Effect of amino group protonation on the carboxyl group in aqueous glycine observed by O 1s X-ray emission spectroscopy

• Published in: Physical chemistry chemical physics : PCCP 2018, Vol.2 (36), p.23214-23221
• Main Authors: Horikawa, Y, Tokushima, T, Takahashi, O, Harada, Y, Hiraya, A, Shin, S
• Format: Article
• Language: English
• Subjects: Acetic acid; Carboxyl group; Electron states; Emission analysis; Emission spectra; Emission spectroscopy; Excitation; Functional groups; Glycine; Molecular orbitals; Protonation; Spectrum analysis; X ray spectra
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c7cp08305j

The valence electronic structures of the amino acid glycine in aqueous solution were investigated in detail through X-ray emission spectroscopy at O 1s excitation under selective excitation conditions of the C&z.dbd;O site in the carboxyl group. The X-ray emission spectra of glycine were similar to that of acetic acid (CH 3 COOH), suggesting a resemblance between the molecular orbitals associated with the carboxyl groups in the two molecules. The changes of O 1s X-ray emission spectra as a function of pH were investigated in detail. In addition to spectral changes due to protonation/deprotonation of the carboxyl group for lower pH-values around the p K a value (∼2.3), the spectra of glycine exhibited further changes in the higher-pH region near the p K b value of glycine (dissociation constant of amino group ∼9.5). These results show the effects of amino group protonation on the electronic state around the carboxyl group. X-ray emission spectroscopy might be a tool to investigate intramolecular interactions between functional groups in a molecule. The valence electronic structures of the amino acid glycine in aqueous solution were investigated in detail through X-ray emission spectroscopy at O 1s excitation under selective excitation conditions of the C&z.dbd;O site in the carboxyl group.