Nanosheet Formation by an Anionic Surfactant-like Peptide and Modulation of Self-Assembly through Ionic Complexation.

Ian W Hamley,Janne Ruokolainen,Jessica Hutchinson,Mehedi Reza,Steven Kirkham,Amanpreet Kaur,Valeria Castelletto

doi:10.1021/acs.langmuir.6b02180

Abstract

The surfactant-like peptide (Ala)6-(Asp) (A6D) is shown to self-assemble into ultrathin (3 nm thick) nanosheets in aqueous solution above a critical aggregation concentration. A combination of circular dichroism and FTIR spectroscopy and X-ray diffraction shows that the nanosheets comprise interdigitated bilayers of the peptide with β-sheet conformation. The self-assembly can be modulated by addition of hexamethylenediamine which is expected to interact with the anionic C terminus (and C-terminal D residue) of the peptide. Multiple ordered nanostructures can be accessed depending on the amount of added diamine. Nanosheet and bicontinuous network structures were observed using cryogenic-TEM and small-angle X-ray scattering. Addition of hexamethylenediamine at a sufficiently large molar ratio leads to disruption of the ordered nanostructure and the formation of a solution of A6D-diamine molecular complexes with highly charged end groups. The multiple acid-functionalized nanostructures that are accessible in this system are expected to have many applications in the fabrication of new nanomaterials.

Highlights

Surfactant-like peptides (SLPs) are a class of amphiphilic peptide comprising a headgroup which is a short sequence of charged residues attached to a tailgroup of neutral residues.[17−19] Pioneering work on SLPs has been conducted by the Zhang group including a series of peptides with neutral amino acid tails and charged residues such as D or K.20−23 We previously reported the unexpected observation that the SLP A6R self-assembles into free-floating ultrathin (3 nm thick) nanosheets in dilute aqueous solution.[12]
We report on the self-assembly of the related SLP A6D with oppositely charged headgroup compared to A6R, and we show that the self-assembly can be modulated by addition of diamine, expected to bind to the peptide C terminus and the carboxyl group in the D residue
We first determined whether the addition of hexamethylenediamine has any influence on the potential aggregation behavior of A6D via critical aggregation concentration (CAC) assays

Summary

■ INTRODUCTION

The formation of free-floating ultrathin nanosheet structures has been observed for several classes of systems including assemblies with graphene or graphene oxide,[1,2] two-dimensional metal oxide sheets,[3,4] various exfoliated inorganic structures,[5] polymer[6] and block copolymer crystals,[7] and peptoids.[8−10] Nanosheet formation by peptides is rarely reported,[11−16] such structures offer the potential to prepare functional nanostructures coated at high density with bioactive amino acids. Surfactant-like peptides (SLPs) have a remarkable ability to self-assemble into different nanostructures, primarily due to their amphiphilic nature. PH was measured using a Mettler Toledo Five Easy FE20 instrument with a Sigma-Aldrich micro pH combination electrode, at various points as more hexamethylenediamine solution was added, until a ratio of 1:2 peptide:diamine was reached. Spectra were recorded using a Thermo Scientific Nicolet IS5 or a Nexus-FTIR spectrometer, both equipped with a DTGS detector In the former, a PEARL liquid cell was used (sample contained between fixed CaF2 plates). Thereafter 3 mL of sample solution was applied onto the grid, and it was blotted twice for 5 s and vitrified in a 1/1 mixture of liquid ethane and propane at a temperature of −180 °C.

■ RESULTS

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES