Inner shell excitation of glycine, glycyl-glycine, alanine and phenylalanine

Oscillator strengths for C 1s excitation spectra of gaseous glycine (Gly), alanine (Ala), phenylalanine (Phe) and glycyl-glycine (Gly-Gly), plus the N 1s and O 1s spectra of Gly and Gly-Gly, have been derived from inner shell electron energy-loss spectroscopy recorded under scattering conditions where electric dipole transitions dominate (2.5keV residual energy, θ:∼2°). X-ray absorption spectra of solid glycine and glycyl-glycine were recorded in a scanning transmission X-ray microscope. Complications in solid state measurements associated with sample crystallinity and the benefits of spectroscopy in a microscope to resolve them are illustrated. Inner shell excitation spectral features characteristic of the peptide bond, readily identified by comparison of the spectra of glycine and glycyl-glycine, include: a ∼0.3eV shift of the C 1s→πCO∗ peak, and introduction of a new pre-edge feature in the N 1s spectrum. These effects are due to 1s→πamide∗ transitions introduced with formation of the peptide bond. The concept of spectral additivity (building block model) is tested by a comparison of the C 1s spectrum of phenylalanine with those of benzene and alanine.