Aqueous Domains Research Articles

Electrostatic modeling of ion pores. Energy barriers and electric field profiles

This paper presents calculations of the image potential for an ion in an aqueous pore through lipid membrane and the electric field produced in such a pore when a transmembrane potential is applied. The method used is one introduced by Levitt (1978, Biophys. J. 22:209), who solved an equivalent problem, in which a surface charge density is placed at the dielectric boundary. It is shown that there are singularities in this surface charge density if the model system has sharp corners. Numerically accurate calculations require exact treatment of these singularities. The major result of this paper is the development of a projection method that explicitly accounts for this behavior. It is shown how this technique can be used to compute, both reliably and efficiently, the electrical potential within a model pore in response to any electrical source. As the length of a channel with fixed radius is increased, the peak in the image potential approaches that of an infinitely long channel more rapidly than previously believed. When a transmembrane potential is applied the electric field within a pore is constant over most of its length. Unless the channel is much longer than its radius, the field extends well into the aqueous domain. For sufficiently dissimilar dielectrics the calculated values for the peak in the image potential and for the field well within the pore can be summarized by simple empirical expressions that are accurate to within 5%.

Spin trapping in heterogeneous electron transfer processes

An overview of applications of spin trapping to electrochemical investigations is presented. Cited studies include characteriza­tions of primary electrode reaction products (e.g., electrooxidations of hal.ide and pseudohalide species, electroreduction of N-methylpyridinium ion) as well as the identification of transient intermediates arising from homogeneous reactions which are electroinitiated. The validity of spatially resolved spin trapping as a probe in the investigation of interfacial processes is demonstrated with examples drawn both from the previously reported co valent attachment of nitrone derived spin traps to silaceous surfaces and from recent studies of spin trapping in micelle and vesicle systems. Amphiphilic nitrone spin traps have been shown to coassemble with both micelles and vesicles such that the nitrone functionality resides in the interfacial region of the ordered system. The ability of these interfacially localized nitrones to trap transient radicals generated both in the hydrophobic domain of the micelle or vesicle and in the aqueous exterior domain is demonstrated.