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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.


Temperature-dependent positions of H atoms interacting with the syn lone pairs of the l-Nva carboxylate group. From their starting positions at 100 K (blue), both H1B and H1A move towards more central positions as the temperature increases through 220 K (green) to 270 K (orange). After transition 2 in Fig. 1 ▸, the distinction between molecule A and B (Fig. 5 ▸) has disappeared, giving a single type of three-centered interaction (dashed lines) at 293 K (red).
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fig7: Temperature-dependent positions of H atoms interacting with the syn lone pairs of the l-Nva carboxylate group. From their starting positions at 100 K (blue), both H1B and H1A move towards more central positions as the temperature increases through 220 K (green) to 270 K (orange). After transition 2 in Fig. 1 ▸, the distinction between molecule A and B (Fig. 5 ▸) has disappeared, giving a single type of three-centered interaction (dashed lines) at 293 K (red).

Mentions: While all other amino acids investigated so far retain their basic hydrogen-bonding pattern during a phase transition, transition 2 of l-Nva at 273 K (Fig. 1 ▸) concerns a shift from L2–L2 to Lx–Lx with Z′ being reduced from 2 (space group I2, Z = 8) to 1 (C2, Z = 4, Fig. 5 ▸c). Major shifts in molecular positions upon the change of crystal symmetry are not required, but minor reorientations can be seen by comparing Figs. 6 ▸(b) and (c). This unprecedented transition appears sharp in DSC, but a plot of the positions of H atoms accepted by the carboxylate syn lone pairs (Fig. 7 ▸) makes it evident that H1A and H1B atoms gradually migrate towards more centered positions between 100 and 270 K (i.e. through transition 1). An abrupt final shift results in a single three-centered interaction in an Lx sheet at 293 K (Fig. 4 ▸).


An exceptional series of phase transitions in hydrophobic amino acids with linear side chains
Temperature-dependent positions of H atoms interacting with the syn lone pairs of the l-Nva carboxylate group. From their starting positions at 100 K (blue), both H1B and H1A move towards more central positions as the temperature increases through 220 K (green) to 270 K (orange). After transition 2 in Fig. 1 ▸, the distinction between molecule A and B (Fig. 5 ▸) has disappeared, giving a single type of three-centered interaction (dashed lines) at 293 K (red).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig7: Temperature-dependent positions of H atoms interacting with the syn lone pairs of the l-Nva carboxylate group. From their starting positions at 100 K (blue), both H1B and H1A move towards more central positions as the temperature increases through 220 K (green) to 270 K (orange). After transition 2 in Fig. 1 ▸, the distinction between molecule A and B (Fig. 5 ▸) has disappeared, giving a single type of three-centered interaction (dashed lines) at 293 K (red).
Mentions: While all other amino acids investigated so far retain their basic hydrogen-bonding pattern during a phase transition, transition 2 of l-Nva at 273 K (Fig. 1 ▸) concerns a shift from L2–L2 to Lx–Lx with Z′ being reduced from 2 (space group I2, Z = 8) to 1 (C2, Z = 4, Fig. 5 ▸c). Major shifts in molecular positions upon the change of crystal symmetry are not required, but minor reorientations can be seen by comparing Figs. 6 ▸(b) and (c). This unprecedented transition appears sharp in DSC, but a plot of the positions of H atoms accepted by the carboxylate syn lone pairs (Fig. 7 ▸) makes it evident that H1A and H1B atoms gradually migrate towards more centered positions between 100 and 270 K (i.e. through transition 1). An abrupt final shift results in a single three-centered interaction in an Lx sheet at 293 K (Fig. 4 ▸).

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.