Molecular Structure of Proline

Abstract

The molecular structures of the two lowest-energy conformers of proline, Pro-I and Pro-II, have been characterized by ab initio electronic structure computations. An extensive MP2/6-31G* quartic force field for Pro-I, containing 62,835 unique elements in the internal coordinate space, was computed to account for anharmonic vibrational effects, including total zero-point contributions to isotopomeric rotational constants. New re and improved r0 least-squares structural refinements were performed to determine the heavy-atom framework of Pro-I, based on experimentally measured (A. Lesarri, S. Mata, E. J. Cocinero, S. Blanco, J. C. Lopez, J. L. Alonso, Angew. Chem. 2002, 114, 4867; Angew. Chem. Int. Ed. 2002, 41, 4673) rotational constant sets of nine isotopomers and our ab initio data for structural constraints and zero-point vibrational (ZPV) shifts. Without the ab initio constraints, even the extensive set of empirical rotational constants cannot satisfactorily fix the molecular structure of the most stable conformer of proline, a 17-atom molecule with no symmetry. After imposing the ab initio constraints, excellent agreement between theory and experiment is found for the heavy-atom geometric framework, the root-mean-square (rms) residual of the empirical rotational constant fit being cut in half by adding ZPV corrections. The most significant disparity, about 0.07 A, between the empirical and the best ab initio structures, concerns the r(N...H) distance of the intramolecular hydrogen bond. Some of the experimental quartic centrifugal distortion constants assigned to Pro-II have been corrected based on data obtained from a theoretical force field.

Molecular structures of the two most stable conformers of free glycine

“…5 to derive the equilibrium structure of Gly-IIn, for which all dependable ab initio computations, including those of the present study, indicate a nonplanar atomic arrangement. Following our recent successful determination of the r BO e structure for the considerably larger and even less rigid amino acid, L-proline, 15 here we report results of a similar study on the two lowest-energy conformers of neutral Gly.…”

“…29 In our previous work on proline, 15 we found the 6-31G* MP2 method to be both a sufficiently accurate and economical approach to computing zero-point vibrational effects on the structures of larger molecules. To ensure accuracy of the anharmonic force fields of glycine, geometries were tightly optimized to better than 10 À6 Å and 10 À5 degrees, and analytic second derivative techniques were invoked.…”

Photoelectron spectra and structures of three cyclic dipeptides: PhePhe, TyrPro, and HisGly

“…In proline, the four lowest energy conformers are constructed from two structural elements: the rotation of the carboxylic acid group and the puckering of the pyrrolidine ring. 21 The carboxylic group is not present in the dipeptide, so we expect that the main conformational element will be the puckering of the proline ring to an "up" or "down" geometry with respect to the twist in the DKP ring, giving rise to two possible conformers. In addition, the phenol moiety of cTyrPro contributes to the conformational landscape.…”

Asymmetric Organocatalysis: A New Stream in Organic Synthesis

“…Indeed, there is a steadily growing number of such organic compounds included in this list, the details for some of which are outlined in the following sections. 56] l-Proline is perhaps the most well-known organocatalyst. Although the natural l-form is normally used, proline is available in both enantiomeric forms [57], this being somewhat of an asset when compared to enzymatic catalysis [58].…”