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 as ¹³C¹⁸O and ¹³C¹⁵N has been reported so far. Here, we establish triple ¹³C₁¹⁸O¹⁵N 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 same ¹³C¹⁸O¹⁵N-labeled sample.