Conformational ensembles of flexible beta-turn mimetics in DMSO-d(6)

b-Turns play an important role in peptide and protein chemistry, biophysics, and bioinformatics. The aim of this research was to study short linear peptides that have a high propensity to form b-turn structures in solution. In particular, we examined conformational ensembles of b-turn forming peptides with a general sequence CBz-L-Ala-L-Xaa-Gly-L-Ala-OtBu. These tetrapeptides, APGA, A(4R)MePGA, and A(4S)MePGA, incorporate proline, (4R)-methylproline, and (4S)-methylproline, respectively, at the Xaa position. To determine the influence of the 4-methyl substituted prolines on the b-turn populations, the NAMFIS (NMR analysis of molecular flexibility in solution) deconvolution analysis for these three peptides were performed in DMSO-d6 solution. The NBO (natural bond orbital) method was employed to gain further insight into the results obtained from the NAMFIS analysis. The emphasis in the NBO analysis was to characterize remote intramolecular interactions that could influence the backbone-backbone interactions contributing to b-turn stability. NAMFIS results indicate that the enantiospecific incorporation of the methyl substituent at the Cg (C4) position of the proline residue can be used to selectively control the pyrrolidine ring puckering propensities and, consequently, the preferred f,y angles associated with the proline residue in b-turn forming peptides. The NAMFIS analyses show that the presence of (4S)-methylproline in A(4S)MePGA considerably increased the type II b-turn population with respect to APGA and A(4R)MePGA. The NBO calculations suggest that this observation can be rationalized based on an n→p* interaction between the N-terminus alanine carbonyl oxygen and the proline carbonyl group. Several other interactions between remote orbitals in these peptides provide a more detailed explanation for the observed population distributions.Peer reviewe

Locked Conformations for Proline Pyrrolidine Ring: Synthesis and Conformational Analysis of cis- and trans-4-tert-4Butylprolines

The motional restrictions of the proline pyrrolidine ring allow this secondary amine amino acid to act as a turn inducer in many peptides and proteins. The pyrrolidine ring is known to exhibit two predominant pucker modes (i.e., C-4 (Cç) exo and endo envelope conformers whose ratio can be controlled by proper substituents in the ring). In nature, the exo puckered 4(R)-hydroxy-L-proline plays a crucial role as a building block in collagen and collagen-like structures. It has been previously concluded that the electronegativity of the 4-cis-substituent increases the endo puckering while the electronegativity of the 4-trans-substituent favors the exo puckering. Here, we have introduced a sterically demanding tert-butyl group at C-4 in trans- and cis-configurations. In the case of transsubstitution, the induced puckering effect on the pyrrolidine ring was studied with X-ray crystallography and 1H NMR spectral simulations. Both cis- and trans-4-tert-butyl groups strongly favor pseudoequatorial orientation, therebycausing opposite puckering effects for the pyrrolidine ring, cis-exo and trans-endo for L-prolines, in contrast to the effects observed in the case of electronegative C-4 substituents. The syntheses and structural analysis are presented for the conformationally constrained 4-tert-butylprolines. The prolines were synthesized from 4-hydroxy-L-proline, substitution with t-BuCuSPhLi being the key transformation. This reaction gave N-Boctrans-4-tert-butyl-L-proline tert-butyl ester in 94% ee and 57% de. Enantioselectivity was increased to 99.2% ee by crystallization of N-Boc-trans-4-tert-butyl-L-proline in the final step of the synthesis.Peer reviewe