Peptide stapling by late-stage Suzuki-Miyaura cross-coupling.

The development of peptide stapling techniques to stabilise α-helical secondary structure motifs of peptides led to the design of modulators of protein-protein interactions, which had been considered undruggable for a long time. We disclose a novel approach towards peptide stapling utilising macrocyclisation by late-stage Suzuki-Miyaura cross-coupling of bromotryptophan-containing peptides of the catenin-binding domain of axin. Optimisation of the linker length in order to find a compromise between both sufficient linker rigidity and flexibility resulted in a peptide with an increased α-helicity and enhanced binding affinity to its native binding partner ß-catenin. An increased proteolytic stability against proteinase K has been demonstrated.

Peptidotriazolamers Inhibit Aß(1-42) Oligomerization and Cross a Blood-Brain-Barrier Model.

In peptidotriazolamers every second peptide bond is replaced by a 1H-1,2,3-triazole. Such foldamers are expected to bridge the gap in molecular weight between small-molecule drugs and protein-based drugs. Amyloid ß (Aß) aggregates play an important role in Alzheimer's disease. We studied the impact of amide bond replacements by 1,4-disubstituted 1H-1,2,3-triazoles on the inhibitory activity of the aggregation "hot spots" K16 LVFF20 and G39 VVIA42 in Aß(1-42). We found that peptidotriazolamers act as modulators of the Aß(1-42) oligomerization. Some peptidotriazolamers are able to interfere with the formation of toxic early Aß oligomers, depending on the position of the triazoles, which is also supported by computational studies. Preliminary in vitro results demonstrate that a highly active peptidotriazolamer is also able to cross the blood-brain-barrier.

Tuning the Biological Activity of RGD Peptides with Halotryptophans†.

An array of l- and d-halotryptophans with different substituents at the indole moiety was synthesized employing either enzymatic halogenation by halogenases or incorporation of haloindoles using tryptophan synthase. Introduction of these Trp derivatives into RGD peptides as a benchmark system was performed to investigate their influence on bioactivity. Halotryptophan-containing RGD peptides display increased affinity toward integrin αvß3 and enhanced selectivity over integrin α5ß1. In addition, bromotryptophan was exploited as a platform for late-stage diversification by Suzuki-Miyaura cross-coupling (SMC), resulting in new-to-nature biaryl motifs. These peptides show enhanced affinity toward αvß3, good affinity to αvß8, and remarkable selectivity over α5ß1 and αIIbß3 while featuring fluorogenic properties. Their feasibility as a probe was demonstrated in vitro. Extensive molecular dynamics simulations were undertaken to elucidate NMR and high-performance liquid chromatography (HPLC) data for these late-stage diversified cyclic RGD peptides and to further characterize their conformational preferences.

Cyclization of RGD Peptides by Suzuki-Miyaura Cross-Coupling.

Halogenated l- or d-tryptophan obtained by biocatalytic halogenation was incorporated into RGD peptides together with a variety of alkyl or aryl boronic acids. Suzuki-Miyaura cross-coupling either in solution or on-resin results in side chain-to-tail-cyclized RGD peptides, for example, with biaryl moieties, providing a new dimension of structure-activity relationships. An array of RGD peptides differing in macrocycle size, the presence of d-amino acid, N-methylation, or connectivity between the indole moiety and the boronic acid showed that, in particular, connectivity exhibits a major impact on affinities toward integrins, for example, αVß3. Structure-activity relationship studies yielded peptides with affinities toward αVß3 in the low nanomolar range, good selectivity, and high plasma stability. Structural characteristics of representative molecules have been investigated by molecular dynamics simulations, which allowed understanding the observed activity differences.

1,4-Disubstituted 1H-1,2,3-Triazole Containing Peptidotriazolamers: A New Class of Peptidomimetics With Interesting Foldamer Properties.

Peptidotriazolamers are hybrid foldamers with features of peptides and triazolamers, containing alternation of amide bonds and 1,4-disubstituted 1H-1,2,3-triazoles with conservation of the amino acid side chains. We report on the synthesis of a new class of peptidomimetics, containing 1,4-disubstituted 1H-1,2,3-triazoles in alternation with amide bonds and the elucidation of their conformational properties in solution. Based on enantiomerically pure propargylamines bearing the stereogenic center in the propargylic position and α-azido esters, building blocks were obtained by copper-catalyzed azide-alkyne cycloaddition. With these building blocks the peptidotriazolamers were readily available by solution phase synthesis. A panel of homo- and heterochiral tetramers, hexamers, and heptamers was synthesized and the heptamer Boc-Ala-Val-Ψ[4Tz]Phe-LeuΨ[4Tz]Phe-LeuΨ[4Tz]Val-OAll as well as an heterochiral and a Gly-containing equivalent were structurally characterized by NMR-based molecular dynamics simulations using a specifically tailored force field to determine their conformational and solvation properties. All three variants adopt a compact folded conformation in DMSO as well as in water. In addition to the heptamers we predicted the conformational behavior of similar longer oligomers i.e., Boc-Ala-(AlaΨ[4Tz]Ala)6-OAll as well as Boc-Ala-(d-AlaΨ[4Tz]Ala)6-OAll and Boc-Ala-(GlyΨ[4Tz]Ala)6-OAll. Our calculations predict a clear secondary structure of the first two molecules in DMSO that collapses in water due to the hydrophobic character of the side chains. The homochiral compound folds into a regular helical structure and the heterochiral one shows a twisted "S"-shape, while the Gly variant exhibits no clear secondary structure.

Synthesis and antimicrobial activity of binaphthyl-based, functionalized oxazole and thiazole peptidomimetics.

Thirty two new binaphthyl-based, functionalized oxazole and thiazole peptidomimetics and over thirty five novel leucine-containing intermediate oxazoles and thiazoles were prepared in this study. This includes the first examples of the direct C-5 arylation of an amino acid dipeptide-derived oxazole. Moderate to excellent antibacterial activity was observed for all new compounds across Gram positive isolates with MICs ranging from 1-16 µg mL(-1). Results for Gram negative E. coli and A. baumannii were more variable, but MICs as low as 4 µg mL(-1) were returned for two examples. Significantly, the in vitro results with a fluoromethyl-oxazole derivative collectively represent the best obtained to date for a member of our binaphthyl peptide antimicrobials.

Glass-forming properties of 3-methylbutane-1,2,3-tricarboxylic acid and its mixtures with water and pinonic acid.

3-Methylbutane-1,2,3-tricarboxylic acid (3-MBTCA) is an atmospheric oxidation product of α-pinene and has been identified as the most relevant tracer compound for atmospheric terpene secondary organic aerosol (SOA) particles. Little is known, however, of its physicochemical properties such as water solubility and phase state (e.g., liquid, crystalline, glassy). To gain knowledge, we synthesized 3-MBCTA from methyl 2-methylpropanoate and dimethyl maleate via a Michael addition and subsequent hydrolysis with 78% overall yield. It was found that 3-MBTCA transforms into anhydrides upon melting at Tm = 426 ± 1 K, thus preventing a determination of the glass transition temperature Tg by differential scanning calorimetry (DSC) through melting and subsequent cooling. Therefore, we designed the novel technique MARBLES (metastable aerosol by low temperature evaporation of solvent) for transferring a substance into a glassy state without heating. In MARBLES an aqueous solution is atomized into wet aerosol particles that are subsequently dried in several diffusion dryers resulting in glass formation of the residual particles for several solutes. The glassy aerosol particles are collected in an impactor until enough mass has accumulated that the sample's Tg can be determined by DSC. Using this method, the glass transition temperature of 3-MBTCA was found to be Tg ≈ 305 ± 2 K. Moreover, we have determined the glass transition Tg' of the maximal freeze-concentrated aqueous solution of 3-MBTCA, and Tg of mixtures of 3-MBTCA with water and pinonic acid. The latter data indicate a dependence of Tg upon the atomic oxygen-to-carbon ratio of the mixture, with implications for parametrizing the glass-forming behavior of α-pinene SOA particles in the atmosphere.