Water-Sensitive High-Frequency Molecular Vibrations in Self-Assembled Diphenylalanine Nanotubes

High-frequency molecular vibrations in bioinspired peptide nanostructures provide insight into the important interactions between peptides and water molecules. Raman spectra acquired from diphenylalanine (FF) nanotubes show that water bonded weakly to FF molecules in the nanochannel cores leads to splitting of the molecular vibrational mode of benzene rings at 1034 cm^–1 into a doublet with the separation diminishing with decreasing water content. X-ray diffraction discloses that loss of water results in noticeable lattice expansion in the subnanometer crystalline structure comprising hexagonal unit cells, and derivation based on the density functional theory shows that the Raman-active phonon modes often appear in pairs due to the duality of the major components in the FF molecules. Without water, the two typical peaks in the vicinity of 1034 cm^–1 from the vibrations of two benzene rings in the FF molecule are very close and usually cannot be distinguished experimentally, but with the addition of water, the two peaks are gradually separated and the relative intensities change. Our results demonstrate that Raman scattering can be used to probe the quantity of water molecules in FF NTs via the linear dependence of the Raman mode position at the low-frequency side of the double-peak mode at 1034 cm^–1 on water molecule number bonded to each FF molecule. This knowledge is important to the fundamental understanding of the interactions between FF nanotubes and water, device design, as well as applications to biochemistry, medicine, and molecular sensing.