![8 hydrophobic amino acids 8 hydrophobic amino acids](https://dfzljdn9uc3pi.cloudfront.net/2019/7055/1/fig-1-full.png)
An accurate refinement now reveals extensive disorder for both molecules in the asymmetric unit, while two previously unknown phases occur above room temperature. L-Methionine has previously been described as being fully ordered at room temperature. Ordering of L-norleucine upon cooling even proceeds via an incommensurately modulated structure.
![8 hydrophobic amino acids 8 hydrophobic amino acids](https://d3i71xaburhd42.cloudfront.net/b601fb50ae67a0aebe895f4c17db93e14d2a396e/3-Table1-1.png)
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. Unlike other enantiomeric amino acids investigated until now, this group featuring linear side chains displays up to five distinct phases.
![8 hydrophobic amino acids 8 hydrophobic amino acids](https://slidetodoc.com/presentation_image_h2/70a4b6b2dfaed5897aa7a9b502b8390a/image-8.jpg)
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.