Aligned peptoid-based macrodiscs for structural studies of membrane proteins by oriented-sample NMR.

Development of a robust, uniform, and magnetically orientable lipid mimetic will undoubtedly advance solid-state NMR of macroscopically aligned membrane proteins. Here, we report on a novel lipid membrane mimetic based on peptoid belts. The peptoids, composed of 15 residues, were synthesized by alternating N-(2-phenethyl)glycine with N-(2-carboxyethyl)glycine residues at a 2:1 molar ratio. The chemically synthesized peptoids possess a much lower degree of polydispersity versus styrene-maleic acid polymers, thus yielding uniform discs. Moreover, the peptoid oligomers are more flexible and do not require a specific folding, unlike lipoproteins, in order to wrap around the hydrophobic membrane core. The NMR spectra measured for the membrane-bound form of Pf1 coat protein incorporated in this new lipid mimetics demonstrate a higher order parameter and uniform linewidths compared with the conventional bicelles and peptide-based macrodiscs. Importantly, unlike bicelles, the peptoid-based macrodiscs are detergent free.

N-Arylation of Amino Acid Esters to Expand Side Chain Diversity in Ketoxime Peptide Ligations.

Palladium-catalyzed N-arylations of amino acid tert-butyl esters using 4-bromo-N,N-dimethylaniline as a coupling partner are reported. The resulting N-aryl amino acid esters are suitable building blocks for the synthesis of electron-rich N-aryl peptides, which undergo oxidative couplings to aminooxy groups to afford ketoxime peptides under mild conditions. N-aryl amino acid tert-butyl esters possessing unnatural side chains were also accessed via glycine Schiff base alkylation, further increasing the scope of Cα-substitution in ketoxime peptides.

Ketoxime peptide ligations: oxidative couplings of alkoxyamines to N-aryl peptides.

Chemoselective ligation methods that preserve or introduce side chain diversity are critical for chemical synthesis of peptides and proteins. Starting from ketone substrates instead of aldehydes in oxime ligation reactions would allow substitution at the site of ligation; however, synthetic challenges to readily access ketone derivatives from common amino acid building blocks have precluded the widespread use of ketoxime peptide ligation reactions thus far. Moreover, ketones are typically much slower to react in condensation reactions compared to aldehydes. Here, one-pot catalyst-free oxidative couplings of α-substituted N-aryl peptides with alkoxyamines provide access to oxime linkages with diverse side chains. Electron-rich N-(p-Me2N-phenyl)-amino acids possessing substituents at the α-carbon were found to be uniquely capable of undergoing site-selective α-C-H oxidations in situ under an O2 atmosphere at neutral pH. Comparative studies with N-arylglycinyl peptides revealed that substitution at the α-carbon caused notable changes in reactivity, with greater sensitivity to solvent and buffer salt composition.

Oxime Ligation via in situ Oxidation of N-Phenylglycinyl Peptides.

Mild conditions for oxime ligations via in situ generation of α-imino amide intermediates are reported. The evaluation of a variety of N-terminal N-phenylglycine residues revealed that a metal-free, chemoselective oxidation was possible using oxygen as the only oxidant in buffer at pH 7.0. Moreover, selective unmasking of an inert residue by addition of potassium ferricyanide is demonstrated. These simple and mild conditions, which can be fine-tuned by the electronic properties of the N-phenylglycine residue, offer unique advantages over conventional approaches for oxime ligations.

Hydrogen peroxide activated quinone methide precursors with enhanced DNA cross-linking capability and cytotoxicity towards cancer cells.

Quinone methide (QM) formation induced by endogenously generated H2O2 is attractive for biological and biomedical applications. To overcome current limitations due to low biological activity of H2O2-activated QM precursors, we are introducing herein several new arylboronates with electron donating substituents at different positions of benzene ring and/or different neutral leaving groups. The reaction rate of the arylboronate esters with H2O2 and subsequent bisquinone methides formation and DNA cross-linking was accelerated with the application of Br as a leaving group instead of acetoxy groups. Additionally, a donating group placed meta to the nascent exo-methylene group of the quinone methide greatly improves H2O2-induced DNA interstrand cross-link formation as well as enhances the cellular activity. Multiple donating groups decrease the stability and DNA cross-linking capability, which lead to low cellular activity. A cell-based screen demonstrated that compounds 2a and 5a with a OMe or OH group dramatically inhibited the growth of various tissue-derived cancer cells while normal cells were less affected. Induction of H2AX phosphorylation by these compounds in CLL lymphocytes provide evidence for a correlation between cell death and DNA damage. The compounds presented herein showed potent anticancer activities and selectivity, which represent a novel scaffold for anticancer drug development.