Phenyl ring dynamics of the insulin fragment Gly-Phe-Phe(B23-B25) by solid state deuterium NMR

The phenyl ring dynamics of the insulin fragment Gly-Phe-Phe(B23-B25) were investigated using solid state deuterium NMR spectroscopy. It was found that the phenyl rings of the two phenylalanine residues Phe² and Phe³ were rigid even up to 100°C both for the Gly-[ring-d₅]Phe-Phe and the Gly-The-[ring-d₅]Phe in the hydrated crystals. When the temperature was raised to 120°C, the hydrated water evaporated from the crystal, resulting in the onset of the flipping motion of the phenyl rings. Spectral simulation of the deuterium NMR spectra was performed to better characterize the motion of the phenyl rings in the peptides. It was found that the phenyl ring motion of the fragments is consistent with a 180° flip about the C_βC_γ bonds. The phenyl ring of Ph² of Gly-[d₅]Phe-Phe was more mobile than that of Phe³ of Gly-Phe-[d₅]Phe when the tripeptide crystal was in the dehydrated state. The Phe-Phe residues in the tripeptide were quite rigid when the hydrophobic interaction around the Phe-Phe moiety was strong.