Low Temperature Physics: 45, 649 (2019); https://doi.org/10.1063/1.5103258
Valine infrared absorbance at cryogenic temperatures
C.A.P. da Costa and E.F. da Silveira
Physics Department, Pontifícia Universidade Católica do Rio de Janeiro Rua Marquês de São Vicente 225, 22453-900, Rio de Janeiro, RJ, Brazil
Received February 4, 2019, published online April 26, 2019
The amino acid L-valine is analyzed by Fourier Transform InfraRed Spectroscopy (FTIR) in the 40–300 K temperature range. A valine sample was deposited onto a ZnSe substrate, cooled down to 40 K and warmed up back to 300 K, annealed at 120 °C for 12 h, cooled down to 40 K and warmed up again to 300 K. FTIR spectra were acquired in transmission mode during this thermal excursion. It was observed that: i) no phase transition has occurred; ii) as temperature decreases, the band absorbances increase linearly and the band widths become narrower; iii) compared to 300 K values, the integrate absorbances (peak areas) at 40 K are 40–100% higher and bandwidths are about half; iv) the FTIR spectrum behaviors of the annealed and non-annealed samples are similar. The sample is considered to be constituted only by valine zwitterions; bandwidth decrease is explained partially by Doppler effect; absorbance enhancement at low temperatures is caused by stronger Van der Walls forces. These general conclusions are expected to be similar for other amino acids.
Key words: amino acid L-valine, FTIR spectrа, zwitterions.