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

Abstract

The valence electronic structures of the amino acid glycine in aqueous solution were investigated in detail through X-ray emission spectroscopy at O1s excitation under selective excitation condition of the C=O site in the carboxyl group. The X-ray emission spectra of glycine were similar to that of acetic acid (CH3COOH), 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 (acidic solution) around pKa value (pH~2.3), the spectra of glycine exhibited further changes in the higher-pH region (basic solution) near the pKb value of glycine (pH ~9.5). Ratio analysis between three forms of glycine, cationic (NH3+CH2COOH), zwitterionic (NH3+CH2COO-) and anionic (NH2CH2COO-) were performed, and the spectra obtained between pH 0.22 and 12.48 could be reproduced by superpositions of the three states. These results show effects of amino group protonation on the carboxyl group electronic state. X-ray emission spectroscopy might be a tool to investigate intramolecular interactions between functional groups in a molecule.

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Publication details

The article was received on 12 Dec 2017, accepted on 11 Jul 2018 and first published on 11 Jul 2018


Article type: Paper
DOI: 10.1039/C7CP08305J
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Effect of amino group protonation on carboxyl group in aqueous glycine observed by O 1s X-ray emission spectroscopy

    Y. Horikawa, T. Tokushima, O. Takahashi, Y. Harada, A. Hiraya and S. Shin, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C7CP08305J

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