Sum frequency generation (SFG) vibrational spectroscopy has been employed in biomaterials research and protein adsorption studies with growing success in recent years. A number of studies focusing on understanding SFG spectra of proteins and peptides at different interfaces have laid the foundation for future, more complex studies. In many cases, a strong NH mode near 3300 cm(-1) is observed in the SFG spectra, but the relationship of this mode to the peptide structure is Uncertain, since it has been assigned to either a backbone amide mode or a side chain related amine resonance. A thorough understanding of the SFG spectra of these first model systems is an important first step for future experiments. To clarify the origin of the NH SFG mode, we studied N-15 isotopically labeled 14-amino acid amphiphilic model peptides composed of lysine (K) and leucine (L) in an alpha-helical secondary structure (LK alpha 14) that were adsorbed onto charged surfaces in situ at the solid-liquid interface. N-15 substitution at the terminal amine group of the lysine side chains resulted in a red-shift of the NH mode of 9 cm(-1) on SiO2 and 13 cm(-1) on CaF2. This clearly shows the 3300 cm(-1) NH feature is associated with side chain NH stretches and not with backbone amide modes.