X-ray Emission Spectroscopy of Proteinogenic Amino Acids at All Relevant Absorption Edges

Nonresonant
N K, O K, C K, and S L2,3 X-ray emission
spectra of the 20 most common proteinogenic amino acids in their solid
zwitterionic form are reported. They represent a comprehensive database
that can serve as a reliable basis for the X-ray absorption spectroscopy
(XES) studies of peptides and proteins. At the most important N and
O K edges, clear similarities and differences between the spectra
of certain amino acids are observed and associated with the specific
chemical structure of these molecules and their functional groups.
Analysis of these spectra allows the generation of spectral fingerprints
of the protonated amino group, the deprotonated carboxylic group,
and, using a building block approach, the specific nitrogen- and oxygen-containing
functional groups in the side chains of the amino acids. Some of these
fingerprints are compared to the spectra of reference compounds with
the respective functional groups; they exhibit reasonable similarity,
underlining the validity of the spectral fingerprint approach. The
C K and S L2,3 XES spectra are found to be specific for
each amino acid, in accordance with the different local environments
of the involved C and S atoms, respectively.