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.

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, α V β 3 . Structure−activity relationship studies yielded peptides with affinities toward α V β 3 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.

Peptidotriazolamers are hybrid foldamers with features of peptides and triazolamers, containing alternation of amide bonds and 1,4-disubstituted 1 H -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 1 H -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.

The amide moiety of peptides can be replaced for example by a triazole moiety, which is considered to be bioisosteric. Therefore, the carbonyl moiety of an amino acid has to be replaced by an alkyne in order to provide a precursor of such peptidomimetics. As most amino acids have a chiral center at Cα, such amide bond surrogates need a chiral moiety. Here the asymmetric synthesis of a set of 24 N-sulfinyl propargylamines is presented. The condensation of various aldehydes with Ellman’s chiral sulfinamide provides chiral N-sulfinylimines, which were reacted with (trimethylsilyl)ethynyllithium to afford diastereomerically pure N-sulfinyl propargylamines. Diverse functional groups present in the propargylic position resemble the side chain present at the Cα of amino acids. Whereas propargylamines with (cyclo)alkyl substituents can be prepared in a direct manner, residues with polar functional groups require suitable protective groups. The presence of particular functional groups in the side chain in some cases leads to remarkable side reactions of the alkyne moiety. Thus, electron-withdrawing substituents in the Cα-position facilitate a base induced rearrangement to α,β-unsaturated imines, while azide-substituted propargylamines form triazoles under surprisingly mild conditions. A panel of propargylamines bearing fluoro or chloro substituents, polar functional groups, or basic and acidic functional groups is accessible for the use as precursors of 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.