Precision Measurement of the p(e,e

New results are reported from a measurement of π^{0} electroproduction near threshold using the p(e,e^{'}p)π^{0} reaction. The experiment was designed to determine precisely the energy dependence of s- and p-wave electromagnetic multipoles as a stringent test of the predictions of chiral perturbation theory (ChPT). The data were taken with an electron beam energy of 1192 MeV using a two-spectrometer setup in Hall A at Jefferson Lab. For the first time, complete coverage of the ϕ_{π}^{*} and θ_{π}^{*} angles in the pπ^{0} center of mass was obtained for invariant energies above threshold from 0.5 up to 15 MeV. The 4-momentum transfer Q^{2} coverage ranges from 0.05 to 0.155 (GeV/c)^{2} in fine steps. A simple phenomenological analysis of our data shows strong disagreement with p-wave predictions from ChPT for Q^{2}>0.07 (GeV/c)^{2}, while the s-wave predictions are in reasonable agreement.

Voltage-clamp experiments on oxidized cholesterol membranes modified with excitability-inducing material and comparison with a model.

The conductance of oxidized cholesterol membranes modified with excitability-inducing material was observed in membranes containing either single conductance channels or 100-1000 channels. Membranes containing single channels have several conductance states for each voltage polarity, and the current through membranes containing manychannels decays with at least two, and probably three, time constants following a step change in voltage (voltage-clamp). The time constants differ by about an order of magnitude. The multi-state behavior seems more pronounced in membranes made from highly oxidized cholesterol. Although a given conductance state could occur at either positive or negative voltages, each state was much more frequent at one polarity or the other. The most frequently observed single-channel conductance states in 0.1 M NaCl are about 0.3, 0.1, 0.03, and 0.0 n-1 for negative voltages and 0.25, 0.05, 0.03, and 0.0 n-1 for positive voltages. The current following a voltage clamp decays to a quasi-steady state within 1 min for positive voltages and 1-15 min for negative voltages. When the holding voltage is --20 mV, the decay constants and quasi-steady state conductances as functions of clamping voltage are reasonably well described by either a three-state model of the conductance or a two-state model applied independently at negative and positive voltages. However, for high voltages, the quasi-steady state does not appear to approach a state in which all the channels are in a low conductance state.

Numerical solution of steady-state electrodiffusion equations for a simple membrane.

A technique is given for obtaining numerical solutions to the steady-state electrodiffusion equations for a simple membrane. Solutions are given for several membrane boundary conditions in terms of ratios of current density to mobility for each ion type.

An exact constant-field solution for a simple membrane.

We show that the exact steady-state solution to the electrodiffusion equations for a simple membrane is the constant electric field solution when the ion environment is electroneutral on both sides of the membrane and the total numbers of ions of the same valence on both sides are equal.