Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Structure factor file (CIF format) https://doi.org/10.1107/S2053229614019809/yf3069ga295ksup5.hkl Contains datablock ga295k

The starting model for the structure refinement at 12 K was based on the atomic coordinates for the heavy atoms taken from an X-ray structure determination at the same temperature (Capelli et al., 2014). The refined 12 K neutron structural model was then used as a starting model for the higher temperatures. The refinement of all structures was done by full-matrix least-squares on F2 using SHELXL97 (Sheldrick, 2008), using the coherent scattering amplitudes tabulated by Rauch & Waschkowski (2003). All H atoms were located in a difference Fourier map and all atoms were refined anisotropically.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Computer programs: ILL-MAD software, ILL-RAFD9 software, RACER (Wilkinson et al., 1988), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), Mercury (Macrae et al., 2008).

Note that the absolute structure Flack parameter is meaningless in this structure determination because in a neutron diffraction experiment there is no anomalous effect.

Chemical Markup Language (CML) file https://doi.org/10.1107/S2053229614019809/yf3069ga150ksup8.cml Supplementary material

A crystal of about 3 × 3 × 1.5 mm3 was sealed in a 2 K four-circle cryo-refrigerator and cooled slowly (at a rate of 2 K min-1) to 12 K while monitoring a very strong reflection (600) in order to monitor crystal quality. No significant changes in the crystal mosaic spread nor splitting of peaks was observed during cooling. The space group P212121, already determined by Wang & Paul (1979), was confirmed at low temperature.

Crystallographic Information File (CIF) https://doi.org/10.1107/S2053229614019809/yf3069sup1.cif Contains datablocks ga12k, ga50k, ga150k, ga295k, publication_text

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Chemical Markup Language (CML) file https://doi.org/10.1107/S2053229614019809/yf3069ga50ksup7.cml Supplementary material

Note that the absolute structure Flack parameter is meaningless in this structure determination because in a neutron diffraction experiment there is no anomalous effect.

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Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

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Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Chemical Markup Language (CML) file https://doi.org/10.1107/S2053229614019809/yf3069ga295ksup9.cml Supplementary material

Crystal data, data collection and structure refinement details are summarized in Table 1. Neutron diffraction measurements were conducted at the Institut Laue–Langevin, Grenoble, on the high-resolution four-circle diffractometer D9 equipped with a small-size position-sensitive 3He-gas detector (Lehmann et al., 1989) that allows an optimal discrimination of the Bragg peak from the background. A wavelength of 0.8313 (2) Å from a Ge(220) monochromator was chosen for the parametric data collection at four temperatures, i.e. 12, 50, 150 and 295 K.

Structure factor file (CIF format) https://doi.org/10.1107/S2053229614019809/yf3069ga150ksup4.hkl Contains datablock ga150k

Structure factor file (CIF format) https://doi.org/10.1107/S2053229614019809/yf3069ga50ksup3.hkl Contains datablock ga50k

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Conformations of simple oligopeptides have been extensively studied to aid in the understanding of the secondary structure of proteins. The dipeptide glycyl-L-alanine, whose X-ray structure was first reported in 1979 (Wang & Paul, 1979), has been used as model system for studying several phenomena, viz. photo-induced electron transfer (Hill et al., 1996), correlation of thermal properties and solid-state behaviour (Barone & Puliti, 1999), solvent effects on conformation and vibrational spectra (Nandini & Sathyanarayana, 2003). More recently, the flexibility of the chain of glycylalanine has been used to build a MOF-like (MOF is metal–organic framework) crystalline material with adaptable pore sizes (Rabone et al., 2010). In the present work, accurate positional and anisotropic displacement parameters for the H atoms have been obtained from neutron diffraction data collected at 12, 50, 150 and 295 K. The values of these parameters serve as a benchmark for assessing the positional and anisotropic displacement parameters of glycyl-L-alanine obtained from a so-called Hirshfeld refinement of X-ray diffraction data as described elsewhere (Capelli et al., 2014). Also of inter­est are the flexibility of the glycyl-L-alanine molecule in the solid and the hydrogen-bonding inter­actions as a function of temperature.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

For all compounds, data collection: ILL-MAD software; cell refinement: ILL-RAFD9 software; data reduction: RACER (Wilkinson et al., 1988); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008).

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Note that the absolute structure Flack parameter is meaningless in this structure determination because in a neutron diffraction experiment there is no anomalous effect.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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Reflections were measured in equatorial geometry with ω-X-θ scans. The scan width was increased with increasing θ angle of the reflection corresponding to the resolution function of the D9 instrument. Counting times were 4–7 s per step. Reflections at very low θ angle were measured with simple ω-scans. Reflections of the type ±h, +k, ±l were measured to a maximum diffracting angle 2θmax = 80° at 12 K and 2θmax = 76° at other temperatures.

Analogous considerations on the orientation of ADPs can be applied to the atoms in both the methyl and the ammonium groups. In both cases, the largest elements of the high-frequency tensor point in the direction of librational motions that increase in amplitude with increasing temperature. In the case of the –NH3+ group, the motion show a reduced amplitude as compared to the –CH3 group; this is most likely due to the hydrogen-bonding inter­actions that reduce the movements of the H atoms.

Note that the absolute structure Flack parameter is meaningless in this structure determination because in a neutron diffraction experiment there is no anomalous effect.

The glycyl-L-alanine molecule (Fig. 1) exists in a zwitterionic form, with the ammonium group bearing three H atoms in a tetra­hedral configuration at an average distance of 1.044 (2) Å from the N atom (average over all temperatures); the two carboxyl­ate O atoms are almost equidistant from the C atom at the three lowest temperatures [on average 1.251 (2) Å for C1—O1 and 1.259 (2) Å for C1—O2]. These C1—O bond lengths become virtually indistinguishable at 295 K [1.248 (5) and 1.249 (4) Å]. The main chain of the molecule, from H2N2—N2 to C3—H3C, shows an overall trans–planar conformation and does not change over the range of temperatures analysed. The largest deviation from planarity is associated with the C4—N1—C2—C3 torsion angle [161.6 (2)° at 12 K]. The carboxyl­ate C atom has a gauche conformation with respect to the main chain, with a tC4—N1—C2—C1 orsion angle of -77.1 (2)°. The conformation of the main chain allows the three groups involved in hydrogen bonding, i.e. NH, NH3+ and COO-, to form a complex three-dimensional network with eight neighbouring molecules in the first coordination sphere (Fig. 2). Both carboxyl­ate O atoms are acceptors in two hydrogen bonds each: atom O2 inter­acts with two H atoms of different ammonium groups, while atom O1 has inter­actions with an ammonium H atom and the H atom on the peptidic N atom. The geometries of the shortest hydrogen bonds are reported in Table 2 and show no significant variations across the four temperatures.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Note that the same molecular chain adopts different conformations in order to optimize the inter­actions with its environment: the conformation adopted in the structure of the dipeptide alone maximizes the number of hydrogen-bonding inter­actions in three dimensions, while the conformation in the metal–dipeptide framework permits tetra­hedral coordination of the Zn counter-ion and formation of the framework structure.

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For related literature, see: Bürgi & Capelli (2000); Bürgi et al. (2004); Barone & Puliti (1999); Capelli et al. (2000, 2014); Coppens et al. (1965); Hill et al. (1996); Howard et al. (1987); Lehmann et al. (1989); Nandini & Sathyanarayana (2003); Rabone et al. (2010); Rauch & Waschkowski (2003); Schomaker & Trueblood (1968); Sheldrick (2008); Wang & Paul (1979); Wilkinson et al. (1988).

The title compound was purchased from Sigma–Aldrich as a powder and was recrystallized by slow evaporation from an aqueous solution.

Bragg intensities were integrated in 3D following the method of Wilkinson et al. (1988) as implemented in the ILL program RACER and corrected for the Lorentz effect. Absorption corrections were applied based on Gaussian integration (Coppens et al., 1965) using as boundary planes the indexed faces of the crystal and the calculated attenuation coefficient, taking also into account the wavelength dependence of the incoherent scattering of H atoms (Howard et al., 1987).

Structure factor file (CIF format) https://doi.org/10.1107/S2053229614019809/yf3069ga12ksup2.hkl Contains datablock ga12k

Chemical Markup Language (CML) file https://doi.org/10.1107/S2053229614019809/yf3069ga12ksup6.cml Supplementary material

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A detailed discussion of the evolution of the anisotropic displacement parameters with temperature is beyond the scope of this paper but few considerations can be made. Preliminary calculations performed with the program NKA (Bürgi et al., 2004) show that using a classic TLS rigid-body description at a single temperature (Schomaker & Trueblood, 1968), the fitting between calculated and observed ADPs deteriorates with lowering temperature (agreement factors of 30% at 295 K, 34% at 150 K, 45% at 50 K and 48% at 12 K). This result suggest that inter­nal motions in glycyl-L-alanine becomes increasingly more important compared to rigid-body translations and librations. Using the six degrees of freedom of a rigid body in a molecular Einstein model that describe the ADPs at all temperatures with a unique set of frequencies/eigenvectors and additional additive tensors accounting for the inter­nal high-frequency motions (Bürgi & Capelli, 2000), the fitting between observed and modelled ADPs becomes more reasonable, with an agreement factor of 20% but the tensors accounting for the high-frequency motions turn out to be much larger than expected for some atoms in specific directions. For example, the ellipsoids of the O atoms of the carboxyl­ate group (Fig. 1) look elongated in the direction orthogonal to the O1—C1—O2 plane, suggesting a librational motion of this group around the C1—C2 direction. The diagonal components of the tensor corresponding to atoms O1 and O2 are: ε11 = 0.0025 (9) Å2, ε22 = 0.0015 (9) Å2, ε33 = 0.0070 (9) Å2, where ε11 and ε22 represent the in-plane directions and ε33 the out-of-plane one. The corresponding calculated value from spectroscopic data and ab initio calculations for the two in-plane motions for the O atom in the carbonyl group of urea is 0.0008 Å2 (see Table 7 in Capelli et al., 2000), much smaller than these diffraction-derived values. Although the comparison is done between a carboxyl and a carbonyl group, the large discrepancy between calculated and observed values for the high-frequency tensor of the O atoms strongly indicates that a more flexible model of motion has to be used to describe the motion of the carboxyl group, especially in the out-of-plane direction.

In the reported structure of the metal–dipeptide framework [Zn(Gly-Ala)2].(solvent) (Rabone et al., 2010), the dipeptide molecule also adopts a trans–planar conformation, but with the two extremes being the ammonium N atom on one side and the carboxyl­ate C atom on the other. The largest deviation from planarity is associated with the C4—N1—C2—C1 torsion angle (-143.5°). The methyl group is in a distorted gauche position relative to the N1···C1 chain. In this configuration, four dipeptide molecules can tetra­hedrally coordinate a Zn atom forming a grid-like layered structure with pores that accommodate solvent molecules.