Alkali-metal Ion Catalysis and Inhibition in Nucleophilic Displacement Reactions of Phosphorus-, Sulfur- and Carbon-based Esters

摘要

Selectivity among alkali-metal ions is a feature of a number of important biological processes. High potassium-ion and low sodium-ion concentrations are maintained in mammalian cells by Na~+-K~+ pumps which consume one third to one half of the ATP required by the resting cells.1-2 Selective transport of these ions across membranes by Na~+-K~+ ATPase enzymes creates ionic concentration gradients which control cell volume, allow for the excitability of nerve and muscle cells, and drive the active transport of sugars and amino acids across cell membranes. Transport by the enzyme involves phosphate transfer from ATP to form a phospho-enzyme, a process that requires Na~+ and Mg~(2+) to proceed and whose reversal is catalysed by K~+. Nerve impulses are electrical signals produced by the flow of alkali-metal ions across the plasma membrane of neurons. The action potential arises from large, selective, and transient changes in the permeability of the axon membrane to Na~+ and K~+. These fluxes are created by ion channels; the sodium channel selects for Na~+ by providing a negatively-charged site with a small radius. The creation of artificial ion channels that mimic this process is a subject of current interest, as is the understanding of alkali-metal ion selectivity in chemical systems, where the same or similar underlying principles must come into play.