Frequency-selective enhancement of water permeation through an aquaporin-inspired fluctuating nanochannel

Abstract

Aquaporins are generally regarded as passive water channels that mediate rapid, selective water permeation in response to osmotic gradients. However, their microscopic permeability may be affected by intrinsic conformational fluctuations. Here, we investigate the effect of pore-radius fluctuations on osmotic water permeation through an aquaporin-inspired nanochannel using coarse-grained molecular simulations. The narrowest region of an hourglass-shaped nanochannel was forced to fluctuate according to white Gaussian, 1/f, or 1/f2 noise while a density difference was imposed between two water reservoirs. White-noise fluctuations had little effect on the relaxation of the density difference, whereas temporally correlated 1/f and 1/f2 fluctuations markedly enhanced water permeation. Frequency-resolved simulations using sinusoidal radius oscillations revealed a characteristic frequency window in which the permeation rate increased. This enhancement was accompanied by an increase in the work rate estimated from pressure–volume hysteresis curves, indicating that channel-radius fluctuations perform mechanical work on confined water molecules. The effective frequency range was found to correspond to the relaxation times associated with wetting and drying within the nanochannel. These results establish a generic physical mechanism by which the frequency spectrum of pore fluctuations can modulate water permeation in a simplified fluctuating nanopore. Its possible relevance to aquaporins remains to be examined using molecularly detailed models.