ABSTRACT
Monte Carlo simulations are used to study ion and polymer chain dynamic properties in a simplified lattice model with only one species of mobile ions. The ions interact attractively with specific beads in the host chains, while polymer beads repel each other. Cross linking of chains by the ions reduces chain mobilities which in turn suppresses ionic diffusion. Diffusion constants for ions and chains as a function of temperature follow the Vogel-Tammann-Fulcher (VTF) law with a common VTF temperature at low ion concentration, but both decouple at higher concentrations, in agreement with experimental observations. Our model allows us to introduce pressure as an independent variable through calculations of the equation of state using the quasichemical approximation, and to detect an exponential pressure dependence of the ionic diffusion.
ABSTRACT
The dynamics of the expulsion of the last liquid monolayer of molecules confined between two surfaces (measured recently for the first time) has been analyzed by solving the two-dimensional Navier-Stokes equation combined with kinetic Monte Carlo simulations. Instabilities in the boundary line of the expelled film were observed. We show that the instabilities produce a rough boundary for all length scales above a critical value and a smooth boundary for shorter lengths. The squeezing out of all but a few trapped islands of liquid is shown to be the result of the pressure gradient in the contact area.
ABSTRACT
Nitric oxide (NO) acts as a signal molecule in the nervous system, as a defense against infections, as a regulator of blood pressure, and as a gate keeper of blood flow to different organs. In vivo, it is thought to have a lifetime of a few seconds. Therefore, its direct detection at low concentrations is difficult. We report on a new type of hybrid, organic-semiconductor, electronic sensor that makes detection of nitric oxide in physiological solution possible. The mode of action of the device is described to explain how its electrical resistivity changes as a result of NO binding to a layer of native hemin molecules. These molecules are self-assembled on a GaAs surface to which they are attached through a carboxylate binding group. The new sensor provides a fast and simple method for directly detecting NO at concentrations down to 1 microM in physiological aqueous (pH=7.4) solution at room temperature.
Subject(s)
Biosensing Techniques , Hemin/chemistry , Nitric Oxide/analysis , Algorithms , Animals , Arsenic , Brain/metabolism , Carboxylic Acids/chemistry , Electrochemistry/instrumentation , Gallium , Isomerism , Metalloporphyrins/chemistry , Models, Molecular , Rats , SemiconductorsABSTRACT
Electron transmission through molecules and molecular interfaces has been a subject of intensive research due to recent interest in electron-transfer phenomena underlying the operation of the scanning-tunneling microscope on one hand, and in the transmission properties of molecular bridges between conducting leads on the other. In these processes, the traditional molecular view of electron transfer between donor and acceptor species gives rise to a novel view of the molecule as a current-carrying conductor, and observables such as electron-transfer rates and yields are replaced by the conductivities, or more generally by current-voltage relationships, in molecular junctions. Such investigations of electrical junctions, in which single molecules or small molecular assemblies operate as conductors, constitute a major part of the active field of molecular electronics. In this article I review the current knowledge and understanding of this field, with particular emphasis on theoretical issues. Different approaches to computing the conduction properties of molecules and molecular assemblies are reviewed, and the relationships between them are discussed. Following a detailed discussion of static-junctions models, a review of our current understanding of the role played by inelastic processes, dephasing and thermal-relaxation effects is provided. The most important molecular environment for electron transfer and transmission is water, and our current theoretical understanding of electron transmission through water layers is reviewed. Finally, a brief discussion of overbarrier transmission, exemplified by photoemission through adsorbed molecular layers or low-energy electron transmission through such layers, is provided. Similarities and differences between the different systems studied are discussed.
ABSTRACT
A lattice relaxation algorithm is developed to solve the Poisson-Nernst-Planck (PNP) equations for ion transport through arbitrary three-dimensional volumes. Calculations of systems characterized by simple parallel plate and cylindrical pore geometries are presented in order to calibrate the accuracy of the method. A study of ion transport through gramicidin A dimer is carried out within this PNP framework. Good agreement with experimental measurements is obtained. Strengths and weaknesses of the PNP approach are discussed.
Subject(s)
Algorithms , Gramicidin/chemistry , Ion Channels/chemistry , Ion Transport/physiology , Biological Transport , Computer Simulation , Diffusion , Dimerization , Models, Molecular , Static ElectricityABSTRACT
We study a deprotonation reaction by an enzyme with activity dependent on pH. The rate and transport equations are simplified with a number of assumptions, are analyzed according to the presence of different time scales, and are solved numerically to show relaxation oscillation and threshold excitation, for different choices of parameters. The imposition of fluctuations (noise) on the deterministic equations for threshold excitation conditions leads to random occurrence of an excitation and return to steady state at low noise level and to large, random variations in concentrations at high noise level. At intermediate noise levels (of the order of the threshold excitation), however, we find quasi-periodic concentration oscillations. Thus, critical values of external constraints necessary for oscillations are altered by the presence of noise.
