PROBING ALAMETHICIN CHANNELS WITH WATER-SOLUBLE POLYMERS - SIZE-MODULATED OSMOTIC ACTION

Contrary to expectations based on heightened solution viscosity, alame thicin channels appear to speed up in the presence of water soluble po lyethylene glycols (PEGs) and dextrans. Specifically, added polymers r educe the probabilities of transition to higher-conductance states but do not change channel lifetimes They thereby shorten the duration of current ''bursts.'' These modified probabilities and kinetics reveal t he action of polymer osmotic stress to suppress channel formation. The osmotic action of large, fully excluded polymers shows that some 3,00 0 angstrom3 of water are taken up by the channel from the solution upo n each transition to an adjacent higher-conductance state. The partial osmotic action of incompletely excluded polymers reveals the extent o f exclusion for different-size polymers. The partial exclusion thus me asured agrees remarkably well with estimates using data on reduction o f single-channel conductance by current-impeding polymers. One can rel ate the degree of each polymer's exclusion to its size and to the radi us of the channel pore.