The Effect Of C-terminal Amino Acid Chirality On Self-assembly Of Short Amphiphilic Peptides

Based on the peptide supramolecular nanostructures formed in vivo and in vitro and their functions in many biological processes,researchers have been devoted to the self-assembly of peptide in the past two decades.Peptide is the primary structure of protein in which amino acids are linked in a certain sequence.Peptide self-assembly refers to the process in which peptide molecules spontaneously form ordered nanostructures through hydrogen bonding interations between peptide chains,hydrophobic interactions,?-?stacking interactions and electrostatic interactions.As the structure and property of the amino acid residues determine the self-assembly process and the supramolecular nanostructures,the peptides self-assembly could be regulated by a rational molecular design.At present,the research works in this field are mainly focused on the mechanism of molecular interactions,and the development of novel biomaterials based on peptide self-assemblies.Chirality plays an important role in the fields of life,new drug design,and advanced materials manufacturing.The special effects of D-type amino acids in peptide sequences have attracted increasing interests from researchers.On the basis of I₃K,we have designed and synthesized a series of amphiphilic peptides by changing the number of amino acids and the type of hydrophilic amino acids.The effect of C-terminal amino acid chirality on peptide self-assembly has been investigated.The morphologies of peptide self-assembly have been observed with atomic force microscopy(AFM)and transmission electron microscopy(TEM).The secondary structures were characterized with circular dichroism spectroscopy(CD),Fourier transform infrared spectroscopy(FTIR)and fluorescence spectroscopy.(1)Three short peptides,I₄^LK^DK,I₄^DK^LK and I₄^DK^DK,were designed by changing the chirality of lysins of I₄^LK^LK.AFM and TEM results showed that the morphologies of I₄^LK^LK,I₄^LK^DK and I₄^DK^DK self-assemblies were left-handed helical nanofibers.The diameter of I₄^LK^DK nanofibers was a bit larger than that of I₄K₂ and I₄^DK^DK.The morphologies of big nanotubes and nanofibers coexisted in the I₄^DK^LK self-assemblies.And the nanofibers were right-handedly twisted.The morphology diffences of the self-assemblies were mainly caused by the different steric hindrance and electrostatic repulsion.The results indicated that the morphology and handness of the self-assemblies could be controlled by the chirality of the amino acids at the hydrophilic head.(2)Due to the pH-sensitivity of the imidazole group on the histidine side chain and the formation of H-H pairing,I₃H and I₃^DH were designed to investigate the effect of histidine chirality and pH on the peptide self-assembly.It was found that I₃H formed right-handed twist nanofibers,while I₃^DH formed left-handed twist nanofibers at pH 7.0.The chirality of C-terminal amino acids controled the handness of the nanofibers,although it was opposite to that of I₃K series.The two peptides also formed nanofibers at pH 5.0.At pH 3.0,I₃H didn't form ordered nanostructures,while I₃^DH formed nanofibers,indicating that the D-amino acid made the peptide molecules self-assemble more easily.At pH 10.0,neither of these two peptides formed ordered structures.In the alkaline environment the H residue was deprotonated without any charge,two peptide molecules gradually aggregated and deposited to form random aggregates.(3)We have synthesized another short peptide molecules,I₃E and I₃^DE by replacement of Lys with Glu,to investigate the effects of Glu chirality and pH on peptide self-assembly.The results showed that both I₃E and I₃^DE formed left-handed twist nanofibers at pH 7.0,indicating that the handness of the nanofibers was independent to C-terminal amino acid chirality.When the pH was increased to 9.0 or 10.0,I₃E and I₃^DE also formed nanofibers.But the handness of the nanofibers disappeared.