Elucidation of the structure of supramolecular polymorphs in peptide nanofibres using Raman spectroscopy

Abstract

Peptide fibre formation via molecular self-assembly is a key step in a range of cellular processes and increasingly considered as an approach to produce supramolecular biomaterials. We previously demonstrated the self-assembly of the tripeptide lysine-dityrosine (KYY) as a substrate for the formation of proton-conducting melanin-like materials. Point based Raman scattering is one of several techniques which were used to characterise the secondary structure of the KYY nanofibre but as is often the case with this type of fibre, the spectra are rather complex and in addition there were variations in intensity between samples making interpretation difficult. Using Raman mapping we show that, as a drop of KYY in solution dries, it self-assembles into two different fibre forms and the simpler spectra obtained for each are easier to interpret. The tyrosine amide marker bands, 852 and 828 cm−1, are present in both forms with similar intensities indicating the formation of a similar secondary structure in both forms with some stacking of the tyrosine rings. However, the tyrosine marker bands at 1614 and 1661 cm−1 vary considerably in intensity between the two forms. It is concluded that both forms consist of stacked polypeptide units joined by hydrogen bonds to form structures similar to β-sheet structures in longer peptides. There are other clear differences such the large intensity difference in the lysine side chain band at 1330 cm−1 and the relative intensities of the bands at 982 and 1034 cm−1. These differences are attributed to changes in the conformation of tyrosine side chains causing different electron withdrawing effects on the ring.