Theoretical calculations of Raman and SERS (cysteine-Ag4 complexes) spectra were performed to evaluate the effect of isotopic substitution of C with C and of N to N on the vibrational signature of cysteine. Results of our DFT calculations (see Fig. and Tables ) have suggested that cysteine molecules in both forms considered here exhibit a profound change in the energy of vibration at 1664 cm -1 (zwitterion) or 1800 cm -1 (cysteine-Ag4) upon the isotopic substitution of the carboxyl carbon from C to Other substitutions had a comparatively minor effect on this vibrational mode, as all isotopic shifts were smaller than 0.2 cm -1 (see Tables ). Detailed results of the DFT isotopic analysis are presented in Chapter 1 of the Supporting Information. In particular, a distinct pattern was observed upon increasing the number of atoms substituted for their heavier counterparts, as shown in Fig. . The relative changes visualized in Fig. were calculated according to equation (1), presented in Chapter 1 of the Supporting Information. Such a large isotopic shift, as presented by the carboxyl carbon substitution, warranted distinct and well-separated features in the experimental SERS spectra. Although isotopic substitution of the carboxyl carbon alone would be sufficient according to the DFT results, the compound with all C and 15 N atoms, Cys-CABN, was chosen for the experimental part of the study, as it was readily available commercially. .