Conformational preference and cis-trans isomerization of 4-methylproline residues

Imidazole-amino acids. Conformational switch under tautomer and pH change

Replacement of the main chain peptide bond by imidazole ring seems to be a promising tool for the peptide-based drug design, due to the specific prototropic tautomeric as well as amphoteric properties. In this study, we present that both tautomer and pH change can cause a conformational switch of the studied residues of alanine (1-4) and dehydroalanine (5-8) with the C-terminal peptide group replaced by imidazole. The DFT methods are applied and an environment of increasing polarity is simulated. The conformational maps (Ramachandram diagrams) are presented and the stability of possible conformations is discussed. The neutral forms, tautomers τ (1) and π (2), adapt the conformations αRτ (φ, ψ = - 75°, - 114°) and C7eq (φ, ψ = - 75°, 66°), respectively. Their torsion angles ψ differ by about 180°, which results in a considerable impact on the peptide chain conformation. The cation form (3) adapts both these conformations, whereas the anion analogue (4) prefers the conformations C5 (φ, ψ = - 165°, - 178°) and β2 (φ, ψ ~ - 165°, - 3°). Dehydroamino acid analogues, the tautomers τ (5) and π (6) as well as the anion form (8), have a strong tendency toward the conformations β2 (φ, ψ = - 179°, 0°) and C5 (φ, ψ = - 180°, 180°). The preferences of the protonated imidazolium form (7) depend on the environment. The imidazole ring, acting as a donor or acceptor of the hydrogen bonds created within the studied residues, has a profound effect on the type of conformation.

Thiazole-amino acids: influence of thiazole ring on conformational properties of amino acid residues

Post-translational modified thiazole-amino acid (Xaa-Tzl) residues have been found in macrocyclic peptides (e.g., thiopeptides and cyanobactins), which mostly inhibit protein synthesis in Gram + bacteria. Conformational study of the series of model compounds containing this structural motif with alanine, dehydroalanine, dehydrobutyrine and dehydrophenylalanine were performed using DFT method in various environments. The solid-state crystal structure conformations of thiazole-amino acid residues retrieved from the Cambridge Structural Database were also analysed. The studied structural units tend to adopt the unique semi-extended β2 conformation; which is stabilised mainly by N-H⋯NTzl hydrogen bond, and for dehydroamino acids also by π-electron conjugation. The conformational preferences of amino acids with a thiazole ring were compared with oxazole analogues and the role of the sulfur atom in stabilising the conformations of studied peptides was discussed.

Polarity effects in 4-fluoro- and 4-(trifluoromethyl)prolines

Fluorine-containing analogues of proline are valuable tools in engineering and NMR spectroscopic studies of peptides and proteins. Their use relies on the fundamental understanding of the interplay between the substituents and the main chain groups of the amino acid residue. This study aims to showcase the polarity-related effects that arise from the interaction between the functional groups in molecular models. Properties such as conformation, acid-base transition, and amide-bond isomerism were examined for diastereomeric 4-fluoroprolines, 4-(trifluoromethyl)prolines, and 1,1-difluoro-5-azaspiro[2.4]heptane-6-carboxylates. The preferred conformation on the proline ring originated from a preferential axial positioning for a single fluorine atom, and an equatorial positioning for a trifluoromethyl- or a difluoromethylene group. This orientation of the substituents explains the observed trends in the pK a values, lipophilicity, and the kinetics of the amide bond rotation. The study also provides a set of evidences that the transition state of the amide-bond rotation in peptidyl-prolyl favors C4-exo conformation of the pyrrolidine ring.

Uncovering the Networks of Topological Neighborhoods in β-Strand and Amyloid β-Sheet Structures

Although multiple hydrophobic, aromatic π–π, and electrostatic interactions are proposed to be involved in amyloid fibril formation, the precise interactions within amyloid structures remain poorly understood. Here, we carried out detailed quantum theory of atoms-in-molecules (QTAIM) analysis to examine the hydrophobic core of amyloid parallel and antiparallel β-sheet structures, and found the presence of multiple inter-strand and intra-strand topological neighborhoods, represented by networks of through-space bond paths. Similar bond paths from side chain to side chain and from side chain to main chain were found in a single β-strand and in di- and tripeptides. Some of these bond-path networks were enhanced upon β-sheet formation. Overall, our results indicate that the cumulative network of weak interactions, including various types of hydrogen bonding (X-H—Y; X, Y = H, C, O, N, S), as well as non-H-non-H bond paths, is characteristic of amyloid β-sheet structure. The present study postulated that the presence of multiple through-space bond-paths, which are local and directional, can coincide with the attractive proximity effect in forming peptide assemblies. This is consistent with a new view of the van der Waals (vdW) interactions, one of the origins of hydrophobic interaction, which is updating to be a directional intermolecular force.

Which DFT levels of theory are appropriate in predicting the prolyl cis–trans isomerization in solution?

DFTs were assessed for the conformational preferences of the peptides containing Pro and its derivatives in chloroform and water.

Comparative effects of trifluoromethyl- and methyl-group substitutions in proline

What is the outcome of trifluoromethyl-/methyl-substitution in each position of the proline ring? Look inside to find out.

Effects of isosteric substitutions on the conformational preference and cis–trans isomerization of proline-containing peptides

Isosteric substitutions of the peptide CO group by CS and CSe groups increased thetranspopulation and rotational barrier to the prolylcis–transisomerization of proline-containing peptides.

Studies of viomycin, an anti-tuberculosis antibiotic: copper(ii) coordination, DNA degradation and the impact on delta ribozyme cleavage activity

Viomycin is a basic peptide antibiotic, which is among the most effective agents against multidrug-resistant tuberculosis.

Helix foldamers of γ-peptides based on 2-aminocyclopentylacetic acid

Oligo-γ-peptides based on 2-aminocyclopentylacetic acid (γAc₅a) with a cyclopentyl constraint on the C^β–C^γ bond and homochiral (1S,2S) configurations preferentially adopt the right-handed 14-helix foldamers in the gas phase and in solution.

Conformational properties of oxazole-amino acids: effect of the intramolecular N-H···N hydrogen bond

Oxazole ring occurs in numerous natural peptides, but conformational properties of the amino acid residue containing the oxazole ring in place of the C-terminal amide bond are poorly recognized. A series of model compounds constituted by the oxazole-amino acids occurring in nature, that is, oxazole-alanine (L-Ala-Ozl), oxazole-dehydroalanine (ΔAla-Ozl), and oxazole-dehydrobutyrine ((Z)-ΔAbu-Ozl), was investigated using theoretical calculations supported by FTIR and NMR spectra and single-crystal X-ray diffraction. It was found that the main feature of the studied oxazole-amino acids is the stable conformation β2 with the torsion angles φ and ψ of -150°, -10° for L-Ala-Ozl, -180°, 0° for ΔAla-Ozl, and -120°, 0° for (Z)-ΔAbu-Ozl, respectively. The conformation β2 is stabilized by the intramolecular N-H···N hydrogen bond and predominates in the low polar environment. In the case of the oxazole-dehydroamino acids, the π-electron conjugation that is spread on the oxazole ring and C(α)═C(β) double bond is an additional stabilizing interaction. The tendency to adopt the conformation β2 clearly decreases with increasing the polarity of environment, but still the oxazole-dehydroamino acids are considered to be more rigid and resistant to conformational changes.

Solvent effects on the conformational preferences of model peptoids. MP2 study

The influence of aqueous environment on the main-chain conformation (ω0 , ϕ, and ψ dihedral angles) of two model peptoids: N-acetyl-N-methylglycine N'-methylamide (Ac-N(Me)-Gly-NHMe) (1) and N-acetyl-N-methylglycine N',N'-dimethylamide (Ac-N(Me)-Gly-NMe₂) (2) was investigated by MP2/6-311++G(d,p) method. The Ramachandran maps of both studied molecules with cis and trans configuration of the N-terminal amide bond in the gas phase and in water environment were obtained and all energy minima localized. The polarizable continuum model was applied to estimate the solvation effect on conformation. Energy minima of the Ac-N(Me)-Gly-NHMe and Ac-N(Me)-Gly-NMe₂ have been analyzed in terms of the possible hydrogen bonds and C = O dipole attraction. To validate the theoretical results obtained, conformations of the similar structures gathered in the Cambridge Crystallographic Data Centre were analyzed. Obtained results indicate that aqueous environment in model peptoids 1 and 2 favors the conformation F (ϕ and ψ = -70º, 180º), and additionally significantly increases the percentage of structures with cis configuration of N-terminal amide bond in studied compounds.