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Solubility Equilibria and Crystallographic Characterization of the L-Threonine/L-allo-Threonine System, Part 2: Crystallographic Characterization of Solid Solutions in the Threonine Diastereomeric System Nikolay Taratin, Heike Lorenz, Daniel Binev, Andreas Seidel-Morgenstern, and Elena Kotelnikova Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg5010566 • Publication Date (Web): 01 Dec 2014 Downloaded from http://pubs.acs.org on December 6, 2014

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Crystal Growth & Design

Solubility Equilibria and Crystallographic Characterization of the L-Threonine/L-alloThreonine System, Part 2: Crystallographic Characterization of Solid Solutions in the Threonine Diastereomeric System Nikolay Taratin,† Heike Lorenz,*,‡ Daniel Binev,‡ Andreas Seidel-Morgenstern,‡ and Elena Kotelnikova† †

Department of Crystallography, Saint Petersburg State University, Saint Petersburg, Russia

‡

Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany Features of crystal structures and their thermal

deformations in the L-threonine/L-allo-threonine

с L-aThr

L-aThr

L-aThr

system were investigated by means of single crystal 5.17

5.17

X-ray diffraction and temperature resolved XRPD.

2.79 3.84

The L-threonine/L-allo-threonine system belongs to

3.30

5.30 7.18

the systems with continuous solid solutions. The L-aThr

3.84

L-Thr

L-aThr

comparative analysis of crystal structures of Lthreonine, L-allo-threonine and their mixed crystals was conducted based on literature and own data. The distances between methyl groups are more

Distances between the carbon atoms of methyl groups in the crystal structure of mixed crystals containing 66% L-allothreonine and 34% L-threonine. Contacts between identical and non-identical neighbor molecules are shown as blue and red dashed lines respectively.

sensitive to the crystal composition than the lengths of the hydrogen bonds. Statistical distribution of L-threonine and L-allo-threonine molecules causes the occurrence of shortened and elongated distances between the methyl groups of the neighbor non-identical molecules (L-Thr—L-aThr). Thermal deformations of L-Thr, L-aThr and solid solutions containing 34 and 90 % L-Thr were investigated. In the case of diastereomers the slight negative thermal expansion along the axes a was observed. Correlations between features of crystal structure and thermal behavior of L-Thr, L-aThr and their solid solutions were identified.

∗

Corresponding Author: Heike Lorenz E-mail: [email protected] Tel.: +49 391 6110 293 Max Planck Institute for Dynamics of Complex Technical Systems Sandtorstraße 1, D-39106 Magdeburg, Germany

ACS Paragon Plus Environment

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Solubility Equilibria and Crystallographic Characterization of the L-Threonine/L-alloThreonine System, Part 2: Crystallographic Characterization of Solid Solutions in the Threonine Diastereomeric System Nikolay Taratin,† Heike Lorenz,*,‡ Daniel Binev,‡ Andreas Seidel-Morgenstern,‡ and Elena Kotelnikova† †Department of Crystallography, Saint Petersburg State University, Saint Petersburg, Russia ‡Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany *Corresponding Author, E-mail: [email protected] KEYWORDS: amino acids, threonine, stereoisomers, diastereomers, crystal structure, solid solutions, mixed crystals, hydrogen bonds, short contact, thermal behavior ABSTRACT

Features of crystal structures and their thermal deformations in the L-threonine/L-allothreonine system were investigated by means of single crystal X-ray diffraction and temperature resolved XRPD. The L-threonine/L-allo-threonine system belongs to the systems with continuous solid solutions. The comparative analysis of crystal structures of L-threonine, L-allo-threonine and their mixed crystals was conducted based on literature and own data. It was found that the distances between methyl groups are more sensitive to the crystal


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composition than the lengths of the hydrogen bonds. Statistical distribution of L-threonine and L-allo-threonine molecules causes the occurrence of shortened and elongated distances between the methyl groups of the neighbor non-identical molecules (L-Thr—L-aThr). Thermal deformations of crystal structures of L-Thr, L-aThr and solid solutions containing 34 and 90 % L-Thr were investigated. In the case of diastereomers the slight negative thermal expansion along the axes a was observed. Correlations between features of crystal structure and thermal behavior of L-Thr, L-aThr and their solid solutions were identified.

1. INTRODUCTION The present article is a continuation of the publication series (part 2) related to the investigation of chiral organic systems comprising solid solutions. One of the typical examples of such systems is a diastereomeric system composed of L-threonine (L-Thr) and Lallo-threonine (L-aThr). In part 1 of the study1 it was already mentioned that understanding the mechanisms of formation of solid solutions (mixed crystals) in chiral organic systems is crucial for successful preparation of optically pure substances and materials with engineered properties that can be useful in pharmacy, agricultural chemistry, cosmetic, food industries, etc. Recently, studies of these systems have been covered in a number of publications.2–11 We chose threonine as a model substance not only due to its chiral nature, but also because it is one of the so-called essential amino acids. Consequently, the investigation of L-Thr/L-aThr solid solution represents both fundamental and practical significance. In part 1 of the study1 we reported (1) the results of solubility analyses and demonstrated the ternary solubility diagram, which was obtained for the L-Thr/L-aThr/H2O system using polythermal and isothermal methods, and (2) the values of the unit cell parameters as a function of a sample composition (% L-Thr), which was determined by means of X-ray powder diffraction (XRPD) technique. From the configuration of the ternary phase diagram it


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could be concluded that the system contains solid solutions (designated as (L,L-a)-Thr in the following); the diagram also exhibits an alyotropic point of the phase equilibria. The continuity of dependence of the unit cell parameters upon the sample composition (% L-Thr) confirmed the existence of solid solutions in the whole compositional range. Further, a clear correlation between the shapes of solubility isotherms and the dependence of the unit cell parameters on sample composition was observed1. Part 2 of the study now focuses on investigation of features of crystal structures in the LThr/L-aThr system. Firstly, the comparative analysis of crystal structures of diastereomers and their solid solutions was performed based on own (L-allo-threonine, (L0.34,L-a0.66)-Thr) and literature (L-threonine12,13, L-allo-threonine14 and (L0.45,L-a0.55)-Thr15) single crystal XRD data. Secondly, the thermal deformations of L-Thr, L-aThr and solid solutions (L0.34,L-a0.66)Thr and (L0.90,L-a0.10)-Thr were studied based on temperature resolved XRPD data.

2. EXPERIMENTAL SECTION 2.1. Substances and Samples The samples were prepared from commercially available reactants: L-threonine (SigmaAldrich, >98% purity) and L-allo-threonine (TCI, > 99% purity). A single crystal from the commercially available L-allo-threonine reactant was used to determine the crystal structure. Two single crystals obtained after evaporation of an aqueous solution at room temperature were chosen for investigation of (L,L-a)-Thr mixed crystals. One of them was used to identify the crystal structure (single crystal XRD), while the other was selected to determine the relative proportions of L-Thr and L-aThr in the mixed crystal (high performance liquid chromatography). All the samples selected for X-ray diffraction studies and temperature resolved X-ray diffraction studies were recrystallized as follows. A predetermined amount of L-Thr and/or L-


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aThr was stirred in water until dissolution was complete. The resulting solution was transferred to a Petri dish and kept at 50 °C until evaporation of the solvent was complete and crystals appeared. A high evaporation rate resulted in mass crystallization of the whole solution, which made it possible to avoid a phenomenon known as continuous fractionation of the solution composition. As a result, the averaged composition of the crystals obtained corresponded to the composition of the initial solution. 2.2. Solid Phase Characterization via Single Crystal XRD Crystal structures of L-allo-threonine and a mixed crystal were solved and refined using the data obtained by means of single crystal X-ray diffraction. The L-aThr sample was studied at 100 K (Kappa APEX DUO, Mo radiation, an Oxford Cobra Plus low temperature accessory), while the mixed crystal was investigated at room temperature (STOE IPDS II, Mo radiation, detector distance 100 mm, 2Θ range 3–60°). Solution and refinement of the crystal structure data were performed using ShelX16 and/or SIR201117 software packages. The positions of hydrogen atoms were calculated by means of standard options (riding refinement) of these software packages. Refinement of non-hydrogen and hydrogen atoms was made in anisotropic and isotropic approximations, correspondingly. A Mercury software package was used to visualize and conduct geometric analysis of the crystal structures. 2.3. Determination of Crystal Composition via High Performance Liquid Chromatography Samples of solid solutions of L-Thr and L-aThr were dissolved in ethanol. The relative concentrations of L-Thr and L-aThr molecules in the solution were determined by means of high performance liquid chromatography (HPLC) (1200 Series equipment, Agilent Technologies, Germany; column: Chirobiotic T, 5 µm particles, 250 x 4.6 mm, Astec, USA; eluent: 70/30 v/v, ethanol/water). 2.4. Solid Phase Characterization via Temperature Resolved XRPD


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Polycrystalline powders of the pure diastereomers (L-Thr, L-aThr) and two solid solutions containing 34% L-Thr and 90% L-Thr were selected for temperature resolved X-ray powder diffraction studies. A Rigaku Ultima IV diffractometer (CuKα radiation, reflection mode, 2Θ range 3–50°, 3 sec/step, 2Θ step 0.1°) equipped with a high-temperature Rigaku SHT-1500 chamber was used to perform the measurements. An L-threonine sample was studied within the temperature range of 25–200 °C with the step of 10–20 °C. An L-allo-threonine sample and the specimens containing 34% and 90% of L-Thr were analyzed within the temperature range starting from room temperature (25 °C) up to 200 °C with the step of 5–10 °C. Thermal expansion coefficients were calculated by means of original computer program DTC18.

3. RESULTS AND DISCUSSION 3.1. Crystal Structures of Individual Diastereomers and Mixed Crystals in the LThreonine/L-allo-Threonine System The crystal structure of L-threonine has already been studied at room temperature12 and at 15 K13. The crystal structure of L-allo-threonine was analyzed at room temperature14 and at 100 K by the authors of the present article. We are also aware of a single work related to structural study of mixed crystals15. The object of the cited work was a single crystal containing 45% of L-Thr and the researchers concluded that the molecules within the crystal structure were statistically distributed. We chose to study a mixed crystal containing another proportion of the diastereomers (34% of L-Thr). 3.1.1. Crystal Structure of L-allo-Threonine. The crystal structure of L-allo-threonine was solved in the space group P212121 with the following orthorhombic cell parameters: a = 13.71 Å, b = 7.74 Å and c = 5.14 Å. The structure of its molecule is depicted in Fig. 1a. The hydrogen bond lengths are shown Table 1. The data obtained by the present authors are in good agreement with the results acquired by P. Swaminathan and R. Srinivasan14.


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a

b

O3 C3

C4*

C4*

C3 N1

C2

O2

O3 C4

C4

C1

c

O3

C3

C2

N1

C2

C1 O1

O2

C1

O2

O1

O1

Figure 1: Structures of molecules of (a) L-aThr, (b) L-Thr13, and (c) a hypothetical “averaged” molecule of (L0.34,L-a0.66)-Thr mixed crystal. Table 1. The lengths of hydrogen bonds (HB) in the crystal structures of L-aThr and (L0.34,L-a0.66)-Thr mixed crystal Notation of hydrogen bonds

Bond Length, Å

Contact atoms (D…A)

НВ1 НВ2

O3…O1 N1…O2II

L-aThr 2.655(3) 2.786(3)

НВ3

N1…O2III

2.887(4)

2.925(6)

НВ4

IV

2.918(4)

2.995(5)

I

N1…O3

(L0.34,L-а0.66)-Thr 2.647(4) 2.795(5)

Symmetry codes: (I) x-½, ½-y,1-z; (II) 1-x, y-½, ½-z; (III) x, y, 1+z; (IV) ½-x, -y, ½+z. Atoms are numbered according to Fig. 1.

3.1.2. Crystal Structure of the Mixed Crystal (L0.34,L-a0.66)-Thr. The crystal structure was solved in the space group P212121 with the following orthorhombic cell parameters: a = 13.61 Å, b = 7.85 Å and c = 5.17 Å. According to the results both diastereomer molecules coexisted in the crystal. This co-existence is represented as partial occupancy of methyl groups, which are responsible for the molecular configurations of L-allo-threonine and L-threonine (Figs. 1a and 1b) in a hypothetical “averaged” molecule of the mixed crystal (Fig. 1c). This results from the statistical distribution of L-Thr and L-aThr molecules in the mixed crystal structure. In the case of studied solid solutions (L,L-a)-Thr (this work and literature data15) partial occupied hydrogen atoms of the methyl groups as well as the hydrogen atom


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connected to the C3 carbon could not be localized based on the X-ray experiment. It should be mentioned, that these hydrogen atoms are not involved in hydrogen bonding. Refined partial occupations of carbon atoms C4 and C4*, which are indicative for proportions of L-Thr and L-aThr, were found to be 0.34 and 0.66, correspondingly. HPLC analysis of another mixed crystal of the same sampling (see Section 2.1.) verified the composition to 34% of L-Thr and 66% of L-aThr. Good agreement between single crystal XRD and HPLC data pointed out the correct use of the partial occupancies of C4/C4* atoms as the L-Thr/L-aThr ratio in the mixed crystal composition. Also, the theoretical XRPD pattern calculated for this mixed crystal on the basis of our structural data (Fig. 2a) was virtually identical to the experimental pattern obtained for a crystal having the same composition (Fig. 2b). As it was expected, the peaks in the experimental XRPD pattern are broader than the peaks in the theoretical one.

a

b

2ӨCukα

Figure 2: Theoretical (a) and experimental (b) X-ray diffraction patterns of (L0.34,L-a0.66)-

Thr mixed crystal. In Fig. 3 main features of the crystal structure of the mixed crystal are illustrated; the lengths of hydrogen bonds in the structure of (L0.34,L-a0.66)-Thr are represented in Table 1. The least packed is the region of the structure that is located along the plane ab. Hydrogen bonds (HB) define the framework motif of the structure. The motif can be represented as a


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combination of cavities delineated by the diastereomeric molecules interlinked with hydrogen bonds. The largest cavity is formed by two pairs of hydrogen bonds HB1 and HB2 and a single hydrogen bond HB3. These cavities form the largest channels in the structure, which are extended along the c axis. Methyl groups CH3 of the neighboring molecules are situated inside the channels and related with each other by 21 axis. Similar channels can be seen in the crystal structures of L-Thr and L-aThr. a

a b

c b

a

c

b

c

Figure 3: Projection of crystal structure of the (L0.34,L-a0.66)-Thr mixed crystal on the planes ab (a), ac (b), and bc (c). Light gray rectangles delineate the projections of the largest channel. Red ovals show the shortest distances between the carbon atoms of the methyl groups. Hydrogen atoms are not shown.


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The shortest hydrogen bonds (see Table 1) are the bonds HB1 between the O3 oxygen of the proton donating hydroxyl group OH and the O1 oxygen of the proton accepting carboxylic group COOH. The nitrogen atom N1, which is present in protonated form, is a donor of protons for hydrogen bonds of the three following types: HB2, HB3, and HB4. The O2 atom of COOH group is an acceptor for protons of the bonds HB2 and HB3, while the O3 atom of OH group can be at the same time both a donor (HB1) and an acceptor (HB4) of protons. Besides the arrangement of hydrogen bonds in (L0.34,L-a0.66)-Thr, the short contacts between C4 and C4* carbon atoms in the methyl groups of the statistically distributed L-Thr and L-aThr molecules were detected. These are the contacts between the adjacent molecules arranged in perpendicular and parallel directions to the largest channel (Fig. 3). These distances are shown as red ovals in Figs. 3b and 3c. The distance directed orthogonally to the channel axis is 2.79 Å, which is close to the HB1 length (2.65 Å), while that extending in the parallel direction is 3.30 Å, i.e. by about 0.3 Å exceeds the length of HB4 (3.00 Å). 3.1.3. Comparative Analysis of Crystal Structures of Diastereomers and Mixed Crystals. The crystal structures of L-threonine12, 13, L-allo-threonine14 (and this work), and those of mixed crystals, namely (L0.45,L-a0.55)-threonine15 and (L0.34,L-a0.66)-threonine (this work), are rather similar to each other. Each diastereomer molecule is connected with hydrogen bonds (see Table 1) to four neighbor molecules. Partially occupied methyl groups determining configuration of L-threonine (C4) and L-allo-threonine (C4*) do not participate in formation of hydrogen bonds. Hydrogen bonds. A comparative analysis of hydrogen bond lengths in the crystal structures of L-Thr, L-aThr and mixed crystals was performed with the use of both our results and reported data12–15, obtained by means of single crystal X-ray diffraction. Hydrogen atoms were excluded from the comparison. The comparison results are shown in Fig. 4. It can be seen that at room temperature (literature data12,14,15 and this work) and low temperatures


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(literature data13 and this work) the lengths of hydrogen bonds (D…A) in L-Thr and L-aThr found by different authors are in good agreement. The lengths of hydrogen bonds HB1, HB2, and HB3 in the crystal structures of pure diastereomers are practically the same. It should be mentioned that the lengths of the shortest hydrogen bonds HB1 and HB2 are not influenced by the crystal composition. The length of HB3 of (L0.34,L-a0.66)-Thr and (L0.45,L-a0.55)-Thr15 exceeds that in case of the pure diastereomers by about 0.04 Å. The most sensitive to the compositional variation is the HB4 bond. In L-aThr this bond is shorter than that in L-Thr by 0.19 Å. This fact is inconsistent with another experimental finding, i.e. non-linear diminishing of the volume V of the orthorhombic cell from 552 Å3 in L-aThr to 542 Å3 in L-Thr.1

Figure 4: Correlation between the lengths of hydrogen bonds D…A (Å) and crystal composition (% L-Thr) (this work and literature data12-15). Hydrogen bonds of different types are shown. Note: L-Thr13 and L-aThr (this work) were measured at 12 K and 100 K correspondingly; L-Thr12, L-aThr14 and solid solutions (L0.34,L-a0.66)-Thr (this work) and (L0.45,L-a0.55)-Thr15 at room temperature. Distances between the carbon atoms of the methyl groups. The changes in distances between the carbon atoms of the methyl groups positioned inside the largest channel (see Fig. 3) are discussed below for those groups belonging to L-aThr (Fig.5a), L-Thr (Fig. 5b) and the (L0.34,L-a0.66)-Thr mixed crystal (Fig. 5c). ACS Paragon Plus Environment

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L-allo-Threonine and L-Threonine (Figs. 5a and 5b). Distances between the carbon atoms of the methyl groups in crystals of L-aThr and L-Thr are close enough. In the case of methyl groups situated in transverse direction across the channel (corresponding to the 21 axis) the distances are 3.80 Å (L-aThr) and 3.78 Å (L-Thr). In the case of methyl groups positioned along the channel (over a translation) the distances are 5.14 Å (L-aThr) and 5.11 Å (L-Thr). Consequently, packing of the molecules in the L-aThr crystal structure is not as dense as it is in the L-Thr structure, which is consistent with diminished volume V of the orthorhombic cell of 552 Å3 in comparison to 542 Å3 for L-aThr and L-Thr respectively. с

с

a

b

5.14

5.11

3.80

3.78

3.78

3.80

5.11

5.14

с

5.17 3.84

3.30

5.17

3.84 3.30

2. 79

5.30

5.17 5.30

3.80

3.84

7.18

L-Thr

L-Thr

L-aThr

L-aThr

L-aThr

3.30

7.18

c L-aThr

L-Thr

L-aThr

L-Thr

L-aThr

Figure 5: Distances between the carbon atoms of methyl groups in pairs of identical (grey dashed lines in the case of pure diastereomers, blue dashed lines in the case of the mixed crystal) and non-identical (red dashed lines) neighbor molecules in the crystal structures of LaThr (a), L-Thr (b) and the (L0.34,L-a0.66)-Thr mixed crystal (c). Hydrogen bonds are shown as solid lines.


12

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Mixed crystal (L0.34,L-a0.66)-Threonine (Fig. 5c). In the case of solid solutions, statistical distribution of the L-threonine and L-allo-threonine molecules within the crystal structure should be taken into account. The transverse and longitudinal distances between the methyl groups of the adjacent molecules in the largest channel in the crystal structure may vary depending on what kind of molecules contact each other. Two types of contacts could be considered. The contacts of the first type occur between two identical molecules, i.e. in LaThr—L-aThr or L-Thr—L-Thr pairs (Fig. 5c, blue dashed lines). The distances between the carbons of the methyl groups located across the channel are 3.84 and 3.80 Å for L-aThr—LaThr and L-Thr—L-Thr pairs respectively. The distances between the carbons of the methyl groups located along the channel are 5.17 Å for both pairs. The contacts of the second type occur between non-identical contacting molecules, i.e. in L-aThr—L-Thr pairs (Fig. 5c, red dashed lines). There are two variants of distances for each pair of methyl groups of the nonidentical molecules located across and along the channel. The transverse distances are 2.79 and 5.30 Å, while longitudinal distances are 3.30 and 7.18 Å. These differences can result in “local” disappearance of the 21 axes. It is comprehensible that the relative proportions of identical and non-identical contacts between methyl groups of the neighbor molecules depend upon the solid solution composition. In the case of equimolar composition, the quantitative proportions of the contacts will depend upon the solid-state nature of the crystal. If it corresponds to a binary compound, non-identical molecules would have formed all the contacts. If it corresponds to a solid solution, the contacts can be formed by molecules of both types, i.e. by non-identical molecules (majority of the contacts), and identical ones (minority of the contacts). The fraction of the non-identical contacts decreases with increase of either the L-threonine or Lallo-threonine component relative to the equimolar composition.


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Distances (Å) between the carbon atoms of the methyl groups in the ab plane

Crystal L-aThr

Pair of molecules

3.80

L-aThr—L-aThr L-Thr—L-aThr

2.79

L-Thr—L-aThr

5.30

(L0.34,L-a0.66)-Thr 3.84

L-aThr—L-aThr

3.80

L-Thr—L-Thr

3.78

L-Thr 1

2

L-Thr—L-Thr 3

4

5

6

7

8

Distances (Å) between the carbon atoms of the methyl groups along the axis c

Crystal L-aThr

Pair of molecules

5.14

L-aThr—L-aThr L-Thr—L-aThr

3.30 (L0.34,L-a0.66)-Thr

L-Thr—L-aThr

7.18

L-Thr 1

2

5.17

L-aThr—L-aThr

5.17

L-Thr—L-Thr

5.11

L-Thr—L-Thr

3

4

5

6

7

8

Figure 6: The distances between the carbon atoms of the methyl groups of neighbor molecules in ab plane and along axis c in the case of L-threonine, L-allo-threonine and the (L0.34,L-a0.66)-Thr mixed crystal. Fig. 6 represents a comparison of the distances between the carbon atoms of the methyl groups of neighbor molecules in the ab plane and along the c axis in the crystal structures of the (L0.34,L-a0.66)-Thr mixed crystal with the pure diastereomers L-Thr and LaThr. It can be seen that the distances between identical molecules in the crystal structure of the solid solution (Fig. 6, blue bars) are close to those formed by molecules of the same type in structures of pure L-threonine and L-allo-threonine (Fig. 6, gray bars). Considerable variation of the distances between methyl groups of the neighbor nonidentical molecules can cause distortions of the channels within the real crystal structure of the solid solution. These distances are shown as crimson bars in Fig. 6. Relative deformations


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of the channels in transverse and longitudinal directions can be estimated on the basis of the corresponding contact elongations in accordance with the following known formula:

ε = (l non − ident − l ident ) / l ident , (1) where ε is a relative elongation of the distances between the methyl group carbon atoms, ℓnonident

is an averaged distance between the carbon atoms of the methyl group belonging to non-

identical molecules (shortened and elongated contacts of the L-aThr—L-Thr type), and ℓident is an averaged distance between the carbon atoms of the methyl group belonging to identical molecules (L-aThr—L-aThr and L-Thr—L-Thr). In the case of the (L0.34,L-a0.66)-Thr mixed crystal, the values of the relative elongation ε in the ab plane and along the c axis (transverse and longitudinal directions of the channel) are 0.057 and 0.011 correspondingly. This can explain the fact that the a and b parameters are more sensitive to the compositional changes than parameter c. Thus, it can be concluded that the most sensitive to the changes in crystal composition appeared to be the distances between methyl groups, but not the length of hydrogen bonds as might be expected. 3.2. Thermal Deformations of the Crystal Structure in the L-Threonine/L-alloThreonine System Features of crystal structures of individual diastereomers and their solid solutions are reflected in dependences of the unit cell parameters on temperature. The thermal behavior of crystal structures of the two diastereomers (L-Thr and L-aThr) and the two solid solutions (L0.34,L-а0.66)-Thr and (L0.90,L-а0.10)-Thr were studied by means of temperature resolved XRPD. Their XRPD patterns obtained at various experimental temperatures were indexed in the orthorhombic space group P212121. Fig. 7 depicts some exemplary XRPD patterns of Lthreonine obtained at ambient temperature (25 °C) and in the temperature range of 80–200 °C.


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It can be seen that configurations of all the patterns demonstrate a marked similarity in having the same sets of peaks with similar relative intensities. The most significant dissimilarities in the patterns obtained at different temperatures can be noticed in two regions (indicated by blue rectangles in Fig. 7 and in more detail in Fig. 8a). Gradual elevation of temperature resulted in progressive convergence of peaks 200 and 110 (for the angles 2ΘCuKα = 12–14°) and peaks 310 and 020 (for the angles 2ΘCuKα = 22–24°). At 200 °C the peaks of each pair practically merged.

T(°C) T, °C 200 160 120 80 30

Figure 7: Powder diffraction patterns (2ΘCuKα) of L-Thr obtained at ambient temperature and temperatures ranging from 80 to 200 °C (from bottom to top). Blue rectangles delineate the regions of the patterns shown in Fig. 8.


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200

200, 110

200, 110

a

b

200, 110

110

c

d

200, 110 200 °С

200 °С 200 80 °С 25 °С 12.6

200 °С

200110

110

13.0

200, 110

200 °С

13.4

25 °С

25 °С

25 °С 12.5

13.0

13.5

12.5

13.0

13.5

12.5

310 020

13.0

13.5

310, 020 310, 020

310 020

200 °С

200 °С

200 °С 200 °С

310

020

80 °С 25 °С 22.4

22.6

22.8

23.0

25 °С

22.0

310 22.5

020 23.0

310, 020

25 °С 22.0

22.5

23.0

310 020

25 °С 22.0

22.5

Figure 8: Profiles of the peak pairs 200—110 (top) and 310—020 (bottom) in XRPD patterns (2ΘCuKα) of L-Thr (a), L-aThr (b), (L0.34,L-а0.66)-Thr (c) and (L0.90,L-а0.10)-Thr (d) obtained in the temperature range of 25–200 °C. Similar regularities were observed during heating L-allo-threonine (Fig. 8b) and (L0.34,La0.66)-Thr (Fig. 8c) and (L0.90,L-a0.10)-Thr (Fig. 8d), except that at 200 °C (the ultimate experimental temperature) the coalescence of the peaks was not complete in the L-aThr pattern, while they merged completely in the patterns of (L0.34,L-a0.66)-Thr and (L0.90,L-a0.10)Thr and already at much lower temperatures, i.e. about 25 °C (Fig. 8c) and 70 °C (Fig. 8d), correspondingly. It should be noted that further heating the mixed crystals may result in a “reversal” effect, i.e. splitting the merged peaks, since with elevating the temperature their shape starts to develop a certain asymmetry (Fig. 8c). The temperature dependences of orthorhombic cell parameters and the unit cell volume obtained for L-Thr, L-aThr and their mixed crystals are shown in Fig. 9.


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Figure 9: Temperature dependences of the orthorhombic cell parameters a (a), b (b) and c (c) and the unit cell volume V (d) obtained for L-Thr (red circles), L-aThr (blue circles), and (L0.34,L-a0.66)-Thr (crimson circles) and (L0.90,L-a0.10)-Thr (green circles).

In all cases, the changes in the orthorhombic cell parameters and its volume were observed to be linear. For the individual diastereomers the values of the b and с parameters grow (∆b ≈ 0.07–0.10 Ǻ and ∆с ≈ 0.04–0.05 Ǻ), while the parameter a slightly diminishes (∆a ≈ 0.01– 0.02 Ǻ). In case of the solid solutions an increase in all the parameters was observed (∆a ≈ 0.02–0.05 Ǻ, ∆b ≈ 0.06 Ǻ, and ∆с ≈ 0.04 Ǻ). Polymorph transformations have not been observed in the system of L-threonine / L-allothreonine; that was shown (1) by absence of principle alterations in the diffraction patterns (see Fig. 7) and (2) by linear and continuous (without any jumps and inflections) temperature dependence of their parameters and volume V (Fig. 9). Consequently the crystal structures of all studied samples during the heating demonstrate the thermal deformations only.


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Based on temperature dependences of the orthorhombic parameters thermal expansion coefficients α of crystal structures were calculated according to the following known formula: αa = (1/a)(da/dT), (2) where a is the unit cell parameter and T the temperature. Thermal expansion coefficients for crystal structures of L-Thr, L-aThr, and solid solutions containing 34 and 90 % L-Thr are given in Table 2.

Table 2. Thermal expansion coefficients α for crystal structures of L-Thr, L-aThr and solid solutions (L0.34,L-a0.66)-Thr and (L0.90,L-a0.10)-Thr. Thermal expansion coefficients, α × 10-6 °С-1

Composition, % L-Thr

αa

αb

αc

αv

100

-7

78

39

109

90

7

51

58

117

34

22

38

58

119

0

-3

52

58

107

Crystals in the whole range of studied compositions have rather close values of volumetric coefficient of thermal expansion, αv, viz.: αv ≈ 113±6 × 10-6 °С-1. Crystals of the diastereomers have the minimal and substantially the same value of volumetric coefficient (108±1 × 10-6 °С-1) (Table 2). The solid solutions, on the contrary, have maximum values of volumetric coefficient in the studied range, which are also practically equal to each other (118±1 x 10-6 °С-1). The maximum difference of the α values along the a and b directions is significant: ∆αa ≈ 29 × 10-6 °С-1 and ∆αb ≈ 40 × 10-6 °С-1 (compositions: 100 and 34 % L-Thr), while in the c direction this difference is about a half of the above value and is as follows: ∆αс ≈ 19 × 10-6 °С-1 (compositions: 100 and 90, 34, 0 % L-Thr) (Table 2).


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80 60

a αa -40

-20

60

b

40

40

20

20

αa

c

60

αa

20

0

0

0 -20

20

40

-40

-20

0

20

-20

-40

-40

-60

-60

αb

40

-40

-20

0

0

20

40

20

-40

-20

0 0 -20

20

40

-40

-40 -60

40

αa

-20

αb

60

d

40

αb

-60 -80

αb

Figure 10: Projections of the figures of thermal expansion coefficients on the ab plane of L-aThr (a), (L0.34,L-a0.66)-Thr (b), (L0.90,L-a0.10)-Thr (c) and L-Thr (d).

Data shown in Table 2 were used to plot projections (sections) of the figures of the thermal expansion coefficients on the planes ab, bc, and ac. The most prominent differences in thermal behavior of diastereomer crystals and their solid solutions become apparent from the projections of figures of their thermal expansion on the ab plane (Fig. 10). The crystals of diastereomers show substantially anisotropic thermal expansion: a slight negative (anomalous) thermal expansion is observed along the direction of axes a. The crystals of the solid solutions are characterized by more isotropic thermal expansion: the greatest expansion is shown by the crystal containing the maximum amount of the admixture molecules (34 % LThr, Fig. 10b). R. Lima19 et al. studied thermal expansion of L-threonine using dilatometric analysis and some data were reported. Their volumetric thermal expansion results (100 × 10-6 °С-1) are in a good agreement with our data (109 × 10-6 °С-1). However, the reported linear coefficients of thermal expansion (αa = 30.1 × 10-6 °С-1, αb = 23.8 × 10-6 °С-1, and αс = 45.4 × 10-6 °С-1)20 are notably different from the data shown in Table 2. The negative thermal expansion effect along the a axis observed by us agrees with results obtained by J. Janczak et al.13, as they reported increase in the a parameter with decrease of temperature after having studied crystal structure of L-threonine at room temperature and at 15 K. We, in turn, have found that the a parameter of L-threonine diminishes as the temperature increases (see Fig. 9a). ACS Paragon Plus Environment

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As an example, Fig. 11 shows the cross-section of

a

the thermal expansion figure in projection on the ab plane of the L-threonine crystal structure in

b αa

vicinity of the largest channel. It can be seen that when heated, the L-threonine crystal structure expands in the direction that is perpendicular to the

αb

.

Figure 11: Thermal expansion figure

layers of molecules (along the b axis) and

in projection on the ab plane of the L-

insignificantly contracts in the direction that is

Thr crystal structure in the area of the

parallel to the layers of molecules (along the a

largest channel.

axis) (see Fig. 3a). This sharp anisotropy of thermal deformations of the crystal structure can

be explained by shift of one layer of molecules relative to the molecules of the neighboring layer along the a axis during heating the crystal. This shear deformation conception can also be applied to L-aThr and the solid solutions of L-Thr and L-aThr; in the case of the solid solutions, however, we also observed a compensation effect that consists in diminishing the thermal expansion anisotropy. It seems that the distorted geometry of the largest channels can impede shear deformations in the crystal structure of the solid solutions. It is to be held in mind that deformation of the channels is occurred due to the presence of shortened and elongated contacts between identical and non-identical molecules in crystal structures of the solid solutions (see Figs. 5 and 6).

4. CONCLUSIONS The results of investigating the crystal structure (single crystal XRD) and thermal behavior (temperature resolved XRD) of diastereomers and their solid solutions in the L-threonine/Lallo-threonine system are in a good agreement with both miscibility data obtained for L-


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Page 22 of 27

threonine and L-allo-threonine molecules in the solid phase (XRPD) and the results of analyzing the solubility equilibria in the L-threonine/L-allo-threonine/water system reported in the first part of the study1. Considering all the experimental findings, it can be concluded that the L-threonine/L-allo-threonine system belongs to the systems with continuous solid solutions, while the presence of an alyotropic point (65 % L-Thr)1 shows those solutions to be thermodynamically non-ideal20. The crystal structures of L-threonine, L-allo-threonine, and (L0.34,L-а0.66)-Thr and (L0.45,Lа0.55)-Thr solid solutions (this work and literature data12-15) have the same framework defined by hydrogen bonds. The lengths of hydrogen bonds depend to a slight extent upon the crystal composition. The contacts between methyl groups of the neighbor diastereomer molecules are rather sensitive to the crystal composition. These groups are positioned in the largest channels of the crystal structures. The distances between methyl groups of contacting identical molecules (L-Thr—L-Thr and L-аThr—L-аThr in the case of diastereomers and solid solutions) and non-identical molecules (L-аThr—L-Thr only in the case of solid solutions) are markedly different. The solid solutions with compositions in vicinity of a diastereomer predominately contain the contacts between identical molecules, while in the region of the equimolar composition, which also contains the alyotropic point, the prevalent are the contacts between non-identical molecules. Also, it was observed that the contacts of the latter type were characterized by both shortened and elongated distances between methyl groups and that might result in distortion of the largest channel in crystal structure of a real mixed crystal. Heating L-Thr, L-aThr, and the (L0.34,L-а0.66)-Thr and (L0.90,L-а0.10)-Thr solid solutions did not result in any polymorph transformations. On the contrary, it was found that the crystalline structures of the substances analyzed experienced only thermal deformations, which was evident from the linear temperature dependences of their unit cell parameters and volume. In


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contrast to the solid solutions, the individual diastereomers underwent substantially anisotropic deformations, up to the slight negative thermal expansion along the a axis. At the same time, the most prominent isotropic thermal expansion was observed in the crystal having the maximum content of the admixture molecules studied (34 % L-Thr) and, consequently, containing the maximum number of contacts between non-identical molecules. Correlations between structural features (the lengths of hydrogen bonds, distances between the methyl groups) of solid solutions and their properties (thermal expansion, solubility) as obtained in the case of the L-threonine/L-allo-threonine system can be useful in developing methods to obtain advanced materials. Acknowledgements. This work is dedicated to the memory of the late Professor Arkady E. Glikin. The authors express their thanks to Mrs. J. Kaufmann (Max Planck Institute for Dynamic of Complex Technical Systems), who performed analyses of mixed crystals by means of High Performance Liquid Chromatography, to Dr. O. Siidra (Saint-Petersburg State University), who carried out Single Crystal X-ray Diffraction study of Lallo-threonine, to Dr. M. Krzhizhanovskaya (Saint-Petersburg State University), who performed thermal X-ray diffraction experiments, and to Dr. V. Fundamenskii (Saint-Petersburg State Technical University), who participated in the discussion of the X-ray diffraction results. Nikolay Taratin and Elena Kotelnikova acknowledge supports provided by RFBR Grants 13-05-12053 and 12-05-00876. A part of the experiments was made using the equipment of Resource Centers “X-ray diffraction Methods” and “Geomodel” of SaintPetersburg State University.

References: (1)

Binev, D.; Taratin, N.; Kotelnikova, E.; Seidel-Morgenstern, A; Lorenz, H. Cryst. Growth Des. 2014, 14, 367−373.

(2)

Huang, J.; Chen, S.; Guzei, I. A.; Yu, L. J. Am. Chem. Soc. 2006, 128, 11985−11992.

(3)

Beckmann, W.; Lorenz, H. Chem. Eng. Technol. 2006, 29, 226−232.

(4)

Wermester, N.; Aubin, E.; Pauchet, M.; Coste, S.; Coquerel, G. Tetrahedron: Asymmetry 2007, 18, 821−831. ACS Paragon Plus Environment

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(5)

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Renou, L.; Morelli, T.; Coste, S.; Petit, M.; Berton, B.; Malandain, J.; Coquerel, G. Cryst. Growth Des. 2007, 7, 1599−1607.

(6)

Kaemmerer, H.; Lorenz, H.; Black, S. N.; Seidel-Morgenstern, A. Cryst. Growth Des. 2009, 9, 1851−1862.

(7)

Bredikhin, A. A.; Bredikhina, Z. A.; Zakharychev, D. V.; Gubaidullin, A. T.; Fayzullin, R. R. CrystEngComm 2012, 14, 648−655.

(8)

Taratin, N. V.; Lorenz, H.; Kotelnikova, E. N.; Glikin, A. E.; Galland, A.; Dupray, V.; Coquerel, G.; Seidel-Morgenstern, A. Cryst.Growth Des. 2012, 12, 5882−5888.

(9)

Balawejder, M.; Kiwala, D.; Lorenz, H.; Seidel-Morgenstern, A.; Piatkowski, W.; Antos, D. Cryst. Growth Des. 2012, 12, 2557–2566.

(10) Isakov, A. I.; Kotelnikova, E. N.;. Kryuchkova, L. Yu; Lorenz H. Proceedings of the 20th International Workshop on Industrial Crystallization – BIWIC 2013, H. Qu, J. Rantanen, C. Malwade, Ed.; University of Southern Denmark: Odense, Denmark, 2013; pp 395-402. (11) Bredikhin, A. A.; Zakharychev, D. V.; Gubaidullin, A. T.; Fayzullin, R. R.; Pashagin, A. V. ; Bredikhina, Z. A. Cryst. Growth Des. 2014, 14, 1676−1683. (12) Shoemaker, D. P.; Donohue J.; Schomaker, V.; Corey, R. B. J. Am. Chem. Soc. 1950, 72, 2328–2349. (13) Janczak, J.; Zobel, D.; Luger, P. Acta Cryst. 1997, C53, 1901–1904. (14) Swaminathan, P.; Srinivasan, R. Acta Cryst. 1975, B31, 217–221. (15) Swaminathan, P.; Srinivasan, R. J. Cryst. Mol. Str. 1975, 5, 101–111. (16) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Refinement, Univ. of Göttingen: Göttingen, 1997.


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(17) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini B., Cascarano, G. L.; Giacovazzo, C.; Mallamo, M.; Mazzone, A.; Polidori, G.; Spagna, R. J. Appl. Cryst. 2012, 45, 357−361. (18) Belousov R., Filatov, S. K. Computer Program “DTC”, Saint-Petersburg State University: Saint-Petersburg, 2006. (19) Lima R., Santos-Junior A., Moreno A., Façanha-Filho P., Freire P., Yoshida, M. J. Thermal Analysis and Calorimetry. 2012, 111, 627–631. (20) Prieto, M. Rev. Mineral. Geochem. 2009, 70, 47−85.


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For Table of Contents Use Only


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Solubility Equilibria and Crystallographic Characterization of the L-Threonine/L-alloThreonine System, Part 2: Crystallographic Characterization of Solid Solutions in the Threonine Diastereomeric System N. Taratin, H. Lorenz, D. Binev, A. Seidel-Morgenstern, and E. Kotelnikova

The crystal structure features of solid solutions of

с

L-threonine and L-allo-threonine are discussed

L-aThr

based on the single crystal XRD data and on the

5.17 3.84 3.30

temperature (temperature resolved XRPD). The

L-aThr

distances between methyl groups of neighbor molecules are more sensitive to the crystal composition than the lengths of the hydrogen bonds.

5.17 2.79

dependence of orthorhombic parameters on

L-aThr

L-aThr

5.30

3.84

7.18

L-Thr

L-aThr

Distances between the carbon atoms of methyl groups in the crystal structure of mixed crystals containing 34% L-threonine and 66% L-allothreonine.


27

Solubility Equilibria and Crystallographic ... - ACS Publications

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