Vibrational Spectroscopy of Triple 13C18O15N Isotope-Edited N-Methylacetamide: Interplay between Experiment and Theory (IR, Raman, UVRR)

Isotope editing of amide bonds is a powerful tool for site-specific vibrational spectroscopy of peptides and proteins. However, only single and double isotope labeling such as13C18O and13C15N has been reported so far. Here, we establish triple13C118O15N isotope editing of the amide bond in N-methylacetamide (NMA) as the most widely employed model system for peptides. The effect of successive isotope labeling on the peak position of amide modes is systematically studied by comparing the IR, electronically nonresonant Raman, and UV resonance Raman (UVRR) spectra of NMA-13C1, NMA-13C118O, and NMA-13C118O15N. The experimental spectra agree well with theoretical predictions obtained by using atomistic, fully polarizable multiscale quantum mechanical (QM)/classical modeling of neat NMA (IR/Raman) and NMA in aqueous solution (UVRR). The presented triple-labeling approach paves the way for complementary site-specific studies of individual amide bonds and their prominent amide I, II, and III vibrations by using both IR and UVRR spectroscopy on the same13C18O15N-labeled sample.