  Ion channels are membrane proteins responsible for an enormous range of
biological functions. Ion selectivity and permeation are based on simple laws
of physics and chemistry. Ion channels are therefore ideal candidates for
physical investigation. A reduced model generates the selectivity of
voltage-gated L-type calcium channel under a wide range of ionic conditions
using only two parameters with unchanging values. The reasons behind the
success of this reduced model are investigated. Monte Carlo simulations are
performed investigating the role of flexibility of the side chains in the
selectivity of calcium channels under a wide range of ionic conditions. Results
suggest that the exact location and mobility of oxygen ions have little effect
on the selectivity behavior of calcium channels. The first order determinant of
selectivity in calcium channels is the density of charge in the selectivity
filter. Flexibility seems a second order determinant. Single channel planar
lipid bilayer experiments are performed to determine the selectivity,
permeation, and sialic acid specificity of a bacterial outer-membrane channel
NanC of Ecoli. NanC exhibits a large unit conductance varying with the applied
voltage polarity, anion selectivity, and voltage gating. Unitary conductance of
NanC decreases significantly in presence of the buffer HEPES. Alternate buffers
are tested. Results suggest that the sialic acid specificity of NanC should be
performed in low concentration salt solutions without pH buffers adjusted to
neutral pH 7.0. Neu5Ac changes the gating and considerably increases the ionic
conductance of NanC. The effect of Neu5Ac on the unitary current through NanC
saturates at higher Neu5Ac concentrations. Interestingly, Neu5Ac reduces the
ionic conductance of OmpF: frequent, long closures are seen. Thus, in E.coli
sialic acid translocation is specifically facilitated by NanC, and not by OmpF.
