Solvent effects in ionic transport through transmembrane protein channels

Cellular microtransport processes: intercellular, intracellular, and aggregate behavior

Ionic and molecular transfer among cells occurs by a variety of transport processes operative at different length scales. Cell membrane permeability and electrical conductance derive from channel proteins producing pores at the molecular (ultrastructural) scale. Intracellular mobility involves the dynamics of motion through the complex ultrastructure of the cytoplasm. These phenomena unite in the larger-scale (microscopic) process of gross intercellular transfer. When such movement occurs among sufficiently many cells, it in turn begins to reflect their average collective (macroscopic) behavior as bulk tissue. This article surveys selected aspects of intercellular and intracellular transport, with emphasis on detailed mechanistic theory, experimental probes of cellular permeability, and systematic transcendence from small to large length scales.

The interaction of Cl- with a gramicidin-like channel

Molecular dynamics simulations have been used to study the interaction of Cl- with a gramicidin-like channel. The results suggest that there is a high-energy barrier at the entrance of the channel, which would correspond to a permeability 10(-9)-times that of a cation of the same size. This could account for the cationic selectivity of the gramicidin channel and indicates that valence selectivity is kinetically controlled.

Structure and dynamics of one-dimensional ionic solutions in biological transmembrane channels

The structure and dynamics of solvated alkali metal cations in transmembrane channels are treated using the molecular dynamics simulation technique. The simulations are based on a modified Fischer-Brickmann model (Fischer, W., and J. Brickmann, 1983, Biophys. Chem., 18:323-337) for gramicidin A-type channels. The trajectories of all particles in the channel as well as two-dimensional pair correlation functions are analyzed. It is found from the analysis of the stationary simulation state that one-dimensional solvation complexes are formed and that the number of water molecules in the channel varies for different alkali metal cations.

Simulation of voltage-driven hydrated cation transport through narrow transmembrane channels

Molecular dynamics studies for the voltage-driven transport of the alkali metal ions lithium, sodium, and potassium through gramicidin A-type channels filled with water molecules are presented. The number of water molecules in the channel is obtained from a previous study (Skerra, A., and J. Brickmann, 1987, Biophys. J., 51:969-976). It is shown that the selectivity of the intrachannel ion diffusion through our model pore conforms to the experimentally observed selectivity of the gramicidin A channel. It is demonstrated that the number of water molecules in the channel plays a key role for the selectivity.

Why is gramicidin valence selective? A theoretical study

Calculations contrasting the channel solvation energy for cesium ions and chloride ions associated with water in gramicidin-like channels are presented. The energy profile for the cation exhibits a deep well at the channel entrance; within the single file region the solvation energy is roughly constant. The anion exhibits a totally different energy profile. There is an energy barrier at the channel entrance; if the ion could surmount this barrier, it would be quite stable within the channel. At the channel entrance, the calculated solvation energy difference between anion and cation is approximately 15 kcal mol-1. This is completely consistent with the observation that chloride neither permeates nor blocks the channel since the estimated rate of ion entry would be approximately 0.01-10(-5) s-1, far slower than the rate at which the channel dimer dissociates into monomers.