FIG. 4.

The beta-toxin channel radius. Parameter ν (see Results for the definition of ν) was calculated as a function of the hydrodynamic radii of the nonelectrolyte PEG. The radius of the beta-toxin pore was determined within the transition zone from limited permeation of a nonelectrolyte to impermeability at ν = 0 (at the intersection between the dotted line at zero and linear curve fits). The radii of beta-toxin channels with conductances of 110 and 60 pS were determined to be 12.7 and 11.1 Å, respectively. Open circles represent the parameter ν calculated for the channels with a conductance of 110 pS in a PEG-free solution. Solid circles represent the parameter ν calculated for the channels with a conductance of 60 pS in a nonelectrolyte-free solution. The membrane-bathing solution contained 100 mM NaCl (pH 7.4). The holding potential applied to the membrane was −60 mV. The addition of low-molecular-weight nonelectrolyte molecules with hydrodynamic radii of less than 6 Å to the membrane-bathing solution did not decrease transmembrane current sufficiently, and therefore these were not included in this analysis.