From Fibers to Micelles Using Point-Mutated Amphiphilic Peptides

Abstract

Biocompatible, self-assembled nanostructures are attracting ever more attention, in particular in aqueous media for biomedical applications. Here, we present the successful, solid-phase peptide synthesis (SPPS) and characterization of short amino acid sequences with amphiphilic character with the aim of gaining insight into their self-assembled, supramolecular structures. The peptide design includes three parts: (a) a charged lysine part, (b) an acetylated lysine part, and (c) a constant hydrophobic rodlike helix, based on gramicidin A (gA). By stepwise replacement of free lysine (K) with acetylated lysine (X) we generated a library of a total of 10 peptides, Ac-X(8)-gA and K(m)X(8-m)-gA (m ranging from 0 to 8). By using point mutations, we adjusted the degree of acetylation (DA) and thus the overall amphiphilicity of the peptides, which led to a change in the secondary structure in the aqueous environment from a β-sheet to an α-helix. This transition generated a significant change in the morphology of the self-assembled structures from fibers to micelles. Two different regions were observed for the conformation of the hydrophilic part of the peptide: one region, a β-sheet-like secondary structure, inducing fiber formation (high DA), the other an α-helical-like secondary structure, generating micelle formation (moderate and low DA). The micellar structures depended on the degree of acetylation, which influenced their critical micelle concentration (cmc). These morphology regions were determined by a combination of circular dichroism, dynamic light scattering, surface tension, and transmission electron microscopy, which allowed us to correlate the generated supramolecular architectures with the fine changes obtained by means of the point mutation strategy.

Self-assembled Structures from Amphiphilic Peptides

“…[10] An in situ reversible acetylation could convert micelles into fibers and vice versa, a behavior that could be exploited in molecular switches.…”

“…K 2 X 6 -gA self-assembled into fibers with a length ranging from 30 to 800 nm, while K 3 X 5 -gA formed micelles and worm-like micelles. [10] Fig. 1.…”

“…[1,5] The sequence of amphiphilic peptides is organized into two regions: the hydrophobic and the hydrophilic part. The latter is predominantly occupied by charged and polar amino acids; these are: arginine (R), [6,7] histidine (H), [7,8] lysine (K), [9][10][11][12][13][14][15] aspartic acid (D), [16,17] glutamic acid (E), [11,18] serine (S), threonine (T), asparagine (N), glutamine (Q), and cysteine (C). [13] The design of the hydrophobic part is based on amino acids with neutral and nonpolar side-chains such as glycine (G), [16] alanine (A), [15,19] valine (V), [17,20] leucine (L), [9,17] isoleucine (I), [21] methionine (M), phenylalanine (F), [22,23] tyrosine (Y), and tryptophan (W).…”

“…[13] The design of the hydrophobic part is based on amino acids with neutral and nonpolar side-chains such as glycine (G), [16] alanine (A), [15,19] valine (V), [17,20] leucine (L), [9,17] isoleucine (I), [21] methionine (M), phenylalanine (F), [22,23] tyrosine (Y), and tryptophan (W). [7,[10][11][12][13][14]23] Depending on the hydrophobic to hydrophilic ratio and the sequence, various self-assembled structures can be constructed -as indicated in the associated references for the above amino acids -although the hydrophobicity is moderated by the polar character of the peptide's backbone.…”

Three‐Coordinate Nickel(I) Complexes Stabilised by Six‐, Seven‐ and Eight‐Membered Ring N‐Heterocyclic Carbenes: Synthesis, EPR/DFT Studies and Catalytic Activity

“…Indeed, a decrease in retention time caused by dimerization was found in GPC for the esterified Ac‐X 8 ‐gA‐OEt (SI, Figure S1). As a result of this modification, vesicle formation was observed for Ac‐X 8 ‐gA‐OEt, which was not the case for Ac‐X 8 ‐ gA 21 . Figure a shows collapsed vesicles.…”