Minimalist De Novo Design of an Artificial Enzyme.

We employed a reductionist approach in designing the first heterochiral tripeptide that forms a robust heterogeneous short peptide catalyst similar to the "histidine brace" active site of lytic polysaccharide monooxygenases. The histidine brace is a conserved divalent copper ion-binding motif that comprises two histidine side chains and an amino group to create the T-shaped 3N geometry at the reaction center. The geometry parameters, including a large twist angle (73°) between the two imidazole rings of the model complex, are identical to those of native lytic polysaccharide monooxygenases (72.61°). The complex was synthesized and characterized as a structural and functional mimic of the histidine brace. UV-vis, vis-circular dichroism, Raman, and electron paramagnetic resonance spectroscopic analyses suggest a distorted square-pyramidal geometry with a 3N coordination at pH 7. Solution- and solid-state NMR results further confirm the 3N coordination in the copper center of the complex. The complex is pH-dependent and could catalyze the oxidation of benzyl alcohol in water to benzaldehyde with yields up to 82% in 3 h at pH 7 and above at 40 °C. The catalyst achieved 100% selectivity for benzaldehyde compared to conventional copper catalysis. The design of such a minimalist building block for functional soft materials with a pH switch can be a stepping stone in addressing needs for a cleaner and sustainable future catalyst.

Electric field modulated peptide based hydrogel nanocatalysts.

The ability to modulate self-assembly is the key to manufacture application-oriented materials. In this study, we investigated the effect of three independent variables that can modulate the catalytic activity of self-assembling peptides. The first two variables, amino acid sequence and its stereochemistry, were examined for their specific roles in the epitaxial growth and hydrogelation properties of a series of catalytic tripeptides. We observed that aromatic π-π interactions that direct the self-assembly of designed peptides, and the catalytic properties of hydrogels, are governed by the position and chirality of the proline residue. Subsequently, the influence of the third variable, an external electric field, was also tested to confirm its catalytic efficiency for the asymmetric C-C bond-forming aldol reaction. In particular, the electric field treated pff and PFF gels showed 10 and 36% higher stereoselectivity, respectively, compared with the control. Structure-property analysis using CD and FTIR spectroscopy indicates the electric field-induced beta to non-beta conformational transition in the peptide secondary structure, which corroborates with its reduced cross-link density and fibril width, respectively. Amplitude sweep rheology of the gels suggests a decrease in the storage modulus, with increased field strength. The results showed that an electric field of optimal strength can modulate the physical characteristics of the hydrogel, which in turn is manifested in the observed difference in enantioselectivity.

Pyridine mediated transition-metal-free direct alkylation of anilines using alcohols via borrowing hydrogen conditions.

Herein, we report pyridine and other similar azaaromatics as efficient biomimetic hydrogen shuttles for a transition-metal-free direct N-alkylation of aryl and heteroaryl amines using a variety of benzylic and straight chain alcohols. Mechanistic studies including deuterium labeling and the isolation of dihydro-intermediates of the benzannulated pyridine confirmed the role of pyridine and a borrowing hydrogen process operating in these reactions. In addition, we have extended this methodology for the development of dehydrogenative synthesis of quinolines and indoles, as well as the transfer hydrogenation of ketones.

A nanoscale iron catalyst for heterogeneous direct N- and C-alkylations of anilines and ketones using alcohols under hydrogen autotransfer conditions.

Here, we report a commercially available nanoscale Fe catalyst for heterogeneous direct N- and C-alkylation reactions of anilines and methyl ketones with alcohols. A hydrogen autotransfer mechanism has been found to operate in these reactions by deuterium labelling studies. In addition, dehydrogenative quinoline synthesis has been demonstrated from amino benzyl alcohols and acetophenones.

Visible light promoted thiol-ene reactions using titanium dioxide.

The radical addition of thiols to alkenes is reported under photoredox conditions, using visible light and TiO2 as a cheap and readily available photocatalyst.

Bivalent enzyme inhibitors discovered using dynamic covalent chemistry.

A bivalent dynamic covalent chemistry (DCC) system has been designed to selectively target members of the homodimeric glutathione-S-transferase (GST) enzyme family. The dynamic covalent libraries (DCLs) use aniline-catalysed acylhydrazone exchange between bivalent hydrazides and glutathione-conjugated aldehydes and the bis-hydrazides act as linkers to bridge between each glutathione binding site. The resultant DCLs were found to be compatible and highly responsive to templating with different GST isozymes, with the best results coming from the M and Schistosoma japonicum (Sj) class of GSTs, targets in cancer and tropical disease, respectively. The approach yielded compounds with selective, nanomolar affinity (K(i) =61 nM for mGSTM1-1) and demonstrates that DCC can be used to simultaneously interrogate binding sites on different subunits of a dimeric protein.

Inhibition of the PLP-dependent enzyme serine palmitoyltransferase by cycloserine: evidence for a novel decarboxylative mechanism of inactivation.

Cycloserine (CS, 4-amino-3-isoxazolidone) is a cyclic amino acid mimic that is known to inhibit many essential pyridoxal 5'-phosphate (PLP)-dependent enzymes. Two CS enantiomers are known; D-cycloserine (DCS, also known as Seromycin) is a natural product that is used to treat resistant Mycobacterium tuberculosis infections as well as neurological disorders since it is a potent NMDA receptor agonist, and L-cycloserine (LCS) is a synthetic enantiomer whose usefulness as a drug has been hampered by its inherent toxicity arising through inhibition of sphingolipid metabolism. Previous studies on various PLP-dependent enzymes revealed a common mechanism of inhibition by both enantiomers of CS; the PLP cofactor is disabled by forming a stable 3-hydroxyisoxazole/pyridoxamine 5'-phosphate (PMP) adduct at the active site where the cycloserine ring remains intact. Here we describe a novel mechanism of CS inactivation of the PLP-dependent enzyme serine palmitoyltransferase (SPT) from Sphingomonas paucimobilis. SPT catalyses the condensation of l-serine and palmitoyl-CoA, the first step in the de novo sphingolipid biosynthetic pathway. We have used a range of kinetic, spectroscopic and structural techniques to postulate that both LCS and DCS inactivate SPT by transamination to form a free pyridoxamine 5'-phosphate (PMP) and beta-aminooxyacetaldehyde that remain bound at the active site. We suggest this occurs by ring opening of the cycloserine ring followed by decarboxylation. Enzyme kinetics show that inhibition is reversed by incubation with excess PLP and that LCS is a more effective SPT inhibitor than DCS. UV-visible spectroscopic data, combined with site-directed mutagenesis, suggest that a mobile Arg(378) residue is involved in cycloserine inactivation of SPT.

Nucleophilic catalysis of acylhydrazone equilibration for protein-directed dynamic covalent chemistry.

Dynamic covalent chemistry uses reversible chemical reactions to set up an equilibrating network of molecules at thermodynamic equilibrium, which can adjust its composition in response to any agent capable of altering the free energy of the system. When the target is a biological macromolecule, such as a protein, the process corresponds to the protein directing the synthesis of its own best ligand. Here, we demonstrate that reversible acylhydrazone formation is an effective chemistry for biological dynamic combinatorial library formation. In the presence of aniline as a nucleophilic catalyst, dynamic combinatorial libraries equilibrate rapidly at pH 6.2, are fully reversible, and may be switched on or off by means of a change in pH. We have interfaced these hydrazone dynamic combinatorial libraries with two isozymes from the glutathione S-transferase class of enzyme, and observed divergent amplification effects, where each protein selects the best-fitting hydrazone for the hydrophobic region of its active site.