Molecular Dynamics Simulations On The Behaviors Of Hydrophilic/Hydrophobic Cyclic Peptide Nanotubes At The Water/Hexane Interface

In this work,nine kinds of L-or D-?-amino acids,i.e.,alanine(L-Ala,A),leucine(D-Leu,L),valine(L-Val,V),isoleucine(L-Ile,I),tryptophan(L-Trp,W),glutamine(L-Gln,Q),threonine(D-Thr,T),serine(D-Ser,S)and cysteine(D-Cys,C),were selected to construct seven cyclic peptide nanotubes(CPNTs)with diverse hydrophilic/hydrophobic external surface properties.Molecular dynamics(MD)simulations have been carried out to study their structures and properties in vacuum and their interfacial behaviors at the water/hexane interface.The simulation results reveal that the side chain structures of amino acids affect the tube length of a CPNT,while the inner diameter of a CPNT is only dependent on the number of amino acid residues on the backbone.The H-bonded interactions in a hydrophilic CPNT are stronger than those in a hydrophobic CPNT.Except for the QT-CPNT and VL-CPNT,the other five CPNTs,i.e.,AL?IL?WL?QS and QC-CPNTs,exhibit different degrees of tilt,fracture and shedding at the water/hexane interface.The interactions of individual cyclic peptide(CP)subunits with water and hexane phases and the neighbouring CP subunits disclose the relative stability of a middle CP(mid-CP)and the easy dissociation of a terminal CP(end-CP).A hydrophobic CPNT is prone to stay at the interface,while a hydrophilic CPNT easily enters the water phase,resulting in many H-bonds with water.Results in this work complement the study of a CPNT as an artificial biological nanochannel at a water/oil interface,and will help to provide information for the self-assembly of CPs at a water/oil interface at an atomic level.