Sunnie Myung, Maren Pink, Mu Hyun Baik, David E Clemmer Acta Crystallogr C. 2005 Aug;61(Pt 8):o506-8. doi: 10.1107/S0108270105021001. Epub 2005 Jul 23.
In the structure of DL-proline, C5H9NO2, the molecules are connected via classical intermolecular N-H...O hydrogen bonds involving the amine and carboxyl groups [N...O = 2.7129 (15) and 2.8392 (16) A], and form chains along the b-axis direction and parallel to (-101). The chains are linked into sheets via weak non-classical hydrogen bonds. The conformation of the molecule and its packing are notably different from the monohydrated DL-proline form.
Martin Lutz, Ruud Bakker Acta Crystallogr C. 2003 Jan;59(Pt 1):m18-20. doi: 10.1107/s0108270102021832. Epub 2002 Dec 10.
The title compound, [ZnCl(2)(C(5)H(9)NO(2))(2)], crystallizes in the centrosymmetric space group C2/c with the Zn atom on a twofold axis. The two proline residues in any one complex thus have the same absolute configuration. Hydrogen bonding links the molecules into linear chains, which run in the crystallographic b direction. The proline residues within any one chain also have an identical absolute configuration.
3. Microbial Proline Racemase-Proline Dehydrogenase Cascade for Efficient Production of D-proline and N-boc-5-hydroxy-L-proline from L-proline
Fanfan Zhang, Shiwen Xia, Hui Lin, Jiao Liu, Wenxin Huang Appl Biochem Biotechnol. 2022 Sep;194(9):4135-4146. doi: 10.1007/s12010-022-03980-y. Epub 2022 May 30.
D-proline and N-boc-5-hydroxy-L-proline are key chiral intermediates in the production of eletriptan and saxagliptin, respectively. An efficient proline racemase-proline dehydrogenase cascade was developed for the enantioselective production of D-proline. It included the racemization of L-proline to DL-proline and the enantioselective dehydrogenation of L-proline in DL-proline. The racemization of L-proline to DL-proline used an engineered proline racemase (ProR). L-proline up to 1000 g/L could be racemized to DL-proline with 1 g/L of wet Escherichia coli cells expressing ProR within 48 h. The efficient dehydrogenation of L-proline in DL-proline was achieved using whole cells of proline dehydrogenase-producing Pseudomonas pseudoalcaligenes XW-40. Moreover, using a cell-recycling strategy, D-proline was obtained in 45.7% yield with an enantiomeric excess of 99.6%. N-boc-5-hydroxy-L-proline was also synthesized from L-glutamate semialdehyde, a dehydrogenated product of L-proline, in a 16.7% yield. The developed proline racemase-proline dehydrogenase cascade exhibits great potential and economic competitiveness for manufacturing D-proline and N-boc-5-hydroxy-L-proline from L-proline.