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An exceptional series of phase transitions in hydrophobic amino acids with linear side chains

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ABSTRACT

The solid-state phase transitions and intermediate structures of S-2-amino­butanoic acid (l-2-aminobutyric acid), S-2-aminopentanoic acid (l-norvaline), S-2-aminohexanoic acid (l-norleucine) and l-methionine between 100 and 470 K, identified by differential scanning calorimetry, have been characterized in a comprehensive single-crystal X-ray diffraction investigation. Unlike other enantiomeric amino acids investigated until now, this group featuring linear side chains displays up to five distinct phases. The multiple transitions between them involve a number of different processes: alteration of the hydrogen-bond pattern, to our knowledge the first example of this observed for an amino acid, sliding of molecular bilayers, seen previously only for racemates and quasiracemates, concerted side-chain rearrangements and abrupt as well as gradual modifications of the side-chain disorder. Ordering of l-norleucine upon cooling even proceeds via an incommensurately modulated structure. l-Methionine has previously been described as being fully ordered at room temperature. An accurate refinement now reveals extensive disorder for both molecules in the asymmetric unit, while two previously unknown phases occur above room temperature.

No MeSH data available.


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Molecular structure of l-Nva at (a) 100, (b) 220 and (c) 293 K with atomic coloring scheme as in Fig. 2 ▸. Side-chain H atoms have been omitted for disordered structures, and only the most populated conformation is shown in ellipsoid representation; the side chains of minor orientations being shown in ball-and-stick style with spheres of arbitrary size (the polar heads are omitted due to extensive overlap).
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fig5: Molecular structure of l-Nva at (a) 100, (b) 220 and (c) 293 K with atomic coloring scheme as in Fig. 2 ▸. Side-chain H atoms have been omitted for disordered structures, and only the most populated conformation is shown in ellipsoid representation; the side chains of minor orientations being shown in ball-and-stick style with spheres of arbitrary size (the polar heads are omitted due to extensive overlap).

Mentions: The two molecules in the asymmetric unit of l-Nva at 100 K (Fig. 5 ▸a) participate in L2-type hydrogen-bonded sheets (Fig. 4 ▸), not Lx sheets as l-Abu, l-Nle and most other amino acids that do not branch at Cβ or Cγ (Görbitz et al., 2009 ▸). At low temperatures, l-Nva is thus isostructural to its analog with an iso­propyl side chain, l-Val (Dalhus & Görbitz, 1996a ▸). There is no side-chain disorder below transition 1 (Fig. S5).


An exceptional series of phase transitions in hydrophobic amino acids with linear side chains
Molecular structure of l-Nva at (a) 100, (b) 220 and (c) 293 K with atomic coloring scheme as in Fig. 2 ▸. Side-chain H atoms have been omitted for disordered structures, and only the most populated conformation is shown in ellipsoid representation; the side chains of minor orientations being shown in ball-and-stick style with spheres of arbitrary size (the polar heads are omitted due to extensive overlap).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5391856&req=5

fig5: Molecular structure of l-Nva at (a) 100, (b) 220 and (c) 293 K with atomic coloring scheme as in Fig. 2 ▸. Side-chain H atoms have been omitted for disordered structures, and only the most populated conformation is shown in ellipsoid representation; the side chains of minor orientations being shown in ball-and-stick style with spheres of arbitrary size (the polar heads are omitted due to extensive overlap).
Mentions: The two molecules in the asymmetric unit of l-Nva at 100 K (Fig. 5 ▸a) participate in L2-type hydrogen-bonded sheets (Fig. 4 ▸), not Lx sheets as l-Abu, l-Nle and most other amino acids that do not branch at Cβ or Cγ (Görbitz et al., 2009 ▸). At low temperatures, l-Nva is thus isostructural to its analog with an iso­propyl side chain, l-Val (Dalhus & Görbitz, 1996a ▸). There is no side-chain disorder below transition 1 (Fig. S5).

View Article: PubMed Central - HTML - PubMed

ABSTRACT

The solid-state phase transitions and intermediate structures of S-2-amino­butanoic acid (l-2-aminobutyric acid), S-2-aminopentanoic acid (l-norvaline), S-2-aminohexanoic acid (l-norleucine) and l-methionine between 100 and 470 K, identified by differential scanning calorimetry, have been characterized in a comprehensive single-crystal X-ray diffraction investigation. Unlike other enantiomeric amino acids investigated until now, this group featuring linear side chains displays up to five distinct phases. The multiple transitions between them involve a number of different processes: alteration of the hydrogen-bond pattern, to our knowledge the first example of this observed for an amino acid, sliding of molecular bilayers, seen previously only for racemates and quasiracemates, concerted side-chain rearrangements and abrupt as well as gradual modifications of the side-chain disorder. Ordering of l-norleucine upon cooling even proceeds via an incommensurately modulated structure. l-Methionine has previously been described as being fully ordered at room temperature. An accurate refinement now reveals extensive disorder for both molecules in the asymmetric unit, while two previously unknown phases occur above room temperature.

No MeSH data available.


Related in: MedlinePlus