Kinetic Resolution of ß-Branched Aldehydes through Peptide-Catalyzed Conjugate Addition Reactions.

The catalytic kinetic resolution of racemic ß-branched aldehydes offers a straightforward stereoselective entry to aldehydes and addition products. Yet, control over stereoselectivity is difficult due to the conformational flexibility of ß-branched aldehydes. Here, we show that the peptide catalyst H-dPro-αMePro-Glu-NH2 resolves ß-branched aldehydes through reaction with nitroolefins and provides γ-nitroaldehydes with three consecutive stereogenic centers in high yields and stereoselectivities. Kinetic, NMR spectroscopic, and computational studies provided insights into the selectivity-determining step and origins of the kinetic resolution.

Self-Sorting Collagen Heterotrimers.

Nature uses elaborate methods to control protein assembly, including that of heterotrimeric collagen. Here, we established design principles for the composition and register-selective assembly of synthetic collagen heterotrimers. The assembly code enabled the self-sorting of eight different strands into three─out of 512 possible─triple helices via complementary (4S)-aminoproline and aspartate residues. Native ESI-MS corroborated the specific assembly into coexisting heterotrimers.

Carbohydrate Co-Solutes Stabilize Collagen Triple Helices.

Carbohydrates are common co-solutes for the stabilization of proteins. The effect of carbohydrate solutions on the stability of collagen, the most abundant protein in mammals, is, however, underexplored. In this work, we studied the thermal stability of collagen triple helices derived from a molecularly defined collagen model peptide (CMP), Ac-(Pro-Hyp-Gly)7 -NH2 , in solutions of six common mono- and disaccharides. We show that the carbohydrates stabilize the collagen triple helix in a concentration-dependent manner, with an increase of the melting temperature of up to 17 °C. In addition, we show that the stabilizing effect is similar for all studied sugars, including trehalose, which is otherwise considered a privileged bioprotectant. The results provided insight into the effects of sugar co-solutes on collagen triple helices and can aid the selection of storage environments for collagen-based materials and probes.

Modifying the Catalytic Activity of Lipopeptide Assemblies with Nucleobases.

Biohybrid catalysts that operate in aqueous media are intriguing for systems chemistry. In this paper, we investigate whether control over the self-assembly of biohybrid catalysts can tune their properties. As a model, we use the catalytic activity of functional hybrid molecules consisting of a catalytic H-dPro-Pro-Glu tripeptide, derivatized with fatty acid and nucleobase moieties. This combination of simple biological components merged the catalytic properties of the peptide with the self-assembly of the lipid, and the structural ordering of the nucleobases. The biomolecule hybrids self-assemble in aqueous media into fibrillar assemblies and catalyze the reaction between butanal and nitrostyrene. The interactions between the nucleobases enhanced the order of the supramolecular structures and affected their catalytic activity and stereoselectivity. The results point to the significant control and ordering that nucleobases can provide in the self-assembly of biologically inspired supramolecular catalysts.

Exploring the signaling space of a GPCR using bivalent ligands with a rigid oligoproline backbone.

G protein-coupled receptors (GPCRs) are one of the most important drug-target classes in pharmaceutical industry. Their diversity in signaling, which can be modulated with drugs, permits the design of more effective and better-tolerated therapeutics. In this work, we have used rigid oligoproline backbones to generate bivalent ligands for the gastrin-releasing peptide receptor (GRPR) with a fixed distance between their recognition motifs. This allows the stabilization of GPCR dimers irrespective of their physiological occurrence and relevance, thus expanding the space for medicinal chemistry. Specifically, we observed that compounds presenting agonists or antagonists at 20- and 30-Å distance induce GRPR dimerization. Furthermore, we found that 1) compounds with two agonists at 20- and 30-Å distance that induce dimer formation show bias toward Gq efficacy, 2) dimers with 20- and 30-Å distance have different potencies toward ß-arrestin-1 and ß-arrestin-2, and 3) the divalent agonistic ligand with 10-Å distance specifically reduces Gq potency without affecting ß-arrestin recruitment, pointing toward an allosteric effect. In summary, we show that rigid oligoproline backbones represent a tool to develop ligands with biased GPCR signaling.

Peptide and Enzyme Catalysts Work in Concert in Stereoselective Cascade Reactions-Oxidation followed by Conjugate Addition.

Enzymes and peptide catalysts consist of the same building blocks but require vastly different environments to operate best. Herein, we show that an enzyme and a peptide catalyst can work together in a single reaction vessel to catalyze a two-step cascade reaction with high chemo- and stereoselectivity. Abundant linear alcohols, nitroolefins, an alcohol oxidase, and a tripeptide catalyst provided chiral γ-nitroaldehydes in aqueous buffer. High yields (up to 92 %) and stereoselectivities (up to 98 % ee) were achieved for the cascade through the rational design of the peptide catalyst and the identification of common reaction conditions.

Overcoming Deactivation of Amine-Based Catalysts: Access to Fluoroalkylated γ-Nitroaldehydes.

Organocatalytic conjugate addition reactions of aldehydes to fluoroalkylated nitroolefins with chiral amine catalysts offer a straightforward stereoselective path to fluoroalkylated γ-nitroaldehydes and downstream derivatives. However, amine-based catalysts suffer from deactivation by reaction with electron-poor fluoroalkylated nitroolefin. Here, we show that catalyst deactivation can be overcome by catalysts that bear an intramolecular acid for protonation and release of the alkylated catalyst through ß-elimination of the nitroolefin. NMR spectroscopic, kinetic, and molecular modeling studies provided detailed structural and mechanistic insights into the factors that control reversible catalyst alkylation and facilitate efficient catalysis.

Introducing Azomethine Imines to Chemical Biology: Bioorthogonal Reaction with Isonitriles.

Azomethine imines are valuable substrates for chemical synthesis in organic solvents that often require anhydrous conditions. Here, we introduce C,N-cyclic-N'-acyl azomethine imines (AMIs) to bioorthogonal reactions in an aqueous environment. These AMIs are stable under physiological conditions and react rapidly (k2 = 0.1-250 M-1 s-1, depending on pH) and chemoselectively with isonitriles in the presence of biological nucleophiles, including thiols. Live-cell imaging of cell-surface-bound isonitriles underlines the biocompatibility of the AMI-isonitrile ligation, and simultaneous one-pot triple-protein labeling demonstrates its orthogonality to commonly used bioorthogonal reactions, such as the SPAAC and iEDDA ligations.

Isonitrile-Proline - A Versatile Handle for the Chemoselective Derivatization of Collagen Peptides.

Functional groups that allow for chemoselective and bioorthogonal derivatization are valuable tools for the labelling of peptides and proteins. The isonitrile is such a group but synthetic methods for its incorporation into peptides by solid-phase peptide synthesis are not known. Here, we introduce (4S)- and (4R)-isonitrileproline (Inp) as building blocks for solid-phase peptide synthesis. Conformational studies of (4S)- and (4R)-Inp and thermal stability analysis of Inp-containing collagen triple helices revealed that the isonitrile group exerts a stereoelectronic gauche effect. We showcase the value of Inp for bioorthogonal labelling by derivatization of Inp-containing collagen model peptides (CMPs). Dual labelling with a pair of bioorthogonal reactions of a CMP containing Inp and azidoproline residues further highlights the versatility of the new isonitrile-containing amino acids.

Identification of Isonitrile-Containing Natural Products in Complex Biological Matrices through Ligation with Chlorooximes.

Isonitrile-containing natural products have garnered attention for their manifold bioactivities but are difficult to detect and isolate due to the chemical lability of the isonitrile functional group. Here, we used the isonitrile-chlorooxime ligation (INC) in a reactivity-based screening (RBS) protocol for the detection and isolation of alkaloid and terpene isonitriles in the cyanobacterium Fischerella ambigua and a marine sponge of the order Bubarida, respectively. A trifunctional probe bearing a chlorooxime moiety, a UV active aromatic moiety, and a bromine label facilitated the chemoselective reaction with isonitriles, UV-Vis spectroscopic detection, and mass spectrometric analysis. The INC-based RBS allowed for the detection, isolation, and structural elucidation of isonitriles in microgram quantities.

Imaging and targeting LOX-mediated tissue remodeling with a reactive collagen peptide.

Collagens are fibrous proteins that are integral to the strength and stability of connective tissues. During collagen maturation, lysyl oxidases (LOX) initiate the cross-linking of fibers, but abnormal LOX activity is associated with impaired tissue function as seen in fibrotic and malignant diseases. Visualizing and targeting this dynamic process in healthy and diseased tissue is important, but so far not feasible. Here we present a probe for the simultaneous monitoring and targeting of LOX-mediated collagen cross-linking that combines a LOX-activity sensor with a collagen peptide to chemoselectively target endogenous aldehydes generated by LOX. This synergistic probe becomes covalently anchored and lights up in vivo and in situ in response to LOX at the sites where cross-linking occurs, as demonstrated by staining of normal skin and cancer sections. We anticipate that our reactive collagen-based sensor will improve understanding of collagen remodeling and provide opportunities for the diagnosis of fibrotic and malignant diseases.

The Yin and Yang of How N-Terminal Acyl Caps Affect Collagen Triple Helices.

N-terminal acylation is a common tool for the installation of functional moieties (e.g., sensors or bioactive molecules) on collagen model peptides (CMPs). The N-acyl group and its length are generally assumed to have little or no influence on the properties of the collagen triple helix formed by the CMP. Here, we show that the length of short (C1-C4) acyl capping groups has different effects on the thermal stability of collagen triple helices in POG, OGP, and GPO frames. While the effect of different capping groups on the stability of triple helices in the GPO frame is negligible, longer acyl chains stabilize OGP triple helices but destabilize POG analogues. The observed trends arise from a combination of steric repulsion, the hydrophobic effect, and n → π* interactions. Our study provides a basis for the design of N-terminally functionalized CMPs with predictable effects on triple helix stability.

Catalytic templated length-controlled oligomerization.

Templated synthesis is an intriguing strategy for the length-controlled synthesis of oligomers. Traditionally, such reactions require stoichiometric amounts of the template with respect to the product. Recently we reported catalytic macrocyclic templates that promote oligomerization of a small molecule substrate with a remarkable degree of length control. Herein we present our efforts toward creating linear templates for catalytic length-controlled oligomer synthesis.

Crystal structure analysis of N-acetylated proline and ring size analogs.

Crystal structures of N-acetylated proline and homologs with four- and six-membered rings (azetidine carboxylic acid and piperidine carboxylic acid) were obtained and compared. The distinctly different conformations of the four-, five-, and six-membered rings reflect Bayer strain, n → π* interaction, and allylic strain, and result in crystal lattices with a zigzag structure.

Organocatalytic Synthesis of Triflones Bearing Two Non-Adjacent Stereogenic Centers.

Trifluoromethylsulfones (triflones) are useful compounds for synthesis and beyond. Yet, methods to access chiral triflones are scarce. Here, we present a mild and efficient organocatalytic method for the stereoselective synthesis of chiral triflones using α-aryl vinyl triflones, building blocks previously unexplored in asymmetric synthesis. The peptide-catalyzed reaction gives rise to a broad range of γ-triflylaldehydes with two non-adjacent stereogenic centers in high yields and stereoselectivities. A catalyst-controlled stereoselective protonation following a C-C bond formation is key to control over the absolute and relative configuration. Straightforward derivatization of the products into, e.g., disubstituted δ-sultones, γ-lactones, and pyrrolidine heterocycles highlights the synthetic versatility of the products.

Albert Eschenmoser (1925-2023): A Giant of Organic Chemistry.

Albert Eschenmoser, one of the greatest organic chemists of the past hundred years, died on July 14, 2023 at the age of 97. The extraordinary breadth of his scientific contributions ranged from synthetic methodology, structure elucidation, and synthesis of natural products to the chemical etiology of biomolecular structures.

Predicting Collagen Triple Helix Stability through Additive Effects of Terminal Residues and Caps.

Collagen model peptides (CMPs) consisting of proline-(2S,4R)-hydroxyproline-glycine (POG) repeats have provided a breadth of knowledge of the triple helical structure of collagen, the most abundant protein in mammals. Predictive tools for triple helix stability have, however, lagged behind since the effect of CMPs with different frames ([POG]n , [OGP]n , or [GPO]n ) and capped or uncapped termini have so far been underestimated. Here, we elucidated the impact of the frame, terminal functional group and its charge on the stability of collagen triple helices. Combined experimental and theoretical studies with frame-shifted, capped and uncapped CMPs revealed that electrostatic interactions, strand preorganization, interstrand H-bonding, and steric repulsion at the termini contribute to triple helix stability. We show that these individual contributions are additive and allow for the prediction of the melting temperatures of CMP trimers.

Frame Shifts Affect the Stability of Collagen Triple Helices.

Collagen model peptides (CMPs), composed of proline-(2S,4R)-hydroxyproline-glycine (POG) repeat units, have been extensively used to study the structure and stability of triple-helical collagen─the dominant structural protein in mammals─at the molecular level. Despite the more than 50-year history of CMPs and numerous studies on the relationship between the composition of single-stranded CMPs and the thermal stability of the assembled triple helices, little attention has been paid to the effects arising from their terminal residues. Here, we show that frame-shifted CMPs, which share POG repeat units but terminate with P, O, or G, form triple helices with vastly different thermal stabilities. A melting temperature difference as high as 16 °C was found for triple helices from 20-mers Ac-OG[POG]6-NH2 and Ac-[POG]6PO-NH2, and triple helices of the constitutional isomers Ac-[POG]7-NH2 and Ac-[GPO]7-NH2 melt 10 °C apart. A combination of thermal denaturation, circular dichroism and NMR spectroscopic studies, and molecular dynamics simulations revealed that the stability differences originate from the propensity of the peptide termini to preorganize into a polyproline-II helical structure. Our results advise that care must be taken when designing peptide mimics of structural proteins, as subtle changes in the terminal residues can significantly affect their properties. Our findings also provide a general and straightforward tool for tuning the stability of CMPs for applications as synthetic materials and biological probes.

Peptide-Catalyzed Stereoselective Conjugate Addition Reaction of Aldehydes to C-Substituted Maleimides.

Catalytic stereoselective additions with maleimides are useful one-step reactions to yield chiral succinimides, molecules that are widespread among therapeutically active compounds but challenging to prepare when the maleimide is C-substituted. We present the tripeptide H-Pro-Pro-Asp-NHC12 H25 as a catalyst for conjugate addition reactions between aldehydes and C-substituted maleimides to form succinimides with three contiguous stereogenic centers in high yields and stereoselectivities. The peptidic catalyst is so chemoselective that no protecting group is needed at the imide nitrogen of the maleimides. Derivatization of the succinimides was straightforward and provided access to chiral pyrrolidines, lactones, and lactams. Kinetic studies, including a Hammett plot, provided detailed insight into the reaction mechanism.

Peptide-Metal Frameworks with Metal Strings Guided by Dispersion Interactions.

Despite impressive advances in the construction of metal-organic frameworks (MOFs), the formation of networks from peptidic ligands is difficult, though they are sought after for their modularity and biocompatibility. Herein we present a peptide-metal framework that consists of helical oligoproline ligands and Zn/K (or Zn/Rb). The crystalline network contains pleated nanosheets with the metal ions aligned in strings. This unprecedented architecture derives from under-appreciated London dispersion interactions between the oligoproline ligands that play in concert with the metal coordination to create the network. Hence, the secondary structure of the peptidic ligand represents an additional control element for the creation of new MOF architectures. We anticipate that our results will instruct the design of further peptidic MOFs and enable the generation of versatile biocompatible materials.