Tube to ribbon transition in a self-assembling model peptide system

Axel Rüter,Stefan Kuczera,Thomas Zinn,Joakim Stenhammar,Theyencheri Narayanan,Ulf Olsson

doi:10.1039/d0cp03204b

Abstract

Peptides that self-assemble into β-sheet rich aggregates are known to form a large variety of supramolecular shapes, such as ribbons, tubes or sheets. However, the underlying thermodynamic driving forces for such different structures are still not fully understood, limiting their potential applications. In the AnK peptide system (A = alanine, K = lysine), a structural transition from tubes to ribbons has been shown to occur upon an increase of the peptide length, n, from 6 to 8. In this work we analyze this transition by means of a simple thermodynamic model. We consider three energy contributions to the total free energy: an interfacial tension, a penalty for deviating from the optimal β-sheet twist angle, and a hydrogen bond deformation when the β-sheets adopt a specific self-assembled structure. Whilst the first two contributions merely provide similar constant energy offsets, the hydrogen bond deformations differ depending on the studied structure. Consequently, the tube structure is thermodynamically favored for shorter AnK peptides, with a crossover at n≈ 13. This qualitative agreement of the model with the experimental observations shows, that we have achieved a good understanding of the underlying thermodynamic features within the self-assembling AnK system.

Highlights

The AnK model peptide system studied in this article consists of a chain of n hydrophobic alanine (A) amino acids flanked by a single lysine (K) residue as a head group
The macroscopic aggregate structures vary greatly, detailed analyses have shown that the arrangement of AnK peptides within the aggregates is consistent throughout the peptide family where laminated, antiparallel b-sheets pack in a two-dimensional (2D) oblique crystal lattice regardless of the peptide length.[28,29]
In this paper we have proposed a thermodynamic model for laminated b-sheet aggregates that predicts a transition from hollow tubes to twisted ribbons of finite width, as a function of increased peptide size

Summary

Introduction

The AnK model peptide system studied in this article consists of a chain of n hydrophobic alanine (A) amino acids flanked by a single lysine (K) residue as a head group. Its aggregation is mainly driven by hydrophobic interaction between the alanine amino acids.[25] Studies varying n have revealed that shorter alanine chains (n = 6) assemble into hollow nanotubes, whereas n = 8, 10 peptides instead assemble into twisted ribbons.[25,26,27] the macroscopic aggregate structures vary greatly, detailed analyses have shown that the arrangement of AnK peptides within the aggregates is consistent throughout the peptide family where laminated, antiparallel b-sheets pack in a two-dimensional (2D) oblique crystal lattice regardless of the peptide length.[28,29] This high internal ordering is a result of the hydrogen bonds formed between amino acids of neighboring peptides

Methods

Results

Conclusion