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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.


Molecular structures of l-Abu at (a) 110 (Görbitz, 2010 ▸), (b) 215, (c) 330 and (d) 365 K. Individual molecules in the asymmetric unit are labelled in italics (A, B, …) with illustrative atomic numbering given for one molecule. Thermal displacement ellipsoids are shown at the 50% probability level. Atom color depths reflect the occupancy of each conformation as listed in Table 5 ▸. The asymmetric unit of the P21 form of l-Abu at 110 K looks exactly as the I2 asymmetric unit shown in (a).
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fig2: Molecular structures of l-Abu at (a) 110 (Görbitz, 2010 ▸), (b) 215, (c) 330 and (d) 365 K. Individual molecules in the asymmetric unit are labelled in italics (A, B, …) with illustrative atomic numbering given for one molecule. Thermal displacement ellipsoids are shown at the 50% probability level. Atom color depths reflect the occupancy of each conformation as listed in Table 5 ▸. The asymmetric unit of the P21 form of l-Abu at 110 K looks exactly as the I2 asymmetric unit shown in (a).

Mentions: A P21 α form and a I2 β form, both with Z′ = 4 (Fig. 2 ▸a), were previously identified for l-Abu at 110 K (Görbitz, 2010 ▸). We have now heated and cooled a selected crystal a number of times across the transition temperature at ∼ 207 K. Unit-cell determinations at 190 K always reproduced the β form, while the α form prevailed at 215 K. Accordingly, transition 1 in Fig. 1 ▸ represents a full and reversible conversion between the two l-Abu polymorphs. From this we conclude that the α form is unstable below transition 1, and that the P21 crystal previously investigated at 110 K (Görbitz, 2010 ▸), the only one with this symmetry out of about ten tested, represented a freak occurrence of a specimen that failed to undergo the normal phase transition upon cooling.


An exceptional series of phase transitions in hydrophobic amino acids with linear side chains
Molecular structures of l-Abu at (a) 110 (Görbitz, 2010 ▸), (b) 215, (c) 330 and (d) 365 K. Individual molecules in the asymmetric unit are labelled in italics (A, B, …) with illustrative atomic numbering given for one molecule. Thermal displacement ellipsoids are shown at the 50% probability level. Atom color depths reflect the occupancy of each conformation as listed in Table 5 ▸. The asymmetric unit of the P21 form of l-Abu at 110 K looks exactly as the I2 asymmetric unit shown in (a).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig2: Molecular structures of l-Abu at (a) 110 (Görbitz, 2010 ▸), (b) 215, (c) 330 and (d) 365 K. Individual molecules in the asymmetric unit are labelled in italics (A, B, …) with illustrative atomic numbering given for one molecule. Thermal displacement ellipsoids are shown at the 50% probability level. Atom color depths reflect the occupancy of each conformation as listed in Table 5 ▸. The asymmetric unit of the P21 form of l-Abu at 110 K looks exactly as the I2 asymmetric unit shown in (a).
Mentions: A P21 α form and a I2 β form, both with Z′ = 4 (Fig. 2 ▸a), were previously identified for l-Abu at 110 K (Görbitz, 2010 ▸). We have now heated and cooled a selected crystal a number of times across the transition temperature at ∼ 207 K. Unit-cell determinations at 190 K always reproduced the β form, while the α form prevailed at 215 K. Accordingly, transition 1 in Fig. 1 ▸ represents a full and reversible conversion between the two l-Abu polymorphs. From this we conclude that the α form is unstable below transition 1, and that the P21 crystal previously investigated at 110 K (Görbitz, 2010 ▸), the only one with this symmetry out of about ten tested, represented a freak occurrence of a specimen that failed to undergo the normal phase transition upon cooling.

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.