Amphiphilic Peptoid Block Copolymers:Design,Synthesis And Self-assembly

Two-dimensional(2D)nanomaterials have potential applications in biomedicine,chemistry and other fields due to their unique physical and chemical properties.Cell membrane is a special two-dimensional nanomaterial composed of bilayer phospholipids,which has the functions of selective substance transportation,recognition,etc.In order to mimic the structure,scientists have made a lot of efforts in designing,synthesizing and constructing bimimetic membranes,expecting them to have similar properties.Among them,two-dimensional nanomaterials assembled by molecules with precise and controllable sequences(such as DNA,protein,peptides,etc.)are a new type of two-dimensional nanomaterials with excellent physical and chemical properties.They have unique structural characteristics and self-identification characteristics.However,the molecular sequences and structures are complicated,making the synthesis work hard to achieve.Moreover,the molecules tend to interact with each other and assemble in unpredictable ways and specific raw materials are required,which greatly limits the development of the material.Peptoids(N-substituted glycines)are a new class of protein mimetic biocompatible materials.They have the same backbone structures compared with peptides except that the side chains are transferred from the alpha carbons to the amide nitrogens.Due to the substitution groups moved from C to N,the main chains of peptoids are free of hydrogen bonds and chiral centers,which makes the material have special properties superior to peptides.For example,peptoids have good solubility,strong resistance to enzymatic hydrolysis,good thermal processing performance,and highly controllable chemical structures Therefore,peptidoid is an ideal material for studying 2D nanomaterials.In this study,a series of peptoid diblock and triblock copolymers with hydrophilic/hydrophobic groups(carboxyl,amino,butyl,hexyl and octyl)in side chains were successfully designed and prepared by solid-phase submonomer synthesis method.The self-assembly behaviors of the amphiphilic peptoids were studied,and the influence of hydrophobic block length and long alkyl side chain length on self-assembly and the stability of the self-assembled structures were explored.We found that peptoid diblock copolymers containing hydrophobic groups(six residues with octyl groups(Noc))and hydrophilic groups(six residues with carboxyethyl groups(Nce)or aminoethyl groups(Nae))were able to self-assemble into two-dimensional nanomaterials.6Nce-6Noc self-assembled into two-dimensional nanosheets with the size of about 100 nm,smaller than the nanosheets of about 500 nm formed by 6Nae-6Noc.After mixing these two diblock polymers,larger two-dimensional nanosheets,about 800 nm,were formed due to the coulomb force between amino and carboxyl groups.The peptoid triblock copolymers have hydrophobic blocks with alkyl side chains in the middle of the molecule chains,and hydrophilic blocks with carboxyethyl or aminoethyl side chains at two ends of the molecule chains separately.The triblock copolymers containing six Noc residues(3Nae-6Noc-3Nce)self-assembled into two-dimensional nanosheets with a size of about 200 nm.However,when the hydrophobic block length is reduced from 6 to 3residues(3Nae-3Noc-3Nce),no self-assembled structures were observed.Kept the hydrophobic blocks with 6 residues but reduced side chain length from 8 C to 6 C,3Nae-6Nhe-3Nce were able to self-assemble into nanosheets about 200 nm.However,when the length of alkyl side chain was further reduced to 4 C,3Nae-6Nbu-3Nce had no self-assembly behavior.The results indicate that enough hydrophobicity is critical for the nanomaterial formation.The prepared self-assembled nanosheets are fairly stable.After sonication,the self-assembly structure of the two diblock cluster peptoids did not change much after mixing.However,the structure of 6Nce-6Noc changed after 15 min of sonication,and the self-assembly structure changed to the morphology before sonication after 30 min of sonication.They can survive after sonication.In a word,peptoids,as a new biomimetic materials,can form 2D nanomaterials which combine the advantages of 2D nanomaterials from peptides and proteins(such as highly designed diversity and functionality)and traditional 2D nanomaterials(such as high chemical and thermal stability).Therefore,peptoids have a bright future in the application of biotechnology.This study is expected to enrich the nanomaterial family as well as peptoid science,and expand their applications in chemistry and biotechnology.