Temperature dependence of molecular structure of dissolved glycine as revealed by ATR-IR spectroscopy

Attenuated total reflectance infrared (ATR-IR) spectroscopy was applied to investigate the structural change in dissolved glycine (H3+N–CH2–COO−) with rising temperature from 27 to 150 °C. The spectra were recorded using a heatable ATR-IR system, which was developed by Masuda et al. [1]. This apparatus allows us to obtain the IR spectrum of aqueous solution at temperatures of up to 200 °C and under pressures of up to 30 MPa. Spectral analysis revealed the following changes in the covalent bond strength of glycine with rising temperature: (1) the N–H bonds of NH3+ group became stronger; (2) the structure of COO− group became asymmetric owing to the strengthening of the electrostatic interaction between the NH3+ group and COO− group; (3) the strengths of C–C bond and C–N bond became weaker. These changes can be mainly attributed to the weakening of the hydrogen bond interaction between glycine and water molecules. The relationship between the structural change and the change in decomposition behavior of glycine with rising temperature was then evaluated. It was interpreted that the decarboxylation and the deamination were caused by the weakening of C–C bond and C–N bond as well as the strengthening of NH3+/COO− interaction with rising temperature.