MD Simulations on the Transport Behaviors of Mixed Na⁺ and Li⁺ in a Transmembrane Cyclic Peptide Nanotube under an Electric Field

Abstract

Due to its inherently stronger hydration, Li + faces a higher dehydration energy than Na + at the entrance of the 8×(WL) 4 /POPE-CPNT. Present MD simulations show that it can enter the channel from a NaCl/LiCl solution only under an electric field stronger than 0.3 V nm −1 , while Na + is easier to move into the channel, which is well elucidated by two cations' PMF profiles. The cation-O w radial distribution functions, the electrostatic interactions with water, and the orientations of neighboring water all refer to a more compact solvation structure and stronger hydration of Li + . Regardless of whether there is an external electric field, Na + mainly appears in an α-plane zone, while Li + does so in a midplane region. The increase in the electric field strength significantly accelerates the cations' axial diffusions, shortening the residence times of two cations in the channel. Furthermore, it makes channel water tend to take positive dipole states.

Study on the Assembly Mechanisms and Transport Properties of Transmembrane End-Charged Cyclic Peptide Nanotubes

“…For example, in a neutral CPNT such as 8 × (W L ) 4 /POPE–CPNT, owing to the orientation effects from the negative naked carbonyl groups at two ends of the CPNT, channel water molecules in gap1 and gap7 adopt positive (θ < 90°) and negative (θ > 90°) dipole states with θ distribution peaks around 40 and 150°, respectively, as shown in Figure S4. H-bond defects with θ distribution peaks around 90° may occur in the middle region of the channel. ,, …”

“…An external electric field with a strength not stronger than 0.3 V nm –1 hardly disturbs such a 1-2-1-2 channel water array, but may slightly increase the degree of the chaos in the system. With a further increase of the external electric field, the number of channel water molecules increases obviously, and the dipole orientations of channel water molecules are more inclined to follow the direction of the external electric field. , …”

“…The introduction of an electric field may help to decrease the dehydration energy barrier of an ion at a CPNT entrance. It was reported that it is extremely difficult for Li + to enter an octa-CPNT under no electric field, but can easily enter the channel under an electric field stronger than 0.3 V nm –1 …”

“…With a further increase of the external electric field, the number of channel water molecules increases obviously, and the dipole orientations of channel water molecules are more inclined to follow the direction of the external electric field. 17,18 Transmembrane CPNTs are also often simulated as ion channels. Ghadiri et al reported that the transport rates of K + and Na + in 8 × cyclo[-(Trp-D-Leu-) 3 -Gln-D-Leu-] were as high as 10 7 s −1 , three times that of Gramicidin A (GA) under the same conditions.…”

“…It was reported that it is extremely difficult for Li + to enter an octa-CPNT under no electric field, but can easily enter the channel under an electric field stronger than 0.3 V nm −1 . 17 Changing the number and kinds of amino acid residues or modifying functional groups can obtain a variety of CPNTs with different structures and rich performances. Among them, hetero-CPNTs are composed of individual CPs with different structures.…”