Transport Behavior Of A Single Ca²⁺, K⁺ And Na⁺ In A Water-filled Transmembrane Cyclic Peptide Nanotube

Molecular dynamics(MD) simulations have been performed to investigate the transport properties of a single Ca2+, K+ and Na+ through a water-filled transmembrane cyclic peptide nanotube(CPNT). Two transmembrane CPNTs, i.e., 8×(WL)n=4,5/POPE(with uniform lengths but various radii), were applied to clarify the dependence of ionic transport property on the channel radius, ionic radius, charge and hydration ability of the cation.Huge energy barrier keeps Ca2+ out of the octa-CPNT, while Na+ and K+ can be trapped in two CPNTs and Na+ encounters deeper energy wells than K+, resulting its longer stay in the channels. The dominant electrostatic interaction of a cation with water molecules change the original water-chain structure in the tube, especially in the octa-CPNT, D-defects were found in the first and last gaps. According to calculation, D-defects are most likely to arise under the presence of K+. In addition, the introduction of a cation significantly reduces the axial diffusion of channel water.Analysis of radial distributions reveals that these cations prefer to reside near a tube axis.Water-bridged interactions were mostly found between the artificially introduced Ca2+ and the framework of the octa-CPNT, and direct coordinations with the tube wall mostly occur for K+ in the octa-CPNT. Nevertheless, in the deca-CPNT, water-bridged interactions dominate the bindings between these cations and the framework of the CPNT, due to the relatively longer distance between the cation and the tube wall. The axial diffusions of a cation is relevant with the ionic initial position, a cation may rapidly drift, especially in the octa-CPNT and different cation drifts at various positions. The cation moves more quickly with the increase of the tube, especilly, K+ behaves most actively and can visit the whole deca-CPNT quickly. The first solvation shells of Ca2+ and Na+ are basically saturated in two CPNTs, while the hydration of K+ is much incomplete in the octa-CPNT. The increase of the channel radius induces more interchange attempts between the first-shell water molecules of a cation and the ones in the outer region. Overall, the solvation structure of Ca2+ in the octa-CPNT is most stable, while that of K+ in the deca-CPNT is most labile.