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Redetermined structure of ?-dl-me-thio-nine at 105?K: an example of the importance of freely refining the positions of the amino-group H atoms.

ABSTRACT: Diffraction data were taken from the contribution named '?-dl-Me-thio-nine at 105 K' by Alagar et al. [Acta Cryst. (2005 ?). E61, o1165-o1167]. Refinement of the coordinates of the three amino H atoms, previously constrained to an idealized geometry, shows that the amino group is in fact rotated 13.5° from the perfectly staggered orientation. This apparently modest change has a profound impact on the calculated hydrogen-bond geometries.

SUBMITTER: Gorbitz CH

PROVIDER: S-EPMC4257275 | biostudies-literature | 2014 Nov

REPOSITORIES: biostudies-literature

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Redetermined structure of β-dl-me-thio-nine at 105 K: an example of the importance of freely refining the positions of the amino-group H atoms.

Görbitz Carl Henrik CH

Acta crystallographica. Section E, Structure reports online 20141015 Pt 11

Diffraction data were taken from the contribution named 'β-dl-Me-thio-nine at 105 K' by Alagar et al. [Acta Cryst. (2005 ▶). E61, o1165-o1167]. Refinement of the coordinates of the three amino H atoms, previously constrained to an idealized geometry, shows that the amino group is in fact rotated 13.5° from the perfectly staggered orientation. This apparently modest change has a profound impact on the calculated hydrogen-bond geometries. ...[more]

PMID: 25484740

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Project description:Understanding of solid-to-solid phase transition mechanisms in polymorphic systems is of critical importance for rigorous control over polymorph purity in the pharmaceutical industry to achieve the desired bioavailability and efficacy of drugs. Ubiquitous defects in crystals may play an important role in the pathways of phase transitions. However, such effects remain poorly understood. Here, the effects of crystal defects on the solid-to-solid phase transformations between dl-me-thio-nine polymorphs α and β are investigated by means of experimental and computational approaches. Thermal analyses of polycrystalline powders show two endothermic peaks in the α-to-β phase transition (and two exothermic peaks for the reverse transition), in contrast with one thermal event observed for single crystals. Variable-temperature 1D and 2D Raman spectra, as well as powder X-ray diffraction patterns, reveal the appearance of two peaks that can attributed to a two-step phase transition, and the extent of the second-step phase transition increases with milling time (or defect density). Quantification of transition kinetics unveils a remarkably higher energy barrier in the second-step phase transition than in the first, proceeding by the cooperative molecular motion pathway. The good linear fitting on the kinetic data by the Jeziorny model suggests that the second-step transition follows the nucleation and growth mechanism. Molecular dynamics simulations were also conducted to understand the role of crystal defects in the solid-state phase transition by tracking the atomic distribution and hydrogen bond lifetime during the transition. It was found that the increasing defect density hinders the propagation of cooperative molecular motion, leading to a combined transition mechanism involving both cooperative motion and nucleation and growth. This study highlights the significant impact of crystal defects on solid-state phase transitions, and the two-step transition mechanism postulated may be universal given the ubiquitous presence of defects in crystalline materials.

Dataset Information

Redetermined structure of ?-dl-me-thio-nine at 105?K: an example of the importance of freely refining the positions of the amino-group H atoms.

Publications

Redetermined structure of β-dl-me-thio-nine at 105 K: an example of the importance of freely refining the positions of the amino-group H atoms.

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