Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers

Unravelling the nature of intra-molecular hydrogen bonds in curcumin using in-situ low temperature spectroscopic studies

Curcumin, described as a wonder drug owing to various medicinal, viz. anticancer, antiviral, anti-inflammatory etc., properties, can also be seen as a model molecular system to study strong intra-molecular OH----O hydrogen bonds which govern its physico-chemical properties. The study of these hydrogen bonds is important to understand its binding characteristics. Here, we present systematic in-situ variable temperature studies of curcumin in the range 350-75 K using infrared spectroscopy to analyse the effects of external stresses on molecular structure and hydrogen bonding network. The results have been well supported by Raman spectroscopic studies. Our studies show striking difference in the nature of the two intra-molecular hydrogen bonds, generally considered equivalent, which form at the edges of the molecule. Also, the strongest intra-molecular hydrogen bond involving the enol group, present at the centre of the molecule, depicts a remarkable temperature induced strengthening upon cooling. The studies further indicate that the compound does not show any drastic structural transition in the measured temperature range. However, subtle spectral changes associated with reorientations of the hydrogen bonds are noticed across 210 K. These results will be useful to predict reaction pathways during chemical complexation of curcumin.

Orientational Adaptations Leading to Plausible Phase Transitions in l-Leucine at Low Temperatures: Revealed by Infrared Spectroscopy

Hydrogen bonding is essential for the stability of amino acids. A change in the geometry and conformation of hydrogen bonds in such molecular systems, for example, under varying thermodynamic conditions of temperature/pressure, may lead to subtle or drastic phase transitions. We demonstrate here the mechanism of temperature-induced phase transitions in the polycrystalline solid sample of l-leucine [(CH₃)₂-C(₄)H-C(₃)H₂-C(₂)H(C(₁)OO^-)(NH₃⁺)], an "essential" amino acid, using in situ Fourier transform infrared spectroscopy in the temperature range 300-4.3 K. Unambiguous spectral signatures of preferred microstructural changes have been reported, which are linked to phase transitions at ∼150 and ∼240 K. The transition at 150 K is found to be associated with a sudden change in reorientation dynamics of the torsional vibrations of the (C₃C₄) group. In contrast, the transition at 240 K is associated with the conformational distortions in the NH₃ group, which causes strengthening of the hydrogen bonds in the ac-plane forming two-dimensional sheets, well separated from each other in the b-direction. These findings pave the way toward settling the long-standing debate on the temperature-induced behavior of l-leucine as well as harnessing its physicochemical properties.

Comments on 'Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers'

The recent paper by Belo, Pereira, Freire, Argyriou, Eckert & Bordallo [(2018 ▸), IUCrJ, 5, 6-12] reports observations that may lead one to think of very strong and visible consequences of the parity-violation energy difference between enantiomers of a molecule, namely alanine. If proved, this claim would have an enormous impact for research in structural chemistry. However, alternative, more realistic, explanations of their experiments have not been ruled out by the authors. Moreover, the theoretical calculations carried out to support the hypothesis are unable to differentiate between enantiomers (molecules or crystals). Therefore, the conclusions drawn by Belo et al. (2018 ▸) are deemed inappropriate as the data presented do not contain sufficient information to reach such a conclusion.

Response to comment on 'Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers'

A response is given to comments by Bürgi & Macchi [IUCrJ (2018), 5, 654-657] about Belo et al. [IUCrJ (2018), 5, 6-12.].