Vibrational spectroscopy and phonon‐related properties of monoclinic GABA, a non‐proteinogenic inhibitory neurotransmitter amino acid

• Published in: Journal of Raman spectroscopy 2021-07, Vol.52 (7), p.1294-1307
• Main Authors: Silva, José Barbosa, Costa, Gabriel, Silva, Mauricélio Bezerra, Costa, Stefane Nunes, Santos, Regina Cláudia Rodrigues, Sales, Francisco Adilson Matos, Valentini, Antoninho, Caetano, Ewerton Wagner Santos, Freire, Valder Nogueira
• Format: Article
• Language: English
• Subjects: Amino acids; Crystals; Debye temperature; Density functional theory; Enthalpy; Entropy; gamma‐aminobutyric acid (GABA) crystal; Infrared analysis; Infrared spectra; Lattice vibration; Mathematical analysis; Monoclinic lattice; Neurotransmitters; normal mode assignment; Phonons; planar vibrational signature; Raman spectra; Raman spectroscopy; solid‐state DFT; Spectrum analysis; Vertebrates; γ-Aminobutyric acid
• ISSN: 0377-0486; 1097-4555
• DOI: 10.1002/jrs.6134

The γ‐aminobutyric acid (GABA) is the most important inhibitory neurotransmitter in vertebrate central nervous systems. Taking full account of the measured X‐ray diffractogram of its stable monoclinic polymorph, we have performed density functional theory (DFT) calculations to analyze/understand its infrared and Raman spectra considering its monoclinic polymorph, an isolated stabilized (120) plane, and a single GABA molecule. Besides, phonon dispersion/density of states, heat capacity, Debye temperature, and entropy/enthalpy/free energies were also obtained for the GABA monoclinic crystal. Vibrational signatures due to the GABA (120) planes are pointed out for the first time for several wavenumbers. The results obtained for the GABA monoclinic polymorph reinforce the need of dispersion‐corrected solid‐state calculations to describe the vibrational properties of molecular crystals instead of considering a single isolated molecule picture, even for wavenumbers larger than those usually associated with lattice modes. Measurements of the infrared and Raman spectra of the monoclinic form of the GABA crystal were analyzed through DFT calculations, obtaining its theoretical IR and Raman spectra. DFT calculations were also performed on a 2D GABA crystal and a single isolated neutral GABA molecule. The calculated spectra for the solid state shows the best agreement with the measured data, especially for wavenumbers smaller than 1300 cm−1. Some rich vibrational planar features were found as well.