The Effect Of Amino Acid Chirality And Side Chain Property On Peptide Self-assembly

Peptide self-assembly is a spontaneous process of association of individual molecule into well-organized structure through tuning various noncovalent interactions,such as hydrogen bonds,electrostatic interactions,hydrophobic interactions,?-?stacking and so forth.Peptide and protein fibrils have attracted an enormous amount of interests due to their relevance to many neurodegenerative diseases and their potential applications in nanotechnology.Although twisted fibrils are regarded as the key intermediate structures of thick fibrils or bundles of fibrils,the factors determining their twisting tendency and their handedness development from the molecular to the supramolecular level are still poorly understood.As the basic unit of peptide,amino acid chirality and property are undoubtedly the key factors of influencing self-assembly.How to design a peptide through reasonable and rational choice of amino acids,using noncovalent interactions self-assembly into specific structure in order to achieve controllable self-assembly is the pursuit of many scientific workers.In this research,we designed and synthesized peptides with different chiral amino acids based on our own designed ultra short peptides,and injected different numbers of achiral amino acid,to investigate the effect of amino acid chirality on self-assembly.Besides,the effect of side chain property on self-assembly was also investigated by changing individual amino acid,to understand the role of various noncovalent interaction and clarify the mechanism of self-assembly.?1?Effect of amino acid chirality:in this study,we designed three pairs of enantiomeric short amphiphilic peptides through changing the chirality of Ile and Lys:Ac-^LI₃^LK-NH₂ and Ac-^DI₃^DK-NH₂,Ac-^LaI₃^LK-NH₂ and Ac-^DaI₃^DK-NH₂,Ac-^DI₃^LK-NH₂ and Ac-^LI₃^DK-NH₂.The results indicated that the twisted handedness of the supramolecular nanofibrils was dictated by the chirality of the hydrophilic Lys head at the C-terminal,as Ac-^LI₃^LK-NH₂,Ac-^LaI₃^LK-NH₂ and Ac-^DI₃^LK-NH₂ formed left handed fibrils while Ac-^DI₃^DK-NH₂,Ac-^DaI₃^DK-NH₂ and Ac-^LI₃^DK-NH₂ formed right-handed fibrils.Beisides,their characteristic CD signals were determined by the chirality of hydrophobic Ile residues,as Ac-^LI₃^LK-NH₂,Ac-^LaI₃^LK-NH₂ and Ac-^LI₃^DK-NH₂showed a negative peak at around 220 nm while Ac-^DI₃^DK-NH₂,Ac-^DaI₃^DK-NH₂ and Ac-^DI₃^LK-NH₂ showed positive peak at around 220 nm.MD simulations delineated the handedness development from molecular chirality to supramolecular handedness by showing that the?-sheets formed by Ac-^LI₃^LK-NH₂,Ac-^LaI₃^LK-NH₂ and Ac-^DI₃^LK-NH₂ exhibited a propensity to twist in a left-handed direction while the ones of Ac-^DI₃^DK-NH₂,Ac-^DaI₃^DK-NH₂ and Ac-^LI₃^DK-NH₂ in a right-handed twisting orientation.?2?Effect of achiral amino acid:on the basis of Ac-I₃K-NH₂,we designed six peptides by inserting different numbers of glycine between hydrophobic amino acids and hydrophilic amino acid,and also changed the chirality of lysine,Ac-^LI₃G^LK-NH₂,Ac-^LI₃GG^LK-NH₂,Ac-^LI₃GGG^LK-NH₂,Ac-^LI₃G^DK-NH₂,Ac-^LI₃GG^DK-NH₂,and Ac-^LI₃GGG^DK-NH₂,to investigate the role of glycine played.The results indicated that the all peptide formed nanofibrils after glycine inserted,the width of the nanofibers decreased with the increase of the number of glycine,this is because the addition of glycine can promote the growth in the long axis while not for the lateral direction?the side chain of glycine is hydrogen atom?.Besides,the chirality of lysine becomes unimportant after the addition of glycine,only related to the number of glycine,handedness can be reversed if the number of glycine changed,which may be due to the good flexibility of glycine.?3?Effect of amino acid side chain property on self-assembly:on the basic of Ac-I₃GGK-NH₂,here we replaced glycine into different property amino acids,designed and synthesized peptide contains?-branched hydrophobic amino acid Ac-I₃VGK-NH₂,and hydrophilic amino acids Ac-I₃QGK-NH₂,Ac-I₃SGK-NH₂ and Ac-I₃NGK-NH₂.The results indicated that Ac-I₃VGK-NH₂ formed wider nanoribbons due to the hydrophobic interaction and?-branched structure of valine side chain,nanofibrils of Ac-I₃GGK-NH₂ also illustrate the role of hydrophobic interactions on the lateral growth.Ac-I₃QGK-NH₂,Ac-I₃SGK-NH₂ and Ac-I₃NGK-NH₂ formed nanoribbons,indicating hydrogen bond formed between amide group or hydroxyl group which could promote the growth in lateral direction.As the control peptide for Ac-I₃QGK-NH₂,Ac-I₃^norVGK-NH₂ could only form nanofibril further declared both the hydrogen bond between amide groups and the?-branched structure can prompte the growth in lateral direction.