text: I. Recent experimental and theoretical work demonstrates that solute translocation can be facilitated by attractive interactions between the channel and the penetrating particle. Although the constructive role of attractive interactions between permeating particles and the channel has been appreciated for many years, a comprehensive theory capable to offer clear understanding and a reliable quantitative description of the channel-facilitated metabolite transport is still to be developed. As an important step towards a comprehensive theory of channel-facilitated passive transport, this year we have considered the average lifetimes in the channel for those particles that traverse the channel and those that return, as well as the total average lifetime of the particle in the channel. Exact expressions for the average lifetimes have been derived in the framework of a one-dimensional diffusion model. The validity of our one-dimensional approach was verified by good agreement of the theoretical predictions with the average lifetimes found in three-dimensional Brownian dynamics simulations. To illustrate some qualitative features of the average lifetimes predicted by the general theory, we have studied a special case in which a symmetric square-well potential occupies some part of the channel length. We find that both the total average lifetime in the channel and the average return time are monotonically increasing functions of the well depth and length. Such behavior agrees with the general intuitive ideas. In contrast, the dependence of the average translocation time on the well length is somewhat counter-intuitive. This time increases with the length when the length is small, reaches a maximum when the well occupies half of the channel, and then it starts to decrease. Concerning the dependence on the well depth, the average translocation time monotonically increases with the depth. The deeper the well, the more pronounced is the turnover behavior of the translocation time. II. Water-soluble polymers have proven to be valuable probes of ion channels. By analyzing the channel conductance in the presence of polyethylene glycol (PEG) it is possible to investigate channel geometry. Understanding the mechanism by which PEGs reduce electrolyte conductivity is important for the interpretation of these experiments. This year we have completed the study of polymer-induced variation of the electrolyte conductivity in the bulk solutions. We measured conductivity of potassium chloride solutions containing PEGs of different molecular mass in a wide range of the polymer concentration up to 33 weight percent for PEG 300, 600, 2000, 4600, and 10000. The data were used to find the dependence of microviscosity, which characterizes the decrease of the ion mobility compared to that in the polymer-free solution, on the polymer volume fraction. We have found that the dependence is well approximated by a simple exponential relation that expresses microviscosity of the polymer-containing solution through the polymer volume fraction. Parameters of the relation only weakly depend on the polymer molecular mass. Through these parameters we can derive practical formulae for the PEG effect on electrolyte solution conductivity, formulae that will be helpful in future work with ion channels. III. New methods developed for investigations of metabolite and other large-molecule transport at the single-channel level require the detailed knowledge of stability and noise characteristics of the open single channels. Appreciation of noise generation mechanisms in nanoscale objects is also important for the successful development of single-molecular sensors based on natural ion channels and solid-state micro- and nano-structures. To address these questions, this year we studied conductance, selectivity, step-wise current transients, and open-channel noise of single trimeric channels of the general bacterial porin, OmpF, over a wide range of proton concentrations. We found that OmpF channels reconstituted in planar bilayer membranes can endure extreme pH conditions for hours. Exposed to highly acidic or basic environment of pH 1 or pH 12, they can be returned to their completely functional state when solution pH is brought back to neutral. This robustness makes it possible to study channel transport properties over a broad range of solution pH. From pH 1 to pH 12, channel transport displays three characteristic regimes. First, in acidic solutions, channel conductance is a strong function of pH; it increases by about 3-fold as the proton concentration decreases from pH 1 to pH 5. This rise in conductance is accompanied by a sharp increase in cation transport number and by pronounced open-channel low-frequency current noise with a peak around pH 2.5. Random stepwise transients with amplitudes about one fifth of the monomer conductance are major contributors to this noise. Second, over the middle range (pH 5 to pH 9), channel conductance and selectivity stay virtually constant; open channel noise is at its minimum. Third, over the basic range (pH 9 to pH 12), channel conductance and cation selectivity start to grow again with an onset of a higher frequency open-channel noise. We attribute these effects to the reversible protonation of channel residues whose pH-dependent charge influences transport by direct interactions with the ions passing through the channel.