Amide or Amine: Determining the Origin of the 3300 cm−1 NH Mode in Protein SFG Spectra Using 15N Isotope Labels

Abstract

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 ¹⁵N isotopically labeled 14-amino acid amphiphilic model peptides composed of lysine (K) and leucine (L) in an α-helical secondary structure (LKα14) that were adsorbed onto charged surfaces in situ at the solid−liquid interface. ¹⁵N 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.