Article pubs.acs.org/JPCB
Twin Displacive Phase Transitions in Amino Acid Quasiracemates Carl Henrik Görbitz* and Pavel Karen Department of Chemistry, University of Oslo, N-0315 Oslo, Norway S Supporting Information *
ABSTRACT: 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.
1. INTRODUCTION Among racemates of the 20 “standard” amino acids genetically incorporated into proteins, polymorphism has been observed only for DL-valine and DL-methionine (DL-Met).1 Whereas any transition between triclinic2 and monoclinic3 DL-valine appears unlikely to occur as it would require a 180° rotation of every second molecular bilayer in the crystal, α-DL-Met turns reversibly into β-DL-Met when cooled through Tc ≈ 326 K4−7 upon sliding of molecular bilayers with respect to each other. In addition to DLMet, only three racemates of nonstandard amino acids with linear side chains have been found to display such a displacive transition: 8−12 DL-aminobutyric acid (DL-Abu, ethyl side chain), DL-norvaline 13 (DL-Nva, n-propyl), and DL-norleucine (DL-Nle, n-butyl).14−16 The interplay of their polymorphs is shown in Figure 1. The associated structural changes have, in addition to X-ray crystallography, also been studied by solid-state NMR methods,17 Raman spectroscopy18 and molecular dynamics simulations.19,20 In this paper, we explore whether thermally induced displacive solid-state phase transitions would also occur for quasiracemates. A quasiracemate is a 1:1 mixture of two related but distinct compounds of opposite chirality. They thus tend to form crystal structures with properties similar to those of the true racemates of the individual components. L-Nva:D-Nle, L-Nva:D-Met, and LNle:D-Met, shown in Scheme 1, were selected for this purpose. All have been studied previously at 150 K21,22 and have the same LD−LD hydrogen-bonding pattern at the center of the molecular bilayers23 as the true racemates. We here report 18 additional single-crystal structures, refined from X-ray diffraction data collected at temperatures right above and below transition temperatures recorded by differential scanning calorimetry (DSC).
2.2. Crystal Growth. For each quasiracemate, equimolar amounts of the two amino acids (approximately 0.3 mg of each) were dissolved in 30 μL of water in a 30 × 6 mm test tube, which was then sealed with parafilm. A small hole was pricked in the parafilm and the tube placed inside a larger test tube filled with 1 mL of acetonitrile. The system was capped and left for 5 days at 20 °C. Thin platelike crystals were formed as the organic solvent diffused into the aqueous solution. 2.3. Differential Scanning Calorimetry Measurements. A liquid-nitrogen operated PerkinElmer Pyris 1 instrument was used to perform heating and cooling DSC cycles of 27.1 mg of dry crystals (grown over a period of 14 days by a scaled-up method) sealed in aluminum pans of 30 μL volume; the scanning rate decreased from 40 to 20 to 10 K/min. The instrument was calibrated against the enthalpy and temperature standards of ndodecane, m-nitrotoluene, p-nitrotoluene, and indium of greater than 99.7% purity. Transition temperatures were evaluated by extrapolating peak-top temperatures to the zero scanning rate, transition enthalpies, and entropies via integrating heat-flow peaks versus time as described by Karen.24 2.4. X-ray Data Collection and Structure Solution. Single-crystal X-ray data collections were carried out with APEX2 software25 on Bruker Apex II and D8 Vantage singlecrystal CCD-diffractometers equipped with Oxford Cryosystems Cryostream Plus cooling units and Mo Kα radiation (λ = 0.71069 Å). Data integration and cell refinement was performed with SAINT-Plus25 upon subsequent absorption correction by SADABS,25 and structure solution with SHELXTL.26 Crystal data and refinement results are in Tables 1−3. Illustrations of molecules and their crystal packing arrangements were prepared by Mercury.27
2. EXPERIMENTAL METHODS 2.1. Materials. The amino acids L-Nva, L-Nle, D-Nle, and DMet were purchased from Sigma and used as received.
Received: February 12, 2015 Revised: March 13, 2015 Published: March 20, 2015
© 2015 American Chemical Society
4975
DOI: 10.1021/acs.jpcb.5b01483 J. Phys. Chem. B 2015, 119, 4975−4984
Article
The Journal of Physical Chemistry B
Figure 1. Phase transitions for the racemates of amino acids with linear side chains on cooling (left arrows) and heating (right arrows), updated from ref 8. Low- and high-temperature phases in space group C2/c (or I2/a) are indicated in blue and red, respectively; intermediate phases in space group P21/c (or P21/a) are shown in green. Phase symbols from the literature are given below each racemate; transition temperatures from DSC given on the top, subject to variation depending on the crystal specimen, heating/cooling rate etc., are taken from Coles et al.14 for DL-Nle, from Chatzigeorgiou et al.18 for DL-Nva, and from Görbitz et al. for DL-Abu.8 Hysteresis data are not available for transitions with temperatures listed in italics (α → β → α for DL-Met and DL-Nle). Circles represent previous work: a, ref 12; b, ref 10; c, ref 11; d, ref 9; e, ref 8; f, ref 13; g, ref 15; h, ref 16; i, ref 14; j, ref 4; k, refs 5 and 7; l, ref 6. The gray box represents room-temperature investigations. The low-temperature phase for DL-Nva, labeled X, was not characterized because of delamination of the crystal.13
Scheme 1
Table 1. Crystal Data for L-Nva:D-Nle (C5H11NO2 · C6H13NO2)
a
temp (K)
105
150a
210
270
281
290
space group
P21
P21
P21
P21
C2
C2
C2
a (Å) b (Å) c (Å) β (deg) V (Å3) Z Nmeasured Nunique Nobserved [F2 > 2σ(F2)] npar Rint R [F2 > 2σ(F2)] wR(F2) CCDC
9.896(2) 4.7188(10) 15.256(3) 102.739(2) 694.2(2) 2 6042 1864 1769 172 0.0148 0.0312 0.0882 1048970
9.9166(2) 4.7247(1) 15.3292(1) 102.349(1) 701.60(2) 2
9.908(2) 4.7303(11) 15.381(4) 101.762(3) 705.7(3) 2 6136 1900 1705 172 0.0161 0.0366 0.1056 1048971
9.940(3) 4.7610(16) 15.556(5) 100.765(4) 723.2(4) 2 5736 1894 1446 156 0.0283 0.0537 0.1441 1048972
61.893(9) 4.7406(6) 9.8843(13) 98.184(4) 2870.7(7) 8 13164 5006 3699 328 0.0415 0.0639 0.1688 1048973
31.066(17) 4.764(3) 9.909(6) 95.794(7) 1459.2(14) 4 7673 1778 1261 187 0.0333 0.0513 0.1354 1048974
32.028(12) 4.7797(17) 9.848 98.002(4) 1492.8(9) 4 5552 1530 997 243 0.1515 0.0716 0.2210 1048975
133618
308
Ref 21; CSD refcode GOLVIM.
2.5. Structure Refinement. In absence of disorder, normal anisotropic least-squares refinements were carried out with SHELXTL26 on F2. Carbon atoms in disorder components with occupancy 2σ(F2)] wR(F2) CCDC
9.8790(15) 4.6776(7) 16.037(2) 107.380(3) 707.22(18) 2 13699 4148 3723 174 0.0343 0.0352 0.0861 1048976
9.8965(1) 4.6963(1) 16.1188(1) 106.989(1) 716.46(17) 2
9.891(3) 4.7090(12) 16.164(4) 106.686(3) 721.2(3) 2 6348 3211 3081 174 0.0255 0.0346 0.1002 1048977
9.896(3) 4.7183(13) 16.195(4) 106.507(3) 725.0(3) 2 6377 3233 3083 173 0.0257 0.0347 0.1018 1048978
9.885(3) 4.7426(14) 16.239(5) 105.750(3) 732.7(4) 2 6225 3090 2858 202 0.0240 0.0411 0.1158 1048979
9.849(4) 4.776(2) 16.236(7) 103.123(5) 743.8(6) 2 6595 3340 2714 217 0.0283 0.0582 0.1739 1048980
63.73(3) 4.799(2) 9.857(5) 91.113(5) 3014(2) 8 11719 5635 3547 503 0.0370 0.0660 0.2124 1048981
32.44(2) 4.815(3) 9.882(7) 99.242(7) 1523.5(18) 4 5434 2569 1881 287 0.0283 0.0543 0.1765 1048982
772921
380
ref 22.; CSD refcode URODIP.
Table 3. Crystal Data for L-Nle:D-Met (C6H13NO2 · C5H11NO2S), DL-Nle, and DL-Met temp (K)
105
150a
355
379
386.5
393
DL-Nle
b
c DL-Met
space group
P21
P21
P21
P21
P21
P21
P21/a
P21/a
a (Å) b (Å) c (Å) β (deg) V (Å3) Z Nmeasured Nunique Nobserved [F2 > 2σ(F2)] npar Rint R [F2 > 2σ(F2)] wR(F2) CCDC
9.859(7) 4.693(3) 16.390(11) 107.301(8) 724.0(8) 2 4968 3265 2597 181 0.0575 0.0373 0.0956 1048983
9.8756(2) 4.7029(1) 16.4192(3) 107.3283(7) 727.96(3) 2
9.949(11) 4.771(5) 16.635(19) 103.783(13) 766.9(15) 2 4287 2727 1802 165 0.0608 0.0663 0.1783 1048984
9.886(12) 4.817(6) 16.59(2) 96.485(15) 785.2(16) 2 4607 2833 1423 304 0.0716 0.0589 0.1942 1048985
9.8546(13) 4.8053(6) 33.303(5) 97.977(4) 1561.8(2) 8 20197 5471 3180 591 0.0435 0.0705 0.2108 1048986
9.922(11) 4.842(6) 17.01(2) 101.132(14) 801.7(16) 2 5113 2825 1402 311 0.0980 0.0876 0.2291 1048980
9.9069(13) 4.737(2) 16.382(2) 104.681(11) 743.8(3) 4
9.886(3) 4.7281(14) 16.811(5) 101.950(7) 768.7(4) 4
131837
1028063
133619
a
Ref 21; (L-Met:D-Nle), CSD refcode GOLVOS. bAt 296 K, ref 15.; CSD refcode DLNLUA01. cAt 340 K, converted from space group P21/c, ref 6; CSD refcode DLMETA07.
at the cyclic scan in Figure 2. A strong event 3 of ΔS = 15(2) J/ (mol·K) occurs at 305(1) K, followed by very weak endothermic events 4 and 5, of ΔS = 0.26 and 0.81 J/(mol·K) at 318 and 332 K, respectively. Up to 270 K the individual amino-acid molecules are fully ordered, Figure 3. This is consistent with the previous 150 K data21 in space group P21. Both amino acids have a N1−C2−C3− C4 gauche conformation (or trans for C1−C2−C3−C4, see Supporting Information for detailed lists of torsion-angle values) and extended (trans) side chains. The same conformations are adopted by DL-Nle in the P21/a α-form15 and the C2/c β-form,16 Figure 1. In contrast, DL-Nva has disordered chains in its analogous polymorphs.13 The main difference between the 270 K P21 and the 290 K C2 quasiracemate structures is that every second molecular bilayer of the original 270 K AAAA stacking is shifted by one-half of the shortest unit-cell edge (i.e., by ∼2.40 Å, Table 1) and 1.33 Å along the 9.9 Å axis, Figure 4, forming an ABAB sequence at 290 K. While in the two corresponding regular racemates this is achieved in one single displacive transition, from α-DL-Nva (P21/c) to β-DLNva (C2/c) at 197 K upon heating and back at 188 K13 and with α14 DL-Nle/γ-DL-Nle at 391 K (heating) or 388 K (cooling), the
this was done to avoid unreasonable values as the result of strong correlation. In the presence of one dominating side-chain conformation (occupancy >0.5), a single set of coordinates was used for the amino and carboxylate groups, otherwise whole molecule disorder was applied. The covalent geometries of the independent disorder parts were linked by SHELXTL SAME 0.004 0.006 commands that restrained corresponding covalent bonds (not involving H) to be similar within an effective standard deviation of 0.004 Å and 1,3-distances [e.g., d(X1···X3) in the fragment X1−X2−X3, X = C/N/O] to be similar within an effective standard deviation of 0.006 Å, effectively putting restraints on covalent angles. Furthermore, restraints on 1,2-, 1,3-, and 1,4-distances were applied for massively disordered structures above room temperature. Details are available in the cif files (https://summary.ccdc.cam.ac.uk/structure-summaryform).
3. RESULTS AND DISCUSSION 3.1. L-Nva:D-Nle. Upon warming at 20 K/min from 100 to 440 K, five endothermic events are registered by DSC. The first two are weak, ΔS = 2.0 and 2.2 J/(mol·K), peaking in this introductory scan at 280(1) and 289(1) K, respectively, and are marked 1 and 2 4977
DOI: 10.1021/acs.jpcb.5b01483 J. Phys. Chem. B 2015, 119, 4975−4984
Article
The Journal of Physical Chemistry B
Figure 3. Molecular structures of L-Nva:D-Nle at five temperatures with individual molecules in the asymmetric unit labeled in italics (A, B, ...). Thermal displacement ellipsoids are shown at the 50% probability level. Color depths reflect the occupancy of each conformation as listed in Table 4. H atoms are omitted for side-chain conformations with occupancy
