Polarized raman spectroscopy for determining the orientation of di-d-phenylalanine molecules in a nanotube

Self‐assembly of short peptides into nanostructures has become an important strategy for the bottom‐up fabrication of nanomaterials. Significant interest to such peptide‐based building blocks is due to the opportunity to control the structure and properties of well‐structured nanotubes, nanofibrils, and hydrogels. X‐ray crystallography and solution nuclear magnetic resonance (NMR), two major tools of structural biology, have significant limitations when applied to peptide nanotubes because of their non‐crystalline structure and large weight. Polarized Raman spectroscopy was utilized for structural characterization of well‐aligned di‐d‐diphenylalanine nanotubes. The orientation of selected chemical groups relative to the main axis of the nanotube was determined. Specifically, the C–N bond of CNH3+ groups is oriented parallel to the nanotube axis, the peptides' carbonyl groups are tilted at approximately 54° from the axis, and the COO− groups run perpendicular to the axis. The determined orientation of chemical groups allowed the understanding of the orientation of di‐d‐diphenylalanine molecule that is consistent with its equilibrium conformation. The obtained data indicate that there is only one orientation of di‐d‐diphenylalanine molecules with respect to the nanotube main axis. Copyright © 2016 John Wiley & Sons, Ltd. In this study, polarized Raman spectroscopy has been exploited to acquire quantitative information regarding structural organization of aligned di‐d‐phenylalanine nanotubes. The orientation of selected chemical groups including peptides' carbonyl, NH3+ and COO− relative to the nanotube axis has been obtained. On the basis of the information obtained orientation of the di‐d‐phenylalanine molecule within a nanotube was proposed.