We also calculated diffusion constants, D, by fitting three dimensional (d = 3) mean square displacement to the linear equation MSD = 2dD τ, (5) in linear regions (between 100 ns and 500 ns) of the MSD plots. As we are looking only at qualitative trends, we perform these for fixed box length (L = 9.3 nm for membrane simulation and L = 5.5 nm for bulk simulation), and neglect box-size contributions. The diffusion coefficients are shown in Table 1. In addition to the diffusion coefficients in the Q I membrane, we also compare the diffusion coefficients of these solutes in bulk water for comparison. Diffusion coefficients of these molecules in the membrane are substantially lower than corresponding bulk values. Since the hydrophilic channels are sub-nanoscale (see Fig 8), diffusion of water and the solutes are substantially hindered. However, the steric hindrance is much stronger, and the decrease of diffusivity much larger, for the solutes compared to water since the solute molecules are larger than water molecules. Specifically, diffusion of larger molecules is slowed by 1-2 orders of magnitude more than water is, consistent with selective rejection of larger molecules.