Ring puckering of the pyrrolidine ring of poly (l-proline) form I as studied by variable-temperature high-resolution 13C NMR spectroscopy

A curious case of dynamic disorder in pyrrolidine rings elucidated by NMR crystallography

A pharmaceutical exhibits differing dynamics in crystallographically distinct pyrrolidine rings despite being nearly related by symmetry, with one performing ring inversions while the other is constrained to torsional librations. Using ¹³C solid-state magic-angle spinning (MAS) NMR and DFT calculations, we show that this contrast originates from C-HH-C close contacts and less efficient C-Hπ intermolecular interactions observed in the transition state of the constrained pyrrolidine ring, highlighting the influence of the crystallographic environment on the molecular motion.

Conformational analysis of L-prolines in water

The results of the ring conformational analysis of L-proline, N-acetyl-L-proline, and trans-4-hydroxy-L-proline by NMR combined with calculations using density functional theory (DFT) and molecular dynamics (MD) are reported. Accurate values of 1H-1H J-couplings in water and other solvents have been determined. Using a two-site equilibrium model, the Cgamma-endo conformer of L-proline in water has been identified as intermediate between gammaTdelta [twist(Cgamma-endo, Cdelta-exo)] and gammaE [envelope(Cgamma-endo)] and the Cgamma-exo conformer as betaTgamma. Both conformers were equally populated at room temperature. The N-acetyl [cis-rotamer gammaTbeta(80%)/gammaE(20%) and trans-rotamer gammaTbeta(61%)/gammaE(39%)] and 4-hydroxy (gammaEpsilon) derivatives showed significant changes in both the population and the geometries of the preferred ring conformers. The combination of NMR predicted populations with the DFT B3LYP/6-311+G(2d,p)/IEFPCM calculations proved successful, resulting in fairly accurate predictions of J-couplings. Simulations using MD were mostly in favor of the two-site equilibrium model between Cgamma-endo and Cgamma-exo conformers, similar to that used for the analysis of NMR J-couplings. Various force fields examined for MD simulations failed to reproduce the ring conformational geometries and populations of L-proline in water accurately, while significantly better agreement with NMR was found for trans-N-acetyl-L-proline using GROMOS and AMBER force fields.

Conformational Preferences of Proline Oligopeptides

A conformational study on the terminally blocked proline oligopeptides, Ac-(Pro)(n)()-NMe(2) (n = 2-5), is carried out using the ab initio Hartree-Fock level of theory with the self-consistent reaction field method in the gas phase and in solutions (chloroform, 1-propanol, and water) to explore the preference and transition between polyproline II (PPII) and polyproline I (PPI) conformations depending on the chain length, the puckering, and the solvent. The mean differences in the free energy per proline of the up-puckered conformations relative to the down-puckered conformations for both diproline and triproline increases for the PPII-like conformations and decreases for the PPI-like conformations as the solvent polarity increases. These calculated results indicate that the PPII-like structures have preferentially all-down puckerings in solutions, whereas the PPI-like structures have partially mixed puckerings. The free energy difference per proline residue between the PPII- and PPI-like structures decreases as the proline chain becomes longer in the gas phase but increases as the proline chain becomes longer in solutions and the solvent polarity increases. In particular, our calculated results indicate that each of the proline oligopeptides can exist as an ensemble of conformations with the trans and cis peptide bonds in solutions, although the PPII-like structure with all-trans peptide bonds is dominantly preferred, which is reasonably consistent with the previously observed results. In diproline Ac-(Pro)(2)-NMe(2), the rotational barrier to the cis-to-trans isomerization for the first prolyl peptide bond increases as the solvent polarity increases, whereas the rotational barrier for the second prolyl peptide bond does not show the monotonic increase as the solvent polarity increases. When the rotational barriers for these two prolyl peptide bonds were compared, it could be deduced that the conformational transition from PPI with the cis peptide bond to PPII with the trans peptide bond is initiated at the C-terminus and proceeds to the N-terminus in water. This is consistent with the results from NMR experiments on polyproline in D(2)O but opposite to the results from enzymatic hydrolysis kinetics experiments on polyproline.

Solid-state NMR studies of collagen-based parchments and gelatin

Historical collagen-based parchments have been studied by solid-state NMR. In addition, new parchment (produced according to traditional methods) and gelatin from bovine skin were also studied. Wideline 1H and MAS 13C measurements were carried out directly on intact parchments. A simple approach is proposed for evaluation of the extent of parchment degradation based on the linewidth changes in the 13C CPMAS spectra relative to new parchment and gelatin. Structural (bound) water content was estimated from wideline 1H NMR lineshape and relaxation time measurements. It was found that the relative water content in parchments correlates linearly with 13C MAS linewidths. Its decrease on parchment degradation indicates that structural water molecules are of primary importance in stabilizing higher order collagen structures. Backbone and side chain dynamics of collagen in parchments were compared to those of gelatin based on the 13C dipolar-dephased experiments. Carbonyl 13C chemical shift anisotropies were measured to deduce the geometry of the collagen backbone motion. Unlike previous studies, we found that the collagen backbone motion is similar to that found in other proteins and occurs primarily via small-angle librations about internal bond directions.