Inner-Shell Excitation Spectroscopy of the Peptide Bond:  Comparison of the C 1s, N 1s, and O 1s Spectra of Glycine, Glycyl-Glycine, and Glycyl-Glycyl-Glycine

Oscillator strengths for C 1s, N 1s, and O 1s excitation spectra of gaseous glycine and the dipeptide, glycyl−glycine, have been derived from inner-shell electron energy-loss spectroscopy recorded under scattering conditions where electric dipole transitions dominate (2.5 keV residual energy, θ ≈ 2°). X-ray absorption spectra of solid glycine, glycyl−glycine, glycyl−glycyl−glycine, and a large protein, fibrinogen, were recorded in a scanning transmission X-ray microscope. The experimental spectra are assigned through interspecies comparisons and by comparison to ab initio computed spectra of various conformations of glycine and glycyl−glycine. Inner-shell excitation spectral features characteristic of the peptide bond are readily identified by comparison of the spectra of gas-phase glycine and glycyl−glycine. They include a clear broadening and a ∼0.3 eV shift of the C 1s → π*CO peak and introduction of a new pre-edge feature in the N 1s spectrum. These effects are due to 1s → π*amide transitions introduced with formation of the peptide bond. Similar changes occur in the spectra of the solids. The computational results support the interpretation of the experimental inner-shell spectra and provide insight into electron density distributions in the core excited states. Possible conformational dependence of the inner-shell excitation spectra was explored by computing the spectra of neutral glycine in its four most common conformations, and of glycyl−glycine in planar and two twisted conformations. A strong dependence of the computed C 1s, N 1s, and O 1s spectra of glycyl−glycine on the conformation about the amide linkage was confirmed by additional ab initio calculations of the conformational dependence of the spectra of formamide.