Jennifer L. Hickey

Site-specific integration of amino acid fragments into cyclic peptides.

The concept of site-specific integration of fragments into macrocyclic entities has not yet found application in the realm of synthetic chemistry. Here we show that the reduced amidicity of aziridine amide bonds provides an entry point for the site-specific integration of amino acids and peptide fragments into the homodetic cyclic peptide architecture. This new synthetic operation improves both the convergence and divergence of cyclic peptide synthesis.

Passive Membrane Permeability of Macrocycles Can Be Controlled by Exocyclic Amide Bonds

We have developed a strategy for synthesizing passively permeable peptidomimetic macrocycles. The cyclization chemistry centers on using aziridine aldehydes in a multicomponent reaction with peptides and isocyanides. The linker region in the resulting product contains an exocyclic amide positioned α to the peptide backbone, an arrangement that is not found among natural amino acids. This amide provides structural rigidity within the cyclic peptidomimetic and promotes the creation of a stabilizing intramolecular hydrogen bonding network. This exocyclic control element also contributes to the increased membrane permeability exhibited by multicomponent-derived macrocycles with respect to their homodetic counterparts. The exocyclic control element is employed along with a strategic placement of N-methyl and d-amino acids to produce passively permeable peptides, which contain multiple polar residues. This strategy should be applicable in the pursuit of synthesizing therapeutically relevant macrocycles.

Solid-Phase Parallel Synthesis of Functionalised Medium-to-Large Cyclic Peptidomimetics through Three-Component Coupling Driven by Aziridine Aldehyde Dimers

The first solid-phase parallel synthesis of macrocyclic peptides using three-component coupling driven by aziridine aldehyde dimers is described. The method supports the synthesis of 9- to 18-membered aziridine-containing macrocycles, which are then functionalized by nucleophilic opening of the aziridine ring. This constitutes a robust approach for the rapid parallel synthesis of macrocyclic peptides.

Stereocontrolled Disruption of the Ugi Reaction toward the Production of Chiral Piperazinones: Substrate Scope and Process Development

The factors determining diastereoselectivity observed in the multicomponent conversion of amino acids, aziridine aldehyde dimers, and isocyanides into chiral piperazinones have been investigated. Amino acid-dependent selectivity for either trans- or cis-substituted piperazinone products has been achieved. An experimentally determined diastereoselectivity model for the three-component reaction driven by aziridine aldehyde dimers has predictive value for different substrate classes. Moreover, this model is useful in reconciling the previously reported observations in multicomponent reactions between isocyanides, α-amino acids, and monofunctional aldehydes.

Bicycle synthesis through peptide macrocyclization using aziridine aldehydes followed by late stage disulfide bond installation

We present a method that can be applied to generate medium-sized peptidomimetic macrocycles equipped with disulfide bonds. The reaction hinges on amphoteric aziridine aldehydes and their ability to bridge the ends of linear peptides in the presence of isocyanides. Aziridine aldehyde dimers enable the initial cyclization, which is followed by site-specific aziridine ring-opening with sodium azide. Subsequent to that, thallium-induced oxidative disulfide bond formation furnishes the final product. The NMR characterization of the molecules obtained using this method indicates that conformationally well-behaved systems are readily accessible. The site-specific introduction of azide functionality should open the doors to subsequent functionalization using well-established protocols.

Rational Design of Calpain Inhibitors Based on Calpastatin Peptidomimetics

Our previously reported structures of calpain bound to its endogenous inhibitor calpastatin have motivated the use of aziridine aldehyde-mediated peptide macrocyclization toward the design of cyclic peptides and peptidomimetics as calpain inhibitors. Inspired by natures hint that a β-turn loop within calpastatin forms a broad interaction around calpains active site cysteine, we have constructed and tested a library of 45 peptidic compounds based on this loop sequence. Four molecules have shown reproducibly low micromolar inhibition of calpain-2. Further systematic sequence changes led to the development of probes that displayed increased potency and specificity of inhibition against calpain over other cysteine proteases. Calculated Ki values were in the low micromolar range, rivaling other peptidomimetic calpain inhibitors and presenting an improved selectivity profile against other therapeutically relevant proteases. Competitive and mixed inhibition against calpain-2 was observed, and an allosteric inhibition site on the enzyme was identified for a noncompetitive inhibitor.

Macrocyclic Templates for Library Synthesis of Peptido-Conjugates

Cyclic peptides have wide utility in the biological sciences. As conformationally locked analogs of the parent linear peptides, they possess greater stability under physiological conditions and increased binding affinity for their targets. As investigations of biological processes often require reporter molecules and functional readouts, chemical probes are commonly appended with functional groups that allow for conjugation to biological entities. Herein we describe the functionalization of cyclic peptides prepared via aziridine aldehyde-mediated macrocyclization. These cyclic peptides contain an aziridine ring that can be further functionalized by ring opening with nucleophiles. We report on the methodology used to produce a cyclic peptide analog of Pro-Gly-Leu-Gly-Phe with either azido or sulfhydryl functionality.