Conformational Effects in the Transport of Glucose through a Cyclic Peptide Nanotube: A Molecular Dynamics Simulation Study

The transport behavior of glucose through a cyclic peptide nanotube (CPN), composed of 8 × cyclo[-(Trp-d-Leu)₄-Gln-d-Leu-] rings embedded in DMPC lipid bilayers was examined using all-Atom molecular dynamics (AAMD) simulations. Two conformational isomers of β-d-glucose, equatorial (⁴C₁) and axial (¹C₄) chair conformers, were used to examine conformational effects on the hydrogen bond network, energetics, and diffusivity of glucose transport through the CPN. Calculations of the number of hydrogen bonds of the two glucose conformers with water molecules and with the CPN illustrate that the total number of hydrogen bonds of the conformers decreases inside the channel compared to bulk water due to the confinement characteristics of the interior of the CPNs although new hydrogen bonds between the hydroxyl and hydroxymethyl hydrogens of glucose and the carbonyl oxygens in the CPN backbone are formed. Despite the decrease of the number of hydrogen bonds inside the CPN, intramolecular hydrogen bonds of ¹C₄ are maintained during permeation of ¹C₄ through the CPN. The retention of intramolecular hydrogen bonds and the spherical shape of ¹C₄ give rise to considerably weaker orientational preferences and higher diffusion coefficients for ¹C₄ than those of ⁴C₁ inside and outside the CPN. Due to larger dipole moments induced by the alignment of hydroxyl and hydroxymethyl groups, ¹C₄ has more favorable interactions with the CPN backbone at the channel entrances and inside the channel than ⁴C₁. In the middle of the CPN channel, entropic gains originating from higher orientational and translational degrees of freedom of ¹C₄ than those of ⁴C₁ also contribute to lower free energy wells for ¹C₄ inside the CPN. This work reveals that the conformational variation and intramolecular hydrogen bond formation of β-d-glucose can have important effects on the energetics and dynamics of glucose transport through CPNs, providing insight into the translocation mechanism of d-glucose into the cell through glucose transporters (GLUTs) and the dynamics of glucose confined in silica nanochannels. It is also demonstrated that CPNs can indeed facilitate the permeation of small hydrophilic molecules such as glucose and can be utilized as a novel carrier system for hydrophilic drug compounds into the cell.