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Novel analytical approaches for the study of mobility and relaxation phenomena in positional isomers of GABA

Novel analytical approaches for the study of mobility and relaxation phenomena in positional isomers of GABA

Owusu-Ware, Samuel K., Chowdhry, Babur, Leharne, Stephen A. and Antonijevic, Milan D. (2013) Novel analytical approaches for the study of mobility and relaxation phenomena in positional isomers of GABA. Physical Chemistry Chemical Physics, 15 (46). pp. 20046-20053. ISSN 1463-9076 (Print), 1463-9084 (Online) (doi:10.1039/C3CP52670D)

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Abstract

γ-Aminobutyric acid (GABA), and its positional isomers DL-α-aminobutyric acid (AABA) and DL-β-aminobutyric acid (BABA) have been analysed, in the solid state, using thermally stimulated depolarisation current (TSDC) spectroscopy. Analysis of the global depolarisation current shows that GABA exhibits greater mobility in comparison to AABA and BABA. Two previously undetected dipolar relaxation modes are observed for GABA at 77 ± 2°C and 114 ± 2°C, the latter having contributions from space charge polarisation. On the other hand, AABA and BABA undergo a single global relaxation mode, also previously unreported, at 109 ± 1 and 104 ± 1°C, respectively prior to high temperature thermal events observed via TGA and DSC studies. Analysis using TSDC thermal windowing show that the dipole relaxation observed for the aminobutyric acids requires activation energies of 195.7 ± 0.8 kJ/mol (BABA), 189.9 ± 3.2 kJ/mol (GABA) and 142.4 ± 1.4 kJ/mol (AABA). Compensation analysis revealed that AABA and BABA display compensation behaviour; GABA did not demonstrate such behaviour. The compensation point (Tc) and the relaxation time (τc) was found to be 214 ± 6°C and 0.051 ± 0.024 s, respectively for AABA and 153 ± 3°C and 0.025 ± 0.011 s for BABA. The compensation point is correlated to the starting point of thermal events i.e. sublimation, melt/decomposition, indicating a correlation between secondary relaxation processes and the main thermal transitions, found via TGA and DSC studies. This study demonstrates the ability to analyse and characterise previously undetected relaxation processes in aminobutyric acids and correlate/link them to chemical and physical stability i.e. 1st and 2nd order thermal transition processes at higher temperatures.

Item Type: Article
Uncontrolled Keywords: GABA, γ-Aminobutyric acid, DL-α-Aminobutyric acid, DL-β-Aminobutyric acid, Compensation analysis Differential scanning calorimetry, Thermal gravimetric analysis, Thermally stimulated depolarisation current spectroscopy
Subjects: Q Science > QD Chemistry
Faculty / Department / Research Group: Faculty of Engineering & Science > Department of Pharmaceutical, Chemical & Environmental Sciences
Last Modified: 17 Oct 2016 09:12
Selected for GREAT 2016: None
Selected for GREAT 2017: None
Selected for GREAT 2018: None
URI: http://gala.gre.ac.uk/id/eprint/10444

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