DL-Norleucine: redetermination of structure and observations with synchrotron radiation Laue diffraction on heating towards transformation

Simulation of diffuse scattering in DL-norleucine

The diffraction patterns of DL-norleucine (SR-2-aminohexanoic acid, DL-Nle) crystals may show obvious diffuse scattering, usually described as `streaking', between the Bragg peaks. This phenomenon is obviously related to the non-ideal behaviour of the crystal. The normal interpretation is disorder in the stacking of weakly interacting 2D layers, known also for a number of other racemates of amino acids with linear hydrophobic side chains, as well as 1:1 complexes between different L- and D-enantiomers (quasi-racemates). Presented here is the first attempt to extract the information hidden in the diffuse scattering for this group of compounds by applying Monte Carlo simulations to the site distributions of two polymorphs in a block of 48 × 48 × 48 unit cells (four sites in each unit cell, 442 368 in total). The results demonstrate that it is indeed possible to model the diffuse scattering and relate it to processes expected to take place during phase transitions, characterized by slipping of molecular bilayers (or parts of them) relative to their neighbours. The understanding of the (intermediate) mixed phases in terms of domain size and defect density is consequently brought to a new level.

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

The solid-state phase transitions and intermediate structures of S-2-amino-butanoic 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.

A phase transition from monoclinic C2 with Z' = 1 to triclinic P1 with Z' = 4 for the quasiracemate L-2-aminobutyric acid-D-methionine (1/1)

Racemates of hydrophobic amino acids with linear side chains are known to undergo a unique series of solid-state phase transitions that involve sliding of molecular bilayers upon heating or cooling. Recently, this behaviour was shown to extend also to quasiracemates of two different amino acids with opposite handedness [Görbitz & Karen (2015). J. Phys. Chem. B, 119, 4975-4984]. Previous investigations are here extended to an L-2-aminobutyric acid-D-methionine (1/1) co-crystal, C4H9NO2·C5H11NO2S. The significant difference in size between the -CH2CH3 and -CH2CH2SCH3 side chains leads to extensive disorder at room temperature, which is essentially resolved after a phase transition at 229 K to an unprecedented triclinic form where all four D-methionine molecules in the asymmetric unit have different side-chain conformations and all three side-chain rotamers are used for the four partner L-2-aminobutyric acid molecules.

Do solid-to-solid polymorphic transitions in DL-norleucine proceed through nucleation?

DL-Norleucine is a molecular crystal exhibiting two enantiotropic phase transitions. The high temperature α ↔ γ transition has been shown to proceed through nucleation and growth [Mnyukh et al., J. Phys. Chem. Solids, 1975, 36, 127]. We focus on the low temperature β ↔ α transition in a combined computational and experimental study. The temperature dependence of the structural and energetic properties of both polymorphic forms is nearly identical. Molecular dynamics simulations and nudged elastic band calculations of the transition process itself, suggest that the transition is governed by cooperative movements of bilayers over relatively large energy barriers.

Twin displacive phase transitions in amino acid quasiracemates

Three quasiracemates, L-norleucine:D-methionine, L-norvaline:D-norleucine, and L-norvaline:D-methionine, were crystallized to see how they differ from regular racemates in terms of crystal structure (studied by single-crystal X-ray diffraction) and of thermally induced phase transitions (studied by differential scanning calorimetry). Two types of transitions are detected between 100 and 450 K and structurally characterized: (1) displacive transitions of the molecular bilayers that form the crystal and (2) continuous or discontinuous disordering transitions in the amino acid side chains. Uniquely for the quasiracemates, the displacive transition proceeds in two close steps as only one surface of each molecular bilayer slides at first, upon forming an intermediate phase, while the other surface follows at a slightly higher temperature. Altogether, 18 new single-crystal structure-refinement data sets are reported for these three quasiracemates.

Size-dependent phase stability of a molecular nanocrystal: a proxy for investigating the early stages of crystallization

We make the link between the size-dependent phase stability of a nanocrystal and the phase-transition behavior of emerging crystallites during the earliest stages of crystallization, by using the former as a proxy for the latter. We outline an extension of the classical nucleation theory to describe crystal nucleation and subsequent transformations of competing polymorphic phases that characterize Ostwald's rule of stages. The theoretical framework reveals that the relative stability of the competing phases is a function of cluster size, which in turn varies with time, and therefore explains the complex transformation behavior observed for some systems. We investigated the stability of a nanocrystal of dl-norleucine by means of molecular simulation as a proxy for post-nucleation phase-transformation behavior in emerging crystallites. The simulations reveal that, for nanocrystals, the surface energy of the transition state of a transformation can dominate the barrier to phase change, thus causing metastable phases to be stabilized, not because they are thermodynamically stable, but rather due to kinetic hindering. Therefore, in the context of the earliest stages of crystal growth, not only does phase stability vary as a function of cluster size, and hence time, but thermodynamically feasible transformations are also prone to kinetic hindering.

Solid-solid phase transitions in DL-norvaline studied by differential scanning calorimetry and Raman spectroscopy

The structural modifications of the amino acid DL-Norvaline have been studied using differential scanning calorimetry (DSC) and Raman spectroscopy. DSC results showed that this amino acid undergoes two solid-solid phase transitions at -116.9 and -76.1 degrees C in the temperature range -130 to +300 degrees C. Raman spectroscopy was applied to complement DSC results. The combination of the two methodologies point out that the observed phase transitions correspond to an increment of disordering in the aliphatic side chain of amino acid, an augmentation of the rotational motion of the amino group and a decrease of the strength of the intramolecular hydrogen bonding of the initial dimers at low temperatures. The observed phase transitions of DL-norvaline are compared with those found in DL-norleucine.

Norleucine, a natural occurrence in a novel ergot alkaloid γ-ergokryptinine

A novel natural peptide ergot alkaloid gamma-ergokryptinine containing norleucine has been isolated from ergot sclerotia of the field-growing parasitic fungus Claviceps purpurea CCM 8059. Its structure was deduced from the NMR and mass spectral data. The final structural proof was provided by the crystal structure determination, which is the first X-ray structure of a natural Nle-containing secondary metabolite. The conformations of three ergopeptinines: gamma-ergokryptinine, ergoladinine, and alpha-ergokryptinine were compared.

Cyano-Bridged Structures Based on [MnII(N3O2-Macrocycle)]2+: A Synthetic, Structural, and Magnetic Study

Reactions between the complex [MnII(L)]2+, where L is a N3O2 macrocyclic ligand, and different cyanometalate precursors such as [M(CN)n]m- (M(III) = Cr, Fe; M(II) = Fe, Ni, Pd, Pt) lead to cyano-bridged molecular assemblies exhibiting a variety of structural topologies. The reaction between [MnII(L)]2+ and [FeII(CN)6]4- forms a trinuclear complex with formula [(MnII(L)(H2O))2(FeII(micro-CN)2(CN)4)] x 2MeOH x 10H2O (1) which crystallizes in the triclinic space group P1. The reaction between [MnII(L)]2+ and [M(II)(CN)4]2-, where M(II) = Ni (2), Pd (3), Pt (4), gives rise to three isostructural linear chain compounds with stoichiometry [(MnII(L))(M(II)(micro-CN)2(CN)2)]n and which crystallize in the monoclinic space group C2/c. The self-assembly between [MnII(L)]2+ with [M(III)(CN)6]3-, where M(III) = Cr (5), Fe (6, 7, 8), forms three types of compounds. Compounds 5 and 6 are isostructural (monoclinic, space group P2(1)/n), and the structures comprise anionic linear chains [(MnII(L))(M(III)(micro-CN)2(CN)4)]n(n-) with cationic trinuclear complexes [(MnII(L)(H2O))2(M(III)(micro-CN)2(CN)4)]+ as counterions. Using an excess of K3[FeIII(CN)6], an analogous compound to 6 but with K+ as counterion is obtained (7), which crystallizes in the triclinic space group P1. Compound 8 consists of 2-D layers with formula [(MnII(L))3(FeIII(micro-CN)4(CN)2)(FeIII(micro-CN)2(CN)4)]n x 2nMeOH; it crystallizes in the monoclinic space group P2(1)/n. The magnetic properties were investigated for all samples. In particular, compound 5, which shows antiferromagnetic exchange interactions between Mn(II) and Cr(III) ions through cyanide bridging ligands, has been studied in detail; the magnetic exchange parameter amounts to J = -7.5(7) cm(-1). Compound 8 shows a magnetically ordered phase below 6.4 K which is confirmed by Mössbauer spectroscopy; two hyperfine split spectra were observed below Tc from which IJI values of 2.1 and 1.6 cm(-1) could be deduced.