Redetermination of the solvent-free crystal structure of l-proline

l-proline crystallized, in its zwitterionic form, without the inclusion of any solvent or water molecules through the slow diffusion of diethyl ether into a saturated solution of l-proline in ethanol. In the crystal, the molecules are linked via N—H⋯O hydrogen bonds, resulting in a two-dimensional network.


Chemical context
There are 20 proteinogenic amino acids that form the basis of life. Like most amino acids, l-proline predominantely exists in the zwitterionic form (Boldyreva, 2008;Gö rbitz, 2015). Among those proteinogenic amino acids, l-proline is the only compound featuring a secondary amine that can have a significant influence on the structure of proteins and peptides. For example, l-proline is responsible for the secondary structure of collagen (Hutton et al., 1966) and often acts as a structural disruptor, which leads to structural changes from helical to coil (Tompa, 2002). Another remarkable influence of the secondary amine can be derived from the hydrogenbonding pattern in the solid state. Amino acids with primary amino groups commonly form bilayers incorporating two antiparallel hydrogen-bonded sheets. In contrast, the secondary amino groups in l-proline and its derivatives usually form single-sheet layers, where the amino groups point in the same direction (Gö rbitz, 2015). Similar conclusions were also drawn relying on powder diffraction data (Seijas et al., 2010). Based on the comparison of 40 different amino acids featuring an endocyclic nitrogen atom, Gö rbitz concluded that small changes in the molecular composition can cause a significant change in the hydrogen-bonding pattern (Gö rbitz, 2015).
Within the last decade, l-proline has also attracted significant attention in the field of organic chemistry as an organocatalyst. Following earlier reports on the application of lproline in the Hajos-Parrish-Eder-Sauer-Wiechert reaction (Eder et al., 1971;Hajos & Parrish, 1974), l-proline was rediscovered as an excellent catalyst for asymmetric aldol reactions (List et al. 2000;Feng et al., 2015). Today, proline and various derivatives are frequently used catalysts that are routinely employed for many different transformations including aldol, Mannich, Diels-Alder or epoxidation reactions (Mukherjee et al., 2007). ISSN 2056-9890 So far, crystal structures with R 1 values of less than 0.10 have been published for 19 of the 20 proteinogenic amino acids (Gö rbitz, 2015). However, for l-proline, the only available crystal structure without inclusions dates from 1965 and features a significantly worse R 1 value of 0.169 (Kayushina & Vainshtein, 1965). To overcome this limitation for the last proteinogenic amio acid, we recently succeeded in determining the crystal structure of l-proline without any inclusions with significantly improved R 1 values.

Structural commentary
l-Proline crystallized in its zwitterionic form: the oxygen atoms of the carboxylic acid (O1 and O2) are deprotonated and accordingly, the amine nitrogen atom N1 is protonated. The pyrrolidine ring within the title compound adopts a slightly bent envelope conformation with the C2 atom out of the plane (Fig. 1). Comparing the obtained values with previously reported crystal structures of enantiomerically pure l-and d-proline, the racemic compound, as well as the cocrystalized structures, only marginal differences can be observed for the distances N1-C1, N1-C4, and C1-C5 as well as for the binding angles C4-N1-C1 and N1-C1-C5. This indicates that the inclusion of solvents and formation of co-crystals does not influence the structural properties of proline significantly.

Supramolecular features
As a secondary amine, l-proline carries two hydrogen atoms at the nitrogen atom N1 in its zwitterionic form. These two hydrogen atoms each interact with one of the oxygen atoms of the carboxylic groups (O1 and O2). The different hydrogenbond parameters between the proline molecules are shown in Table 1. As shown in Fig. 2, these hydrogen bonds result in the formation of a single-sheet architecture within the ab plane (also termed sheet L1 in Gö rbitz, 2015). This structure is addionaly stabilized by hydrophobic interactions between the C-H bonds of the pyrrolidine substructure (see Fig. 2). In comparison, the hydrogen-bonding pattern of isoleucin (DAILEU01: Varughese & Srinivasan, 1975) as a typical example of an amino acid with a primary amino group features a double-sheet structure where the hydrophobic and hydrophilic parts interact with each other (Fig. 3). Therefore, the hydrogen-bonding pattern observed for l-proline once again illustrates why proline is considered to be a structural disruptor in proteins. However, as already pointed out above, small structural changes can have a signifcant influence, as the addition of a hydroxy group in 3-hydroxyproline results in the formation of bands in the supramolecular structure (HOPROL12: Koetzle et al., 1973). This again highlights how even small changes such as the addition of a hydroxy group can change the packing in the crystal structure.  Table 1 Hydrogen-bond geometry (Å , ). (3) 165 (3) Symmetry codes: (i) Àx þ 1; y þ 1 2 ; Àz þ 1 2 ; (ii) x þ 1; y; z.

Figure 2
View along the c axis (left) and the a axis (right) showing that l-proline forms layers through hydrogen bonding between the carboxylic group O1 respectively O2 and amine N1.

Figure 1
The molecular structure of the title compound l-proline. Displacement ellipsoids are drawn at the 50% probability level. entry refers to the single crystal of the pure l-isomer without any inclusions (PROLIN: Kayushina & Vainshtein, 1965). However, the determination of this crystal structure was performed in 1965. Nevertheless, Kayushina and Vainshtein could identify the space group as P2 1 2 1 2 1 and determine the cell parameters with a = 5.20 Å , b = 9.02 Å , c = 11.55 Å , which are good, but could be determined with higher precision in this study. Furthermore, the R 1 value has now improved substantially to 0.039. Seijas et al. (2010) investigated the powder diffraction data of enantiopure l-proline and obtained an R 1 value of 0.089 with similar structural features. They further compared the four pseudopolymorphs of l-proline, l-proline monohydrate, dl-proline and dl-proline monohydrate and concluded that all show a layered packing, which is stabilized by van der Waals interactions (PROLIN01: Seijas et al., 2010). Besides the single entry for enantiopure l-proline, one entry refers to enantiopure l-proline with the inclusion of water (RUWGEV: Janczak & Luger, 1997), two entries refer to the racemic compound (QANRUT: Myung et al., 2005;QANRUT01: Hayashi et al., 2006) and the racemic product with water (DLPROM01: Padmanabhan et al., 1995;DLPROM02: Flaig et al., 2002) or chloroform (WERMIQ: Klussmann et al., 2006). The enantiopure l-proline was also crystallized with inclusions of p-aminobenzoic acid (CIDBOH: Athimoolam & Natarajan, 2007), 1,1-dicyano-2-

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms bonded to carbon were placed with idealized geometry and refined using a riding model with C-H = 0.95 Å , U iso (H) = 1.2 U eq (C) for CH, C-H = 0.99 Å U iso (H) = 1.2U eq (C) for CH 2 , C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for CH 3 . N-bound H atoms were located in a difference electron map and refined isotropically. Hydrophilic and hydrophobic layers in the crystal structure of isoleucin (DAILEU01: Varughese & Srinivasan, 1975).

(S)-Pyrrolidine-2-carboxylic acid
Crystal data C 5 H 9 NO 2 M r = 115.13 Orthorhombic, P2 1 2 1 2 1 Hall symbol: P 2ac 2ab a = 5.2794 (4) Å b = 8.8686 (6) Å c = 11.5321 (9) Å V = 539.94 (7)  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )