Cα-Methyl proline: A unique example of split personality

4-Methylpseudoproline analogues of cyclolinopeptide A: Synthesis, structural analysis and evaluation of their suppressive effects in selected immunological assays

The synthesis of new analogues of cyclolinopeptide A (CLA) and their linear precursors modified with (R)- and (S)-4-methylpseudoproline in the Pro³-Pro⁴ fragment are presented. The peptides were tested in comparison with cyclosporine A (CsA) in concanavalin A (Con A) and pokeweed mitogen (PWM)-induced mouse splenocyte proliferation and in secondary humoral immune response in vitro to sheep erythrocytes (SRBC). Their effects on expression of selected signaling molecules in the Jurkat T cell line were also determined. In addition, the structural features of the peptides, applying nuclear magnetic resonance and circular dichroism, were analyzed. The results showed that only peptides 7 and 8 modified with (R)-4-methylpseudoproline residue (c(Leu¹-Val²-(R)-(αMe)Ser(ΨPro)³-Pro⁴-Phe⁵-Phe⁶-Leu⁷-Ile⁸-Ile⁹) and c(Leu¹-Val²-Pro³-(R)-(αMe)Ser(ΨPro)⁴-Phe⁵-Phe⁶-Leu⁷-Ile⁸-Ile⁹), respectively) strongly suppressed mitogen-induced splenocyte proliferation and the humoral immune response, with peptide 8 being more potent. Likewise, peptide 8 more strongly elevated expression of Fas, a proapoptotic signaling molecule in Jurkat cells. We postulate that the increased biological activity of peptide 8, compared to the parent molecule and other studied peptides, resulted from its more flexible structure, found on the basis of both CD and NMR studies. CD and NMR spectra showed that replacement of Pro³ by (R)-(αMe)Ser(¬Pro) caused much greater conformational changes than the same replacement of the Pro⁴ residue. Such a modification could lead to increased conformational freedom of peptide 8, resulting in a greater ability to adopt a more compact structure, better suited to its putative receptor. In conclusion, peptide 8 is a potent immune suppressor which may find application in controlling immune disorders.

Conformational landscape of substituted prolines

The cyclic side chain of the amino acid proline confers unique conformational restraints on its backbone and side chain dihedral angles. This affects two equilibria-one at the backbone (cis/trans) and the other at the side chain (endo/exo). Substitutions on the proline ring impose additional steric and stereoelectronic effects that can further modulate both these equilibria, which in turn can also affect the backbone dihedral angle (ϕ, ψ) preferences. In this review, we have explored the conformational landscape of several termini capped mono-(2-, 3-, 4-, and 5-) substituted proline derivatives in the Cambridge Structural Database, correlating observed conformations with the nature of substituents and deciphering the underlying interactions for the observed structural biases. The impact of incorporating these derivatives within model peptides and proteins are also discussed for selected cases. Several of these substituents have been used to introduce bioorthogonal functionality and modulate structure-specific ligand recognition or used as spectroscopic probes. The incorporation of these diversely applicable functional groups, coupled with their ability to define an amino acid conformation via stereoelectronic effects, have a broad appeal among chemical biologists, molecular biophysicists, and medicinal chemists.

A novel peptide conformation: the γ-bend ribbon

Unlike the extensively investigated relationship between the peptide β-bend ribbon and its prototypical 310-helix conformation, the corresponding relationship between the narrower γ-bend ribbon and its regular γ-helix counterpart still remains to be studied, as the latter 3D-structures have not yet been experimentally authenticated. In this paper, we describe the results of the first characterization, both in the crystal state and in solution, of the γ-bend ribbon conformation using X-ray diffraction and FT-IR absorption, electronic CD and 2D-NMR spectroscopies applied to an appropriate set of synthetic, homo-chiral, sequential dipeptide oligomers based on (S)-Ala and the known γ-bend inducer, Cα-tetrasubstituted, N-alkylated α-amino acid residue (S)-Cα-methyl-azetidine-carboxylic acid.

A structural and mechanistic study of π-clamp-mediated cysteine perfluoroarylation

Natural enzymes use local environments to tune the reactivity of amino acid side chains. In searching for small peptides with similar properties, we discovered a four-residue π-clamp motif (Phe-Cys-Pro-Phe) for regio- and chemoselective arylation of cysteine in ribosomally produced proteins. Here we report mutational, computational, and structural findings directed toward elucidating the molecular factors that drive π-clamp-mediated arylation. We show the significance of a trans conformation prolyl amide bond for the π-clamp reactivity. The π-clamp cysteine arylation reaction enthalpy of activation (ΔH^‡) is significantly lower than a non-π-clamp cysteine. Solid-state NMR chemical shifts indicate the prolyl amide bond in the π-clamp motif adopts a 1:1 ratio of the cis and trans conformation, while in the reaction product Pro3 was exclusively in trans. In two structural models of the perfluoroarylated product, distinct interactions at 4.7 Å between Phe1 side chain and perfluoroaryl electrophile moiety are observed. Further, solution ¹⁹F NMR and isothermal titration calorimetry measurements suggest interactions between hydrophobic side chains in a π-clamp mutant and the perfluoroaryl probe. These studies led us to design a π-clamp mutant with an 85-fold rate enhancement. These findings will guide us toward the discovery of small reactive peptides to facilitate abiotic chemistry in water.

Helical screw-sense preferences of peptides based on chiral, Cα-tetrasubstituted α-amino acids

The preferred helical screw senses of chiral α-amino acids with a C(α)-tetrasubstituted α-carbon atom, as determined in the crystal state by X-ray diffraction analyses on derivatives and peptides, are reviewed. This survey covers C(α)-methylated and C(α)-ethylated α-amino acids, as well as α-amino acids cyclized on the α-carbon, including those characterized by the combination of lack of chirality at the α-carbon with either side-chain or axial chirality. Although, in general, chiral C(α)-tetrasubstituted α-amino acids show a less pronounced bias toward a single helical screw sense than their proteinogenic (C(α)-trisubstituted) counterparts, our analysis highlights significant differences in terms of magnitude and direction of such a bias among the various sub-families of residues, and between individual amino acids within each sub-family as well. The experimental findings can be rationalized, at least in part, on the basis of steric considerations.

Substitution of proline32 by α-methylproline preorganizes β2-microglobulin for oligomerization but not for aggregation into amyloids

Conversion of soluble folded proteins into insoluble amyloids generally proceeds in three distinct mechanistic stages: (1) initial protein misfolding into aggregation-competent conformers, (2) subsequent formation of oligomeric species and, finally, (3) self-assembly into extended amyloid fibrils. In the work reported herein, we interrogated the amyloidogenesis mechanism of human β2-microglobulin (β2m), which is thought to be triggered by a pivotal cis-trans isomerization of a proline residue at position 32 in the polypeptide, with nonstandard amino acids. Using chemical protein synthesis we prepared a β2m analogue in which Pro32 was replaced by the conformationally constrained amino acid α-methylproline (MePro). The strong propensity of MePro to adopt a trans prolyl bond led to enhanced population of a non-native [trans-MePro32]β2m protein conformer, which readily formed oligomers at neutral pH. In the presence of the antibiotic rifamycin SV, which inhibits amyloid growth of wild-type β2m, [MePro32]β2m was nearly quantitatively converted into different spherical oligomeric species. Self-assembly into amyloid fibrils was not observed in the absence of seeding, however, even at low pH (<3), where wild-type β2m spontaneously forms amyloids. Nevertheless, we found that aggregation-preorganized [MePro32]β2m can act in a prion-like fashion, templating misfolded conformations in a natively folded protein. Overall, these results provide detailed insight into the role of cis-trans isomerization of Pro32 and ensuing structural rearrangements that lead to initial β2m misfolding and aggregation. They corroborate the view that conformational protein dynamics enabled by reversible Pro32 cis-trans interconversion rather than simple population of the trans conformer is critical for both nucleation and subsequent growth of β2m amyloid structures.

Single and multiple peptide γ-turns: literature survey and recent progress

Published data on peptide isolated and repetitive γ-turns are reviewed. Advancements in our laboratories on these 3D-structures are also presented.

Is the backbone conformation of C(alpha)-methyl proline restricted to a single region?

C(alpha)-methyl-L-proline, or L-(alphaMe)Pro, is probably the most conformationally constrained alpha-amino acid. In particular, its omega and phi torsion angles are restricted to about 180 and -60 degrees, respectively, and only three ranges of values are theoretically available for psi in mono- or longer peptides, namely, about -30 degrees (cis', 3(10)/alpha-helical structure), 60 degrees (inverse gamma turn), or 140 degrees (trans', poly(L-Pro)(n) II structure). In this work, we examined the tendency of a number of N(alpha)-acyl dipeptide N'-alkylamides of the type RCO-(alphaMe)Pro-Xxx-NHR' or RCO-Xxx-(alphaMe)Pro-NHR', in which Xxx is L (or D)-Ala, Aib (alpha-aminoisoburyric acid), or L (or D)-(alphaMe)Pro, long enough to fold into intramolecularly hydrogen-bonded gamma or beta turns. The results are compared with those obtained for the corresponding dipeptides based on Pro, a well-known turn-forming residue. For the crystal-state 3D-structural analysis we used X-ray diffraction, whereas our solution conformational analysis was heavily based on the FTIR absorption and (1)H and (13)C NMR spectroscopy techniques. We conclude that (alphaMe)Pro is able to explore both trans' and cis' psi areas of the conformational space, but in (alphaMe)Pro the latter is overwhelmingly more populated, in marked contrast to the Pro preference. This finding is a clear indication that in (alphaMe)Pro the major 3D-structural determinant is the C(alpha)-methyl group. The circular dichroism (CD) signature of a peptide type III' beta-turn conformation is also proposed.