Pseudo-Prolines as a Molecular Hinge:  Reversible Induction ofcisAmide Bonds into Peptide Backbones

Abstract

Serine, threonine-derived (4S)-oxazolidine-4-carboxylic acid, and cysteine-derived (4R)-thiazolidinecarboxylic acid, denoted pseudo-proline (Xaa[ΨR 1 ,R 2 pro]), serve as structure disrupting, solubilizing building blocks in peptide synthesis. Variation of the 2-C substituents within the heterocyclic system results in different physicochemical and conformational properties. NMR studies of a series of pseudo-proline (ΨPro)-containing peptides reveal a pronounced effect of the 2-C substituents upon the cis to trans ratio of the adjacent amide bond in solution. 2-C unsubstituted systems show a preference similar to that of the proline residue for the trans form, whereas 2,2-dimethylated derivatives adopt the cis amide conformation in high content. For 2-monosubstituted ΨPro, the cis−trans distribution depends on the 2-C chirality. For the 2-(S)-diastereoisomer, both forms are similarly populated in solution, whereas the 2-(R)-epimer adopts preferentially the trans form. The results are supported by conformational energy calculations and suggest that, by tailoring the degree of substitution, pseudo-prolines may serve as a temporary proline mimetic or as a hinge in peptide backbones.

Solid-State and Solution-Phase Conformations of Pseudoproline-Containing Dipeptides

“…We positioned Val, Phe, and Gly in the Xaa position to examine the effect the Xaa side chain had on the cis:trans ratio of the Xaa-Thr( Me,Me pro) amide bond. We included four d-allo-threonine derivatives (5, 6, 9, and 12) because, while the influence of inverting the relative stereochemistry at the α-position of the residue preceding a Thr( Me,Me pro) has been reported, [4] the effect of changing the relative stereochemistry between the α-and β-positions on Thr( Me,Me pro) formation and conformation has not been previously investigated. Our standard conditions for the synthesis of XaaThr( Me,Me pro) derivatives involve treatment of the Thrcontaining dipeptides with 2-methoxypropene at 0 • C in the presence of an acid catalyst.…”

“…These modified amino acid residues were introduced by Mutter and coworkers as temporary protecting groups for peptide synthesis and were found to exert a pronounced effect on the peptide backbone conformation. [1,2] The oxazolidine derivatives are now extensively used as tools to impart better solubility to a growing peptide chain in solid-phase synthesis because they prevent aggregation of growing peptide chains [3,4] and have been found to significantly increase the yields of difficult peptide sequences [5][6][7][8][9] by improving solvation and peptide coupling kinetics. This is attributed to a conformational preference for cis-amide bond formation.…”

“…This is attributed to a conformational preference for cis-amide bond formation. The ability of R,R pro residues to induce cis-amide bonds is well established by experimental [4,10,11] and theoretical [12] studies. In particular, disubstitution at the 2-C position (e.g.…”

Probing the self‐association, intermolecular contacts, and folding propensity of amelogenin

“…Previous studies have shown that Pro residues can experience cistrans interconversion or function as a molecular ''hinge'' within polypeptide sequence segments 31,41,42,45 and that TFE affects these parameters. 28,29,33,38 Hence, this responsiveness to TFE indicates that this second group of Pro residues act as folding enhancers and play an important role in the structure and dynamics 31,41,42,45 of the intrinsically disordered protein, rP172. These Pro residues may also be responsible for the increased level of conformational exchange observed in the 70-rP172 sample relative to AQ-rP172 as evidenced by the presence of additional HSQC crosspeaks in 70-rP172 ( Figure S1, Electronic Supporting Information).…”

“…1 ppm 9,36,37 we find that only a limited set of seven Pro residues (P2, N-terminus; P127, P130, P139, P154, central domain; P157, P162, C-terminus) respond to TFE, with the largest effects observed within the C-terminal half of this protein. Since the Pro imido ring Ca and Cb chemical shifts are sensitive to polypeptide backbone conformation, 38,41,42 we infer that TFE has a select effect on the conformation of these particular prolines, but that the remaining detectable Pro residues within rP172 are resistant to TFE-induced conformational change.…”