Direct Evidence for CH···π Interaction Mediated Stabilization of Pro-cisPro Bond in Peptides with Pro-Pro-Aromatic motifs

The Realm of Unconventional Noncovalent Interactions in Proteins: Their Significance in Structure and Function

Proteins and their assemblies are fundamental for living cells to function. Their complex three-dimensional architecture and its stability are attributed to the combined effect of various noncovalent interactions. It is critical to scrutinize these noncovalent interactions to understand their role in the energy landscape in folding, catalysis, and molecular recognition. This Review presents a comprehensive summary of unconventional noncovalent interactions, beyond conventional hydrogen bonds and hydrophobic interactions, which have gained prominence over the past decade. The noncovalent interactions discussed include low-barrier hydrogen bonds, C5 hydrogen bonds, C-H···π interactions, sulfur-mediated hydrogen bonds, n → π* interactions, London dispersion interactions, halogen bonds, chalcogen bonds, and tetrel bonds. This Review focuses on their chemical nature, interaction strength, and geometrical parameters obtained from X-ray crystallography, spectroscopy, bioinformatics, and computational chemistry. Also highlighted are their occurrence in proteins or their complexes and recent advances made toward understanding their role in biomolecular structure and function. Probing the chemical diversity of these interactions, we determined that the variable frequency of occurrence in proteins and the ability to synergize with one another are important not only for ab initio structure prediction but also to design proteins with new functionalities. A better understanding of these interactions will promote their utilization in designing and engineering ligands with potential therapeutic value.

Mechanistic Study of Diketopiperazine Formation during Solid-Phase Peptide Synthesis of Tirzepatide

This study focused on investigating diketopiperazine (DKP) and the formation of associated double-amino-acid deletion impurities during linear solid-phase peptide synthesis (SPPS) of tirzepatide (TZP). We identified that the DKP formation primarily occurred during the Fmoc-deprotection reaction and post-coupling aging of the unstable Fmoc-Pro-Pro-Ser-resin active pharmaceutical ingredient (API) intermediate. Similar phenomena have also been observed for other TZP active pharmaceutical ingredient (API) intermediates that contain a penultimate proline amino acid, such as Fmoc-Ala-Pro-Pro-Pro-Ser-resin, Fmoc-Pro-Pro-Pro-Ser-resin, and Fmoc-Gly-Pro-Ser-Ser-Gly-Ala-Pro-Pro-Pro-Ser-resin, which are intermediates for both hybrid and linear synthesis approaches. During post-coupling aging, it is found that Fmoc deprotection can proceed in dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), and acetonitrile (ACN) solvents without any piperidine addition. Density functional theory (DFT) calculations showed that a peptide that has a penultimate proline stabilizes the transition state through the C-H···π interaction during Fmoc decomposition, which causes those peptides to be more prone to cascade-deprotection reactions. Pseudo-reaction pathways are then proposed, and a corresponding macrokinetics model is developed to allow accurate prediction of the TZP peptide intermediate self-deprotection and DKP formation rate. Based on those studies, control strategies for minimizing DKP formation were further investigated and an alternative to Fmoc protection was identified (Bsmoc-protected amino acids), which eliminated the formation of the DKP byproducts. In addition, the use of oxyma additives and lower storage temperature was demonstrated to markedly improve the peptide intermediate stability to DKP degradation pathways.

Understanding the anomaly of cis-trans isomerism in Pro-His sequence

The anomalous absence of cisPro stabilizing C^αH^α_Xaa···π_Aro interactions at Xaa-Pro-Aro exclusively when Aro is His, is understood by NMR structural analyses of model peptides, as due to i → i backbone-side chain C₆ H-bond that forms uniquely when Aro is His, which significantly decreases its χ₁-g^- population essential for C^αH^α_Xaa···π_Aro formation.

Accurate de novo design of membrane-traversing macrocycles

We use computational design coupled with experimental characterization to systematically investigate the design principles for macrocycle membrane permeability and oral bioavailability. We designed 184 6-12 residue macrocycles with a wide range of predicted structures containing noncanonical backbone modifications and experimentally determined structures of 35; 29 are very close to the computational models. With such control, we show that membrane permeability can be systematically achieved by ensuring all amide (NH) groups are engaged in internal hydrogen bonding interactions. 84 designs over the 6-12 residue size range cross membranes with an apparent permeability greater than 1 × 10-6 cm/s. Designs with exposed NH groups can be made membrane permeable through the design of an alternative isoenergetic fully hydrogen-bonded state favored in the lipid membrane. The ability to robustly design membrane-permeable and orally bioavailable peptides with high structural accuracy should contribute to the next generation of designed macrocycle therapeutics.

van der Waals interactions to control amide cis–trans isomerism

We discover the presence of local van der Waals (vdW) interactions at the cis amide bond in crystals of isobutyroyl–Pro–Val–OMe.

Unexpectedly rigid short peptide foldamers in which NH-π and CH-π interactions are preserved in solution

NH-π and CH-π interactions, due to their weak character, are not easily identified in solution. We report a group of isolable short peptides with stable folds, in which NH-π and CH-π main chain-side chain interactions can be detected in solution by means of NMR and ATR-IR spectroscopy.

Chlorine "Equatorial Belt" Activation of CF3Cl by CO2: The C···Cl Tetrel Bond Dominance in CF3Cl-CO2

Besides its typical halogen donor behavior (exhibiting a Cl σ-hole) in forming Cl···B halogen bonds (B is an electron-rich region), CF₃Cl reveals a new interaction site in its complex with CO₂ when explored by rotational spectroscopy. Experimental evidence and theoretical analyses point out irrefutably that CF₃Cl prefers to link to CO₂ through its Cl "equatorial belt" consisting of the lone pairs of the Cl atom, resulting in a C···Cl tetrel bond. In addition, a secondary plausible C···O tetrel bond and a F···O halogen bond might contribute to the relative orientation of the moieties forming the complex. The effects of the Cl "equatorial belt" present in perhalogenated molecules, such as CF₃Cl, have been hitherto overlooked in describing the origin of noncovalent interactions. That left a significant void that the present study tries to fill by outlining its importance.

Leveraging orthogonal mass spectrometry based strategies for comprehensive sequencing and characterization of ribosomal antimicrobial peptide natural products

Covering: Up to July 2020Ribosomal antimicrobial peptide (AMP) natural products, also known as ribosomally synthesized and post-translationally modified peptides (RiPPs) or host defense peptides, demonstrate potent bioactivities and impressive complexity that complicate molecular and biological characterization. Tandem mass spectrometry (MS) has rapidly accelerated bioactive peptide sequencing efforts, yet standard workflows insufficiently address intrinsic AMP diversity. Herein, orthogonal approaches to accelerate comprehensive and accurate molecular characterization without the need for prior isolation are reviewed. Chemical derivatization, proteolysis (enzymatic and chemical cleavage), multistage MS fragmentation, and separation (liquid chromatography and ion mobility) strategies can provide complementary amino acid composition and post-translational modification data to constrain sequence solutions. Examination of two complex case studies, gomesin and styelin D, highlights the practical implementation of the proposed approaches. Finally, we emphasize the importance of heterogeneous AMP peptidoforms that confer varying biological function, an area that warrants significant further development.

Cloning, expression and immunological characterisation of Coc n 1, the first major allergen from Coconut pollen

Coconut pollen has been documented to be a major contributor to the aeroallergen load in India, causing respiratory allergy in a large cohort of susceptible individuals. Here, we report the identification of the first major allergen from Coconut pollen, Coc n 1. The full-length sequence of the allergen was determined from previously identified peptides and overexpressed in E. coli. Recombinant Coc n 1 folded into a trimer and was found to possess allergenicity equivalent to its natural counterpart. Proteolytic processing of Coc n 1 led to the formation of an immunodominant ∼20 kDa C-terminal subunit and the site of cleavage was determined by amino acid microsequencing. Five linear IgE binding epitopes were predicted and mapped on the homology modelled structure of Coc n 1. Amongst three immunodominant epitopes, two were present towards the C-terminal end. Coc n 1 was found to belong to the highly diverse cupin superfamily and mimics its structure with known 7S globulin or vicilin allergens but lacks sequence similarity. Using sequence similarity networks, Coc n 1 clustered as a separate group containing unannotated cupin domain proteins and did not include known vicilin allergens except Gly m Bd 28 kDa, a Soybean major allergen. 7S globulins are major storage proteins and food allergens, but presence of such protein in pollen grains is reported for the first time. Further study on Coc n 1 may provide insights into its function in pollen grains and also in the development of immunotherapy to Coconut pollen allergy.

Improved Modeling of Thioamide FRET Quenching by Including Conformational Restriction and Coulomb Coupling

Thioamide-containing amino acids have been shown to quench a wide range of fluorophores through distinct mechanisms. Here, we quantitatively analyze the mechanism through which the thioamide functional group quenches the fluorescence of p-cyanophenylalanine (Cnf), tyrosine (Tyr), and tryptophan (Trp). By comparing PyRosetta simulations to published experiments performed on polyproline ruler peptides, we corroborate previous findings that both Cnf and Tyr quenching occurs via Förster resonance energy transfer (FRET), while Trp quenching occurs through an alternate mechanism such as Dexter transfer. Additionally, optimization of the peptide sampling scheme and comparison of thioamides attached to the peptide backbone and side chain revealed that the significant conformational restriction associated with the thioamide moiety results in a high sensitivity of the apparent FRET efficiency to underlying conformational differences. Moreover, by computing FRET efficiencies from structural models using a variety of approaches, we find that quantitative accuracy in the role of Coulomb coupling is required to explain contributions to the observed quenching efficiency from individual structures on a detailed level. Last, we demonstrate that these additional considerations improve our ability to predict thioamide quenching efficiencies observed during binding of thioamide-labeled peptides to fluorophore-labeled variants of calmodulin.

CH-π interaction between cross-strand amino acid pairs stabilizes β-hairpins

We identified several CH-π donor-acceptor pairs involving amino acid side chains with less polarized C-H bonds at a solvent-exposed site between the strands of a β-hairpin peptide. Therein, we observe a distance-dependent induction of CH-π interaction within the aliphatic-aromatic amino acid pair. Our results also suggest an interplay of hydrophobicity and CH-π interaction in dictating the stability of β-hairpins, where a leucine-tryptophan pair contributes -1.14 kcal mol-1 to the overall foldedness of the β-hairpin.

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.

Weak Acid-Base Interactions of Histidine and Cysteine Affect the Charge States, Tertiary Structure, and Zn(II)-Binding of Heptapeptides

Zinc fingers are proteins that are characterized by the coordination of zinc ions by an amino acid sequence that commonly contains two histidines and two cysteines (2His-2Cys motif). Investigations of oligopeptides that contain the 2His-2Cys motif, e.g., acetyl-His₁-Cys₂-Gly₃-Pro₄-Tyr₅-His₆-Cys₇, have discovered they exhibit pH-dependent Zn(II) chelation and have redox activities with Cu(I/II), forming a variety of metal complexes. To further understand how these 2His-2Cys oligopeptides bind these metal ions, we have undertaken a series of ion mobility-mass spectrometry and B3LYP/LanL2DZ computational studies of structurally related heptapeptides. Starting with the sequence above, we have modified the potential His, Cys, or C-terminus binding sites and report how these changes in primary structure affect the oligopeptides positive and negative charge states, conformational structure, collision-induced breakdown energies, and how effectively Zn(II) binds to these sequences. The results show evidence that the weak acid-base properties of Cys-His are intrinsically linked and can result in an intramolecular salt-bridged network that affects the oligopeptide properties.

Aromatic interactions in β-hairpin scaffold stability: A historical perspective

Non-covalent interactions between naturally occurring aromatic residues have been widely exploited as scaffold stabilizing agents in de novo designed peptides and in Nature - inspired structures. Our understanding of the factors driving aromatic interactions and their observed interaction geometries have advanced remarkably with improvements in conventional structural studies, availability of novel molecular methods and in silico studies, which have together provided atomistic information on aromatic interactions and interaction strengths. This review attempts to recapitulate the early advances in our understanding of aromatic interactions as stabilizing agents of peptide β-hairpins.

Microwave spectroscopy of 2-(trifluoromethyl)pyridine⋯water complex: Molecular structure and hydrogen bond

In order to explore the -CF₃ substitution effect on the complexation of pyridine, we investigated the 2-(trifluoromethyl)pyridine⋯water complex by using pulsed jet Fourier transform microwave spectroscopy complemented with quantum chemical calculations. Experimental assignment and ab initio calculations confirmed that the observed complex is stabilized through N⋯H-O and O⋯H-C hydrogen bonds forming a five-membered ring structure. The bonding distance in N⋯H-O is determined to be 2.027(2) Å, whilst that in O⋯H-C interaction is 2.728(2) Å. The quantum theory of atoms in molecules analysis indicates that the interaction energy of N⋯H-O hydrogen bond is ∼22 kJ mol^-1 and that for O⋯H-C hydrogen bond is ∼5 kJ mol^-1. The water molecule lies almost in the plane of the aromatic ring in the complex. The -CF₃ substitution to pyridine quenches the tunneling splitting path of the internal motion of water molecule.

Understanding the conformational analysis of gababutin based hybrid peptides

A new class of gababutin-based tetrapeptide shows a C₁₂/C₁₀ hydrogen-bonded hybrid turn.

A Teaching Experiment to Elucidate a Cation-π Effect in an Alkyne Cycloaddition Reaction and Illustrate Hypothesis Driven Design of Experiments

An organic chemistry experiment is described that is based on recent research to elucidate a novel cation-π interaction between tetraalkammonium cations and propargyl hydrazines. This non-bonded interaction is a key component of the mechanism of ammonium-catalyzed intramolecular cycloaddition of nitrogen to the terminal carbon of a C-C triple bond of the propargyl substrate. In this teaching experiment, reactions and control experiments are employed to demonstrate the testing of two alternative mechanistic hypotheses. Specifically, cyclization reactions are performed with a soluble base (sodium phenoxide) with and without tetrabutylammonium bromide under homogeneous conditions. Students observe that the added ammonium salt accelerates the reaction. They are then encouraged to develop a testable hypothesis for the role of the ammonium salt in the cyclization mechanism: typical phase transfer or other. IR spectroscopy is then used to directly observe a dose dependent shift of the alkyne stretching mode due to a cation-π interaction. In this experiment, undergraduate "researchers" were able to practice the scientific method on a contemporary system and see how data are generated and interpreted to adjudicate between rival hypotheses in a way that emulates authentic and current research in a lab setting. This experimental design was tested on students enrolled in the introductory undergraduate Organic Chemistry Lab.

Tuning for Visible Fluorescence and Near-Infrared Phosphorescence on a Unimolecular Mechanically Sensitive Platform via Adjustable CH-π Interaction

CH-π interaction-assisted alignment of organic conjugated systems has played an important role to regulate molecular electronic and photophysical properties, whereas harnessing such a smart noncovalent interaction into the tuning of unimolecular complex emissive bands covering a wide spectral region remains a challenging research topic. Since the tuning for visible and near-infrared emissive properties in a single π-functional platform relates to its multicolor luminescent behaviors and potential superior application in analysis, bioimaging, and sensing, herein, we report a proportional control of the singlet and triplet emissions that cover visible and near-infrared spectral regions, respectively, can be straightforwardly achieved by CH-π interaction-assisted self-assembly at the unimolecular level. Employing an octathionaphthalene-based single luminophore as a prototype, we find that a strength-adjustable CH-π interaction-assisted self-assembly can be established in mixed DMF/H₂O and in the film state. The hybridization of planar local excited and intramolecular charge transfer transitions occurs on the basis, allowing a competitive inhibition to the intersystem crossing process to generate a complex emission composed of visible fluorescence and near-infrared phosphorescence. Furthermore, reversible mechanochromic and mechanoluminescent conversions of the corresponding solid sample can both be observed to rely on a corresponding self-assembly alternation. These results can probably provide new visions for the development of future intelligent and multifunctional luminescent materials.

Assessment of the Presence and Strength of H-Bonds by Means of Corrected NMR

The downfield shift of the NMR signal of the bridging proton in a H-bond (HB) is composed of two elements. The formation of the HB causes charge transfer and polarization that lead to a deshielding. A second factor is the mere presence of the proton-accepting group, whose electron density and response to an external magnetic field induce effects at the position of the bridging proton, exclusive of any H-bonding phenomenon. This second positional shielding must be subtracted from the full observed shift in order to assess the deshielding of the proton caused purely by HB formation. This concept is applied to a number of H-bonded systems, both intramolecular and intermolecular. When the positional shielding is removed, the remaining chemical shift is in much better coincidence with other measures of HB strength.

PPII Helical Peptidomimetics Templated by Cation-π Interactions

Poly-proline type II (PPII) helical PXXP motifs are the recognition elements for a variety of protein-protein interactions that are critical for cellular signaling. Despite development of protocols for locking peptides into α-helical and β-strand conformations, there remains a lack of analogous methods for generating mimics of PPII helical structures. We describe herein a strategy to enforce PPII helical secondary structure in the 19-residue TrpPlexus miniature protein. Through sequence variation, we showed that a network of cation-π interactions could drive the formation of PPII helical conformations for both peptide and N-substituted glycine peptoid residues. The achievement of chemically diverse PPII helical scaffolds provides a new route towards discovering peptidomimetic inhibitors of protein-protein interactions mediated by PXXP motifs.

A novel secondary structure based on fused five-membered rings motif

An analysis of protein structures indicates the existence of a novel, fused five-membered rings motif, comprising of two residues (i and i + 1), stabilized by interresidue N_i+1–H∙∙∙N_i and intraresidue N_i+1–H∙∙∙O=C_i+1 hydrogen bonds. Fused-rings geometry is the common thread running through many commonly occurring motifs, such as β-turn, β-bulge, Asx-turn, Ser/Thr-turn, Schellman motif, and points to its structural robustness. A location close to the beginning of a β-strand is rather common for the motif. Devoid of side chain, Gly seems to be a key player in this motif, occurring at i, for which the backbone torsion angles cluster at ~(−90°, −10°) and (70°, 20°). The fused-rings structures, distant from each other in sequence, can hydrogen bond with each other, and the two segments aligned to each other in a parallel fashion, give rise to a novel secondary structure, topi, which is quite common in proteins, distinct from two major secondary structures, α-helix and β-sheet. Majority of the peptide segments making topi are identified as aggregation-prone and the residues tend to be conserved among homologous proteins.

Type VIa β-turn-fused helix N-termini: A novel helix N-cap motif containing cis proline

Helix N-capping motifs often form hydrogen bonds with terminal amide groups which otherwise would be free. Also, without an amide hydrogen, proline (trans) is over-represented at helix N-termini (N1 position) because this naturally removes the need to hydrogen bond one terminal amide. However, the preference of cisPro, vis-à-vis helix N-termini, is not known. We show that cisPro (α_R or PP_II ) often appears at the N-cap position (N0) of helices. The N-cap cisPro(α_R ) is associated with a six-residue sequence motif - X_(-2) -X_(-1) -cisPro-X₍₁₎ -X₍₂₎ -X₍₃₎ - with preference for Glu/Gln at X_(-1) , Phe/Tyr/Trp at X₍₁₎ and Ser/Thr at X₍₃₎ . The motif, formed by the fusion of a helix and a type VIa β-turn, contains a hydrogen bond between the side chain of X_(-1) and the side chain/backbone of X₍₃₎ , a α-helical hydrogen bond between X_(-2) and X₍₂₎ and stacking interaction between cisPro and an aromatic residue at X₍₁₎ . NMR experiments on peptides containing the motif and its variants showed that local interactions associated with the motif, as found in folded proteins, were not enough to significantly tilt the cis/trans equilibrium towards cisPro. This suggests that some other evolutionary pressure must select the cisPro motif (over transPro) at helix N-termini. Database analysis showed that >C = O of the pre-cisPro(α_R ) residue at the helix N-cap, directed opposite to the N→C helical axis, participates in long-range interactions. We hypothesize that the cisPro(α_R ) motif is preferred at helix N-termini because it allows the helix to participate in long-range interactions that may be structurally and functionally important.

Implications of aromatic-aromatic interactions: From protein structures to peptide models

With increasing structural information on proteins, the opportunity to understand physical forces governing protein folding is also expanding. One of the significant non-covalent forces between the protein side chains is aromatic-aromatic interactions. Aromatic interactions have been widely exploited and thoroughly investigated in the context of folding, stability, molecular recognition, and self-assembly processes. Through this review, we discuss the contribution of aromatic interactions to the activity and stability of thermophilic, mesophilic, and psychrophilic proteins. Being hydrophobic, aromatic amino acids tend to reside in the protein hydrophobic interior or transmembrane segments of proteins. In such positions, it can play a diverse role in soluble and membrane proteins, and in α-helix and β-sheet stabilization. We also highlight here some excellent investigations made using peptide models and several approaches involving aryl-aryl interactions, as an increasingly popular strategy in protein and peptide engineering. A recent survey described the existence of aromatic clusters (trimer, tetramer, pentamer, and higher order assemblies), revealing the self-associating property of aryl groups, even in folded protein structures. The application of this self-assembly of aromatics in the generation of modern bionanomaterials is also discussed.

CH-π hydrogen bonds in biological macromolecules

This is a sequel to the previous Perspective "The CH-π hydrogen bond in chemistry. Conformation, supramolecules, optical resolution and interactions involving carbohydrates", which featured in a PCCP themed issue on "Weak Hydrogen Bonds - Strong Effects?": Phys. Chem. Chem. Phys., 2011, 13, 13873-13900. Evidence that weak hydrogen bonds play an enormously important role in chemistry and biochemistry has now accumulated to an extent that the rigid classical concept of hydrogen bonds formulated by Pauling needs to be seriously revised and extended. The concept of a more generalized hydrogen bond definition is indispensable for understanding the folding mechanisms of proteins. The CH-π hydrogen bond, a weak molecular force occurring between a soft acid CH and a soft base π-electron system, among all is one of the most important and plays a functional role in defining the conformation and stability of 3D structures as well as in many molecular recognition events. This concept is also valuable in structure-based drug design efforts. Despite their frequent occurrence in organic molecules and bio-molecules, the importance of CH-π hydrogen bonds is still largely unknown to many chemists and biochemists. Here we present a review that deals with the evidence, nature, characteristics and consequences of the CH-π hydrogen bond in biological macromolecules (proteins, nucleic acids, lipids and polysaccharides). It is hoped that the present Perspective will show the importance of CH-π hydrogen bonds and stimulate interest in the interactions of biological macromolecules, one of the most fascinating fields in bioorganic chemistry. Implication of this concept is enormous and valuable in the scientific community.

NMR Analysis of Tuning Cross-Strand Phe/Tyr/Trp-Trp Interactions in Designed β-Hairpin Peptides: Terminal Switch from L to D Amino Acid as a Strategy for β-Hairpin Capping

Interaction among the side chains of aromatic amino acids is a well-known mechanism of protein and peptide structure stabilization, particularly in β sheets. Using short β-hairpin models bearing the sequence Ac-Leu-Xxx-Val-DPro-Gly-Leu-Trp-Val-NH2, we report the surprising observation of significant destabilization in aryl–tryptophan interactions, which results in poorly folded peptide populations accompanied by lowering of stability. We find that such destabilization arises from forced occupancy of the indole ring in the shielded Edge position, in T-shaped aryl geometries. We demonstrate that this destabilizing effect can be efficiently salvaged by replacing the N-terminal LLeu with DLeu, which causes an increase in the folded hairpin population, while retaining Trp in the Edge position. Our observation of unique cross strand NOEs and data from temperature-dependent NMR and CD measurements reveals the formation of a locally stabilized aliphatic–aromatic network, leading to an overall increase in ΔGF° by ∼ −0.6 to −1.2 kcal/mol. Our results suggest that a contextual evaluation of stabilization by tryptophan is necessary in β hairpins. Furthermore, we report for the first time that the use of D isomers of aliphatic amino acids at the terminus is stabilizing, which can serve as a new strategy for increasing β-hairpin stability.

Tunable control of polyproline helix (PPII) structure via aromatic electronic effects: an electronic switch of polyproline helix

Aromatic rings exhibit defined interactions via the unique aromatic π face. Aromatic amino acids interact favorably with proline residues via both the hydrophobic effect and aromatic-proline interactions, C-H/π interactions between the aromatic π face and proline ring C-H bonds. The canonical aromatic amino acids Trp, Tyr, and Phe strongly disfavor a polyproline helix (PPII) when they are present in proline-rich sequences because of the large populations of cis amide bonds induced by favorable aromatic-proline interactions (aromatic-cis-proline and proline-cis-proline-aromatic interactions). We demonstrate the ability to tune polyproline helix conformation and cis-trans isomerism in proline-rich sequences using aromatic electronic effects. Electron-rich aromatic residues strongly disfavor polyproline helix and exhibit large populations of cis amide bonds, while electron-poor aromatic residues exhibit small populations of cis amide bonds and favor polyproline helix. 4-Aminophenylalanine is a pH-dependent electronic switch of polyproline helix, with cis amide bonds favored as the electron-donating amine, but trans amide bonds and polyproline helix preferred as the electron-withdrawing ammonium. Peptides with block proline-aromatic PPXPPXPPXPP sequences exhibited electronically switchable pH-dependent structures. Electron-poor aromatic amino acids provide special capabilities to integrate aromatic residues into polyproline helices and to serve as the basis of aromatic electronic switches to change structure.

Local control of cis-peptidyl-prolyl bonds mediated by CH···π interactions: the Xaa-Pro-Tyr motif

Compared to generic peptide bonds, the peptidyl-prolyl bond shows a strong propensity for the cis conformer. The presence of a sequence-contiguous aromatic (Aro) residue can further stabilize the cis conformer, as observed for the Aro-Pro motif. The cis propensity of the reverse sequence motif, Pro-Aro, is not so well understood, especially the effect of N-capping the Pro-Aro motif with different amino acid residues. From a comparative nuclear magnetic resonance study of two peptide series with the general sequences Ac-Xaa-Pro-Tyr-NH2 and Ac-Xaa-Pro-Ala-NH2, we present a relative thermodynamic scale that reflects how the nature of the Xaa side chain influences the cis propensity of the Xaa-Pro-Tyr motif, with Gly, Pro, and Ala at position Xaa giving the greatest enhancement of the cis-peptidyl-prolyl population. We also show that CH···π interaction between Xaa and Tyr is responsible for the enhanced cis population. However, the mere presence of the CH···π interaction does not guarantee that the peptidyl-prolyl bond will have a higher cis content in Xaa-Pro-Tyr than in Xaa-Pro-Ala. Xaa-dependent intramolecular interactions present in Xaa-trans-Pro-Tyr can nullify favorable CH···π interactions in Xaa-cis-Pro-Tyr. The relative cis-peptidyl-prolyl stabilizing propensities of Xaa (Xaa-Pro-Tyr) in proteins and in our peptide series show strong linear correlation except when Xaa is aromatic. We also explore the Xaa-Pro-Gly-Tyr sequence motif and show that mediated by a Pro-Tyr CH···π interaction, the cis-peptidyl-prolyl bond in the motif is stabilized when Xaa is Pro.

Aromatic-proline interactions: electronically tunable CH/π interactions

Proline residues have unique roles in protein folding, structure, and function. Proline and the aromatic amino acids comprise the encoded cyclic protein residues. Aromatic protein side chains are defined by their negatively charged π faces, while the faces of the proline ring are partially positively charged. This polarity results from their two-point connection of the side chain to the electron-withdrawing protein backbone, and the lower electronegativity of hydrogen compared to carbon, nitrogen, and oxygen. The hydrogens adjacent to the carbonyl and amide nitrogen, Hα and Hδ, respectively, are the most partially positive. Proline's side chain is also conformationally restricted, allowing for interaction with aromatic residues with minimal entropic or steric penalty. Proline and aromatic residues can interact favorably with each other, due to both the hydrophobic effect and the interaction between the π aromatic face and the polarized C-H bonds, called a CH/π interaction. Aromatic-proline interactions can occur locally, for example, to stabilize cis-amide bonds, and over larger distances, in the tertiary structures of proteins, and intermolecularly in protein-protein interactions. In peptides and proteins, aromatic-proline sequences more readily adopt cis-prolyl amide bonds, where the aromatic ring interacts with the proline ring in the cis conformation. In aromatic-proline sequences, Trp and Tyr are more likely to induce cis-amide bonds than Phe, suggesting an aromatic electronic effect. This result would be expected for a CH/π interaction, in which a more electron-rich aromatic would have a stronger (more cis-stabilizing) interaction with partial positive charges on prolyl hydrogens. In this Account, we describe our investigations into the nature of local aromatic-proline interactions, using peptide models. We synthesized a series of 26 peptides, TXPN, varying X from electron-rich to electron poor aromatic amino acids, and found that the population of cis-amide bond (Ktrans/cis) is tunable by aromatic electronics. With 4-substituted phenylalanines, we observed a Hammett correlation between aromatic electronics and Ktrans/cis, with cis-trans isomerism electronically controllable by 1.0 kcal/mol. All aromatic residues exhibit a higher cis population than Ala or cyclohexylalanine, with Trp showing the strongest aromatic-proline interaction. In addition, proline stereoelectronic effects can modulate cis-trans isomerism by an additional 1.0 kcal/mol. The aromatic-proline interaction is enthalpic, consistent with its description as a CH/π interaction. Proline-aromatic sequences can also promote cis-prolyl bonds, either through interactions of the aromatic ring with the preceding cis-proline or with the Hα prior to cis-proline. Within proline-rich peptides, sequences commonly found in natively disordered proteins, aromatic residues promote multiple cis-amide bonds due to multiple favorable aromatic-proline interactions. Collectively, we found aromatic-proline interactions to be significantly CH/π in nature, tunable by aromatic electronics. We discuss these data in the context of aromatic-proline and aromatic-glycine interactions in local structure, in tertiary structure, in protein-protein interactions, and in protein assemblies.

Evolutionary conservation of the polyproline II conformation surrounding intrinsically disordered phosphorylation sites

Intrinsically disordered (ID) proteins function in the absence of a unique stable structure and appear to challenge the classic structure-function paradigm. The extent to which ID proteins take advantage of subtle conformational biases to perform functions, and whether signals for such mechanism can be identified in proteome-wide studies is not well understood. Of particular interest is the polyproline II (PII) conformation, suggested to be highly populated in unfolded proteins. We experimentally determine a complete calorimetric propensity scale for the PII conformation. Projection of the scale into representative eukaryotic proteomes reveals significant PII bias in regions coding for ID proteins. Importantly, enrichment of PII in ID proteins, or protein segments, is also captured by other PII scales, indicating that this enrichment is robustly encoded and universally detectable regardless of the method of PII propensity determination. Gene ontology (GO) terms obtained using our PII scale and other scales demonstrate a consensus for molecular functions performed by high PII proteins across the proteome. Perhaps the most striking result of the GO analysis is conserved enrichment (P < 10(-8) ) of phosphorylation sites in high PII regions found by all PII scales. Subsequent conformational analysis reveals a phosphorylation-dependent modulation of PII, suggestive of a conserved "tunability" within these regions. In summary, the application of an experimentally determined polyproline II (PII) propensity scale to proteome-wide sequence analysis and gene ontology reveals an enrichment of PII bias near disordered phosphorylation sites that is conserved throughout eukaryotes.

XH/π (X = C, Si) Interactions in Graphene and Silicene: Weak in Strength, Strong in Tuning Band Structures

The lack of a band gap has greatly hindered the applications of graphene in electronic devices. By means of dispersion-corrected density functional theory computations, we demonstrated that considerable CH/π interactions exist between graphene and its fully (graphane) or patterned partially (C4H) hydrogenated derivatives. Due to the equivalence breaking of two sublattices of graphene, a 90 meV band gap is opened in the graphene/C4H bilayer. The band gap can be further increased to 270 meV by sandwiching graphene between two C4H layers. By taking advantage of the similar SiH/π interactions, a 120 meV band gap also can be opened for silicene. Interestingly, the high carrier mobility of graphene/silicene can be well-preserved. Our theoretical results suggest a rather practical solution for gap opening of graphene and silicene, which would allow them to serve as field effect transistors and other nanodevices.

Glycine rescue of β-sheets from cis-proline

Proline is incompatible with ideal β-sheet geometry, and the incompatibility gets magnified when Pro assumes the cis peptidyl-prolyl conformation. We show that Gly appears with high propensity at pre-cisPro positions in β-sheets and rescues the β-sheet from severe distortions by assuming a right-handed polyproline conformation (β(PR)), effectively increasing the local β-sheet register by one residue. The united residue, Gly(β(PR))-cisPro, is evolutionarily conserved, functionally important, and dynamic in nature.