Diffusion in membranes

In tiiese exfnessions, y is tiie redprocal of tiie unquenched dec time, (Q] is tiie quencher concentration In molecules per angstrom squared, R is the interaction radius, and x, I /D, where f) is tiie mubialdiffusicMi coeffident JoCat) and yo(x) are Zdo-order Bessel functions of tiie first and second kinds, respectively. Even moce complex expressions are needed for the radiation model Because of the difficulties in evaluating [9.16] and [9.17], several approximate analytical expressions have been proposed.  

Bi many instances, sudi as for memtuTuie-bound fhioio-fdiores and quendiers, one wishes to use the diserved bimolecular quendiing constants to edentate the diffusion 

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Studies of the effect of permeant s size on the translational diffusion in membranes suggest that a free-volume model is appropriate for the description of diffusion processes in the bilayers [93]. The dynamic motion of the chains of the membrane lipids and proteins may result in the formation of transient pockets of free volume or cavities into which a permeant molecule can enter. Diffusion occurs when a permeant jumps from a donor to an acceptor cavity. Results from recent molecular dynamics simulations suggest that the free volume transport mechanism is more likely to be operative in the core of the bilayer [84]. In the more ordered region of the bilayer, a kink shift diffusion mechanism is more likely to occur [84,94]. Kinks may be pictured as dynamic structural defects representing small, mobile free volumes in the hydrocarbon phase of the membrane, i.e., conformational kink g tg ) isomers of the hydrocarbon chains resulting from thermal motion [52] (Fig. 8). Small molecules can enter the small free volumes of the kinks and migrate across the membrane together with the kinks. 

Nonequilibrium thermodynamics provides a second approach to combined convection and diffusion problems. The Kedem-Katchalsky equations, originally developed to describe combined convection and diffusion in membranes, form the basis of this approach [6,7] ... 

General Membrane Function Membrane Composition Phospholipid Bilayer Membrane Structure Posttranslational Modification Membrane Fluidity Diffusion in Membranes... 

The insertion of proteins into intracellular membranes has incising effects upon the kinetic and thermodynamic properties of the corresponding biological interactions. Although diffusion in membranes is approx. 100-fold slower than in aqueous solution the probability for two molecules to meet... 

R. J. Cherry, Measurement of protein rotational diffusion in membranes by flash photolysis, Methods Enzymol. L1X, 47-61 (1978). 

Polymer network structure is important in describing the transport through biomedical membranes [139, 140]. The mechanism of diffusion in membranes may be that of pure diffusion or convective transport depending on the mesh size of the polymer network. With this in mind, polymer membranes are typically divided into three major types described below [141]. 

The starting point for the mathematical description of diffusion in membranes is the proposition, solidly based in thermodynamics, that the driving forces of pressure, temperature, concentration, and electrical potential are interrelated and that the overall driving force producing movement of a permeant is the gradient in its chemical potential. Thus, the flux,. /,(g/cm2 s), of a component, i, is... 

J.S. Schultz, J.D. Goddard and S.R. Suchdeo, Facilitated Transport via Carrier-mediated Diffusion in Membranes, AIChE 7. 20, 417 (1974). 

The solubility selectivity of a membrane for a specific gas pair could be increased (in principle) by inducing specific interactions between the polymer and the more soluble component of the gas pair. For example, the substitution of certain polar groups in some rubbery polymers has been found to increase their solubility selectivity for CO2 relative to CH4 (Story and Koros, 1991 Koros, 1985). Unfortunately, the increase in the polarity of a polymer also tends to increase its chain packing density, and as a result, decreases the gas diffusivity in membranes made from that polymer. 

ESR is very useful in the study of dynamic properties of membrane components because of its high sensitivity and favorable time scale. Early ESR studies of short-range lateral diffusion in membranes were based on Heisenberg exchange (HE) effects of nitroxide spin-label line width. The HE contribution to the ESR linewidth is given for nitroxides by = )dDNACf, where d is the encounter dis-... 

Vattulainen 1, Mouritsen OG. Diffusion in membranes. In Diffusion in Condensed Matter Methods, Materials, Models. Heitjans P, Karger I, eds. 2005. Springer-Verlag, Berlin. 

Fluorescence recovery after photobleaching (FRAP) for measuring lateral diffusion in membranes Section 12.6... 

The more conventional method for studying the energetics of diffusion in membranes is to perform permeation experiments as a function of equilibrium temperature. Figure 13 illustrates the eflEect of temperature on the apparent diflEusion coeflScient calculated from the water vapor permeation time lag established by steady-state permeation with a 75 to 0% RH gradient across the membrane. The principles of the time lag permeation method are adequately discussed elsewhere (58). The lower curve corresponds to a sample which was not mechanically supported and was observed to deform into a hemispherical shape. This deformation is the combined result of a small pressure diflEerence across the membrane and a decrease in modulus of stratum corneum as the temperature is increased. The upper curve corresponds to a supported sample. Previous to the experiment, both samples had identical thermal histories. Stresses accompanying deformation of the unsupported cor-... 

Use of liposomes to investigate membrane permeability (p. 334) Use of hydropathy plots to locate transmembrane helices fp. 34U) Fluorescence recovery after photnblcaching (FRAP) for measuring lateral diffusion in membranes (p. 342)... 

Almeida PFF, Vaz WLC (1995) Lateral diffusion in membranes. In Lipowsky R, Sackmann E (eds) Handbook of biological physics, structure and dynamics of membranes, vol. 1A. Elsevier, Amsterdam... 

Diffusion in membranes, polymer solutions, and gels can be described adequately by simple models. 

The ability of agents to permeate through membranes can be varied by modifications that effect diffusion in membranes, engage protein carriers in the membrane, or regulate overall size. 

Peppas, N. and D. Meadows, Macromolecular structure and solute diffusion in membranes An overview of recent theories. Journal of Membrane Science, 1983, 16, 361-377. 

The concept of quasi-three-dimensionil diffusion in membranes is illustrated by pyrene exclmer fonnation. Although excimer formation is not strictly quenching, the monomer emission is decreased by diffusive encounters of excited memomers with ground-atate monomers. Hence, the quenching of the monomer emission is described by Stem-Volmer kinetics. 

Schultz, J.S., Goddard, J.D. and S.R. Suchdeo. "Facilitated transport via carrier-mediated diffusion in membranes. Part I Mechanistic aspects, experimental systems and characteristic regimes." American Institute of Chemical Engineers Journal 20 (1974) 417-445. 

The theory of first passage times has also been employed by us recently in order to introduce a new variant of the method of photobleaching to measure diffusion in membranes. This variant [R. Peters et al., Proc. Natl. Acad. Sci. USA 78, 62 (1981)] employs continuous radiation at an intermediate light level and allows one to monitor in living cells the lateral transport of membrane constituents of very small concentrations, e.g. cell surface receptors. 

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