Diffusion of large particles through small pores: from entropic to enthalpic transport

We present an implicit statistical model for the steric effect on the potential of mean force (PMF) of a molecule diffusing through a flexible nanochannel of varying size. The average cross sectional area profile of the channel and the average minimal projection area of the molecule are the two major quantities determining the steric part of the PMF barrier for the translocation of the particle in the case of a small rigid particle and a large rigid channel. In this case, the description is reduced to the Fick-Jacobs model and the PMF is completely entropic. However, the flexibility of channel's cross section and that of molecule's size play crucial role when a large molecule goes through a narrow channel. The PMF profile changes its statictical nature and becomes enthalpic. We treat the flexibility in terms of the equilibrium fluctuations of the pore and of the molecule, independently. For the case of gaussian fluctuations, we derived simple analytical expressions for the steric barrier.