The experimental SERS spectra of Cys-CABN (see Fig. for molecular structure) and Cyscabn (Fig. ) adsorbed on AgNPs for 30 minutes at 25 °C are juxtaposed to the corresponding calculated Raman spectra of Cys-CABN and Cys-cabn adsorbed on the Ag4 cluster in Fig. . As predicted by the DFT calculations, there is a distinct isotopic shift in the experimental vibrational spectra when comparing the fingerprints of Cys-cabn and Cys-CABN (dark blue and green curves in Fig. ). The vibrational mode involving mostly the νC-O modes (visualized in the inset of Fig. ), discussed above, was assigned to experimental signals at a Raman shift of 1633 cm -1 for Cys-cabn and at 1577 cm -1 for Cys-CABN in experimental SERS spectra. Such a high energy difference confirms the conclusion drawn from DFT calculations that the shift would ensure the existence of distinct features in the experimental SERS spectra. These two bands, corresponding to the same vibrational mode, were selected as marker bands for Cys-cabn (1633 cm -1 ) and Cys-CABN (1577 cm -1 ), respectively. The detailed PED analysis of the vibrational modes observed in SERS spectra is presented in Tables and of the Supporting Information.