. Evidence from the past decade indicates that EGF receptors also form macromolecular complexes with integrin receptors leading to EGF receptor transactivation during cell adhesion. However, relatively little is known about how these complexes form and impact cell migration. Our recent work characterized a molecular complex between EGF receptor and β3 integrin which recognizes RGD motifs in extracellular matrix proteins. Complex formation requires a dileucine motif (679-LL) in the intracellular juxtamembrane region of the EGF receptor that also controls whether or not the receptor undergoes Src kinase-dependent phosphorylation at Tyr-845. In contrast to wild-type receptors, mutant EGF receptors defective for Tyr-845 phosphorylation form complexes with β1 integrin that also binds RGD motifs. In addition, we have discovered that EGF receptor antagonizes small GTPase RhoA by mediating membrane recruitment of its regulatory GAP p190RhoGAP. In this addendum we discuss a potential new role for Src-dependent EGF receptor transactivation in integrin/EGF receptor complex formation. We also discuss how our study fits with previous observations linking p190RhoGAP to RhoA-dependent cytoskeletal rearrangements involved in cell migration, and provide new data that the EGF receptor is compartmentalized to relatively immature zyxin-poor focal adhesions which are the likely site of p190RhoGAP signaling
Amplitude expansion of the binary phase field crystal model
Amplitude representations of a binary phase field crystal model are developed
for a two dimensional triangular lattice and three dimensional BCC and FCC
crystal structures. The relationship between these amplitude equations and the
standard phase field models for binary alloy solidification with elasticity are
derived, providing an explicit connection between phase field crystal and phase
field models. Sample simulations of solute migration at grain boundaries,
eutectic solidification and quantum dot formation on nano-membranes are also
presented.Comment: 11 pages, 8 figure
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