Probing the γ-Turn in a Short Proline Dipeptide Chain

Proline Analogues

Within the canonical repertoire of the amino acid involved in protein biogenesis, proline plays a unique role as an amino acid presenting a modified backbone rather than a side-chain. Chemical structures that mimic proline but introduce changes into its specific molecular features are defined as proline analogues. This review article summarizes the existing chemical, physicochemical, and biochemical knowledge about this peculiar family of structures. We group proline analogues from the following compounds: substituted prolines, unsaturated and fused structures, ring size homologues, heterocyclic, e.g., pseudoproline, and bridged proline-resembling structures. We overview (1) the occurrence of proline analogues in nature and their chemical synthesis, (2) physicochemical properties including ring conformation and cis/trans amide isomerization, (3) use in commercial drugs such as nirmatrelvir recently approved against COVID-19, (4) peptide and protein synthesis involving proline analogues, (5) specific opportunities created in peptide engineering, and (6) cases of protein engineering with the analogues. The review aims to provide a summary to anyone interested in using proline analogues in systems ranging from specific biochemical setups to complex biological systems.

Quantum chemistry meets high-resolution spectroscopy for characterizing the molecular bricks of life in the gas-phase

Computation of accurate geometrical structures and spectroscopic properties of large flexible molecules in the gas-phase is tackled at an affordable cost using a general exploration/exploitation strategy. The most distinctive feature of the approach is the careful selection of different quantum chemical models for energies, geometries and vibrational frequencies with the aim of maximizing the accuracy of the overall description while retaining a reasonable cost for all the steps. In particular, a composite wave-function method is used for energies, whereas a double-hybrid functional (with the addition of core-valence correlation) is employed for geometries and harmonic frequencies and a cheaper hybrid functional for anharmonic contributions. A thorough benchmark based on a wide range of prototypical molecular bricks of life shows that the proposed strategy is close to the accuracy of state-of-the-art composite wave-function methods, and is applicable to much larger systems. A freely available web-utility post-processes the geometries optimized by standard electronic structure codes paving the way toward the accurate yet not prohibitively expensive study of medium- to large-sized molecules by experimentally-oriented researchers.

Side-Chain Polarity Modulates the Intrinsic Conformational Landscape of Model Dipeptides

The intrinsic conformational preferences of small peptides may provide additional insight into the thermodynamics and kinetics of protein folding. In this study, we explore the underlying energy landscapes of two model peptides, namely, Ac-Ala-NH₂ and Ac-Ser-NH₂, using geometry-optimization-based tools developed within the context of energy landscape theory. We analyze not only how side-chain polarity influences the structural preferences of the dipeptides, but also other emergent properties of the landscape, including heat capacity profiles, and kinetics of conformational rearrangements. The contrasting topographies of the free energy landscape agree with recent results from Fourier transform microwave spectroscopy experiments, where Ac-Ala-NH₂ was found to exist as a mixture of two conformers, while Ac-Ser-NH₂ remained structurally locked, despite exhibiting an apparently rich conformational landscape.

Intrinsic folding of the cysteine residue: competition between folded and extended forms mediated by the -SH group

A dual microwave and optical spectroscopic study of a capped cysteine amino acid isolated in a supersonic expansion, combined with quantum chemistry modelling, enabled us to characterize the conformational preferences of Cys embedded in a protein chain. IR/UV double resonance spectroscopy provided evidence for the coexistence of two conformers, assigned to folded and extended backbones (with classical C7 and C5 backbone H-bonding respectively), each of them additionally stabilized by specific main-chain/side-chain H-bonding, where the sulfur atom essentially plays the role of H-bond acceptor. The folded structure was confirmed by microwave spectroscopy, which demonstrated the validity of the DFT-D methods currently used in the field. These structural and spectroscopic results, complemented by a theoretical Natural Bond Orbital analysis, enabled us to document the capacity of the weakly polar -CH2-SH side chain of Cys to adapt itself to the intrinsic local preferences of the peptide backbone, i.e., a γ-turn or a β-sheet extended secondary structure. The corresponding local H-bonding bridges the side chain acceptor S atom to the backbone NH donor site of the same or the next residue along the chain, through a 5- or a 6-membered ring respectively.

Neutral Peptides in the Gas Phase: Conformation and Aggregation Issues

Combined IR and UV laser spectroscopic techniques in molecular beams merged with theoretical approaches have proven to be an ideal tool to elucidate intrinsic structural properties on a molecular level. It offers the possibility to analyze structural changes, in a controlled molecular environment, when successively adding aggregation partners. By this, it further makes these techniques a valuable starting point for a bottom-up approach in understanding the forces shaping larger molecular systems. This bottom-up approach was successfully applied to neutral amino acids starting around the 1990s. Ever since, experimental and theoretical methods developed further, and investigations could be extended to larger peptide systems. Against this background, the review gives an introduction to secondary structures and experimental methods as well as a summary on theoretical approaches. Vibrational frequencies being characteristic probes of molecular structure and interactions are especially addressed. Archetypal biologically relevant secondary structures investigated by molecular beam spectroscopy are described, and the influences of specific peptide residues on conformational preferences as well as the competition between secondary structures are discussed. Important influences like microsolvation or aggregation behavior are presented. Beyond the linear α-peptides, the main results of structural analysis on cyclic systems as well as on β- and γ-peptides are summarized. Overall, this contribution addresses current aspects of molecular beam spectroscopy on peptides and related species and provides molecular level insights into manifold issues of chemical and biochemical relevance.

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.

Energetic switch of the proline effect in collision-induced dissociation of singly and doubly protonated peptide Ala-Ala-Arg-Pro-Ala-Ala

Suppression of the selective cleavage at N-terminal of proline is observed in the peptide cleavage by proteolytic enzyme trypsin and in the fragment ion mass spectra of peptides containing Arg-Pro sequence. An insight into the fragmentation mechanism of the influence of arginine residue on the proline effect can help in prediction of mass spectra and in protein structure analysis. In this work, collision-induced dissociation spectra of singly and doubly charged peptide AARPAA were studied by ESI MS/MS and theoretical calculation methods. The proline effect was evaluated by comparing the experimental ratio of fragments originated from cleavage of different amide bonds. The results revealed that the backbone amide bond cleavage was selected by the energy barrier height of the fragmentation pathway although the strong proton affinity of the Arg side chain affected the stereostructure of the peptide and the dissociation mechanism. The thermodynamic stability of the fragment ions played a secondary role in the abundance ratio of fragments generated via different pathways. Fragmentation studies of protonated peptide AACitPAA supported the energy-dependent hypothesis. The results provide an explanation to the long-term arguments between the steric conflict and the proton mobility mechanisms of proline effect.

The role of amino acid side chains in stabilizing dipeptides: the laser ablation Fourier transform microwave spectrum of Ac-Val-NH2

The steric effects imposed by the isopropyl group of valine in the conformational stabilization of the capped dipeptide N-acetyl-l-valinamide (Ac-Val-NH₂) have been studied by laser ablation molecular beam Fourier transform microwave (LA-MB-FTMW) spectroscopy.

Fourier transform microwave spectroscopy of Ac-Ser-NH2: the role of side chain interactions in peptide folding

Serine capped dipeptide N-acetyl-l-serinamide (Ac-Ser-NH2) has been investigated using Fourier transform microwave spectroscopic techniques combined with laser ablation sources. Spectral signatures originating from one dominant species have been detected in the supersonic expansion. Rotational and nuclear quadrupole coupling constants of the two (14)N nuclei have been used in the characterization of a C/γ-turn structure, which is stabilized by a CO∙∙∙HN intramolecular hydrogen bond closing a seven-membered ring. Two extra hydrogen bonds involving the polar side chain (-CH2OH) further stabilize the structure. The non-observation of C5 species, attributed to the presence of the polar side chain, is in contrast with the previous gas phase observation of the related dipeptides containing glycine or alanine residues. The A-E splitting pattern arising from the internal rotation of the methyl group has been analyzed and the internal rotation barrier has been determined.

Accurate Characterization of the Peptide Linkage in the Gas Phase: A Joint Quantum-Chemical and Rotational Spectroscopy Study of the Glycine Dipeptide Analogue

Accurate structures of aminoacids in the gas phase have been obtained by joint microwave and quantum-chemical investigations. However, the structure and conformational behavior of α-aminoacids once incorporated into peptide chains are completely different and have not yet been characterized with the same accuracy. To fill this gap, we present here an accurate characterization of the simplest dipeptide analogue (N-acetyl-glycinamide) involving peptidic bonds. State-of-the-art quantum-chemical computations are complemented by a comprehensive study of the rotational spectrum using a combination of Fourier transform microwave spectroscopy with laser ablation. The coexistence of the C7 and C5 conformers has been proved and energetically as well as spectroscopically characterized. This joint theoretical-experimental investigation demonstrated the feasibility of obtaining accurate structures for flexible small biomolecules, thus paving the route to the elucidation of the inherent behavior of peptides.