Developing deprotectase biocatalysts for synthesis.

Organic synthesis often requires multiple steps where a functional group (FG) is concealed from reaction by a protecting group (PG). Common PGs include N-carbobenzyloxy (Cbz or Z) of amines and tert-butyloxycarbonyl (OtBu) of acids. An essential step is the removal of the PG, but this often requires excess reagents, extensive time and can have low % yield. An overarching goal of biocatalysis is to use "green" or "enzymatic" methods to catalyse chemical transformations. One under-utilised approach is the use of "deprotectase" biocatalysts to selectively remove PGs from various organic substrates. The advantage of this methodology is the exquisite selectivity of the biocatalyst to only act on its target, leaving other FGs and PGs untouched. A number of deprotectase biocatalysts have been reported but they are not commonly used in mainstream synthetic routes. This study describes the construction of a cascade to deprotect doubly-protected amino acids. The well known Bacillus BS2 esterase was used to remove the OtBu PG from various amino acid substrates. The more obscure Sphingomonas Cbz-ase (amidohydrolase) was screened with a range of N-Cbz-modified amino acid substrates. We then combined both the BS2 and Cbz-ase together for a 1 pot, 2 step deprotection of the model substrate CBz-L-Phe OtBu to produce the free L-Phe. We also provide some insight into the residues involved in substrate recognition and catalysis using docked ligands in the crystal structure of BS2. Similarly, a structural model of the Cbz-ase identifies a potential di-metal binding site and reveals conserved active site residues. This new biocatalytic cascade should be further explored for its application in chemical synthesis.

An active machine learning discovery platform for membrane-disrupting and pore-forming peptides.

Membrane-disrupting and pore-forming peptides (PFPs) play a substantial role in bionanotechnology and can determine the life and death of cells. The control of chemical and ion transport through cell membranes is essential to maintaining concentration gradients. Likewise, the delivery of drugs and intracellular proteins aided by pore-forming agents is of interest in treating malfunctioning cells. Known PFPs tend to be up to 50 residues in length, which is commensurate with the thickness of a lipid bilayer. Accordingly, few short PFPs are known. Here we show that the discovery of PFPs can be accelerated via an active machine learning approach. The approach identified 71 potential PFPs from the 25.6 billion octapeptide sequence space; 13 sequences were tested experimentally, and all were found to have the predicted membrane-disrupting ability, with 1 forming highly stable pores. Experimental verification of the predicted pore-forming ability demonstrated that a range of short peptides can form pores in membranes, while the positioning and characteristics of residues that favour pore-forming behaviour were identified. This approach identified more ultrashort (8-residues, unmodified, non-cyclic) PFPs than previously known. We anticipate our findings and methodology will be useful in discovering new pore-forming and membrane-disrupting peptides for a range of applications from nanoreactors to therapeutics.

Attaching palladium catalysts to antibodies.

Antibody-directed enzyme prodrug therapy (ADEPT) is a powerful concept in which antibody targeting is linked to enzymatic prodrug activation. The work herein describes the first steps in the development of a technology analogous to ADEPT but in which a palladium catalyst is attached of an antibody rather than an enzyme. Antibody-metal conjugates have been used in a variety of contexts including for radiotherapy; however, none of the metals attached to the antibodies have been used for catalytic purposes. This work represents the first example a metal being attached to an antibody for the purposes of carrying a functional catalyst.

Solid-Phase Synthesis of Fluorescent Probes for Plasma Membrane Labelling.

The cellular plasma membrane plays a fundamental role in biological processes, including cell growth, signaling and transport. The labelling of the plasma membrane with targeted fluorescent probes offers a convenient and non-invasive way to image the morphological changes and dynamics of a membrane in real-time and, despite many examples of fluorescent plasma membrane probes, a "universal targeting/anchoring moiety" is still required. In this study, a small number of stearic acid-based probes labelled with 6-carboxyfluorescein was designed and fabricated via solid-phase synthesis in which variations in both charge and hydrophobicity were explored. To ease the synthesis process, a gram-scale synthesis of the Fmoc-Lys(6-carboxyfluoresein diacetate)-OH building block was developed, allowing the discovery of optimal probes that carried a positively charged amino group and a stearic acid tail that exhibited intense plasma membrane brightness and robust retention.

Bioorthogonal Swarming: In Situ Generation of Dendrimers.

With the aid of bioorthogonal chemistry, we demonstrate the fabrication of synthetic dendrimers in situ around living cells. Using tetrazine dienophile and aminooxyl/hydrazide aldehyde chemistries, the density of functional groups on the dendrimers exponentially amplified intensities of fluorescent markers in antibody-targeted live cell imaging. This novel "swarming" approach highlights the power of bioorthogonal chemistry and provides a route to non-natural chemical structures on cells, paving the way for the generation of various artificial cellular nanostructures and scaffolds.

Solid-phase synthesis of biocompatible N-heterocyclic carbene-Pd catalysts using a sub-monomer approach.

Taking inspiration from the assembly of so-called peptoids (N-alkylglycine oligomers) we present a new synthetic methodology whereby N-heterocyclic carbene (NHC) based Pd ligands were assembled using a sub-monomer approach and loaded with Pd via solid-phase synthesis. This allowed the rapid generation a library of NHC-palladium catalysts that were readily functionalised to allow bioconjugation. These catalysts were able to rapidly activate a caged fluorophore and 'switch-on' an anticancer prodrug in 3D cell culture.

Tetrazine-Responsive Self-immolative Linkers.

Molecules that undergo activation or modulation following the addition of benign external small-molecule chemical stimuli have numerous applications. Here, we report the highly efficient "decaging" of a variety of moieties by activation of a "self-immolative" linker, by application of water-soluble and stable tetrazine, including the controlled delivery of doxorubicin in a cellular context.

In-Cell Dual Drug Synthesis by Cancer-Targeting Palladium Catalysts.

Transition metals have been successfully applied to catalyze non-natural chemical transformations within living cells, with the highly efficient labeling of subcellular components and the activation of prodrugs. In vivo applications, however, have been scarce, with a need for the specific cellular targeting of the active transition metals. Here, we show the design and application of cancer-targeting palladium catalysts, with their specific uptake in brain cancer (glioblastoma) cells, while maintaining their catalytic activity. In these cells, for the first time, two different anticancer agents were synthesized simultaneously intracellularly, by two totally different mechanisms (in situ synthesis and decaging), enhancing the therapeutic effect of the drugs. Tumor specificity of the catalysts together with their ability to perform simultaneous multiple bioorthogonal transformations will empower the application of in vivo transition metals for drug activation strategies.

Copper Catalysis in Living Systems and In†Situ Drug Synthesis.

The copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction has proven to be a pivotal advance in chemical ligation strategies with applications ranging from polymer fabrication to bioconjugation. However, application in vivo has been limited by the inherent toxicity of the copper catalyst. Herein, we report the application of heterogeneous copper catalysts in azide-alkyne cycloaddition processes in biological systems ranging from cells to zebrafish, with reactions spanning from fluorophore activation to the first reported in situ generation of a triazole-containing anticancer agent from two benign components, opening up many new avenues of exploration for CuAAC chemistry.

Flow and Microwave-Assisted Synthesis of N-(Triethylene glycol)glycine Oligomers and Their Remarkable Cellular Transporter Activities.

Peptidomimetics, such as oligo-N-alkylglycines (peptoids), are attractive alternatives to traditional cationic cell-penetrating peptides (such as R9) due to their robust proteolytic stability and reduced cellular toxicity. Here, monomeric N-alkylglycines, incorporating amino-functionalized hexyl or triethylene glycol (TEG) side chains, were synthesized via a three-step continuous-flow reaction sequence, giving the monomers N-Fmoc-(6-Boc-aminohexyl)glycine and N-Fmoc-((2-(2-Boc-aminoethoxy)ethoxy)ethyl)glycine in 49% and 41% overall yields, respectively. These were converted into oligomers (5, 7, and 9-mers) using an Fmoc-based solid-phase protocol and evaluated as cellular transporters. Hybrid oligomers, constructed of alternating units of the aminohexyl and amino-TEG monomers, were non-cytotoxic and exhibited remarkable cellular uptake activity compared to the analogous fully TEG or lysine-like compounds.

Photoluminescent carbon dots from 1,4-addition polymers.

Photoluminescent carbon dots were synthesised directly by thermopyrolysis of 1,4-addition polymers, allowing precise control of their properties. The effect of polymer composition on the properties of the carbon dots was investigated by TEM, IR, XPS, elemental analysis and fluorescence analysis, with carbon dots synthesised from nitrogen-containing polymers showing the highest fluorescence. The carbon dots with high nitrogen content were observed to have strong fluorescence in the visible region, and culture with cells showed that the carbon dots were non-cytotoxic and readily taken up by three different cell lines.

Fluorophore-Tagged Poly-Lysine RAFT Agents: Controlled Synthesis of Trackable Cell-Penetrating Peptide-Polymers.

The conjugation of a fluorophore and a variety of cell-penetrating peptides onto a RAFT agent allowed for the synthesis of polymers of defined sizes with quantifiable cell-uptake. Each peptide-RAFT agent was used to polymerize acrylamide, acrylate, and styrene monomers to form high or low molecular weight polymers (here 50 or 7.5 kDa) with the peptide having no influence on the RAFT agent's control. The incorporation of a single fluorophore per polymer chain allowed cellular analysis of the uptake of the size-specific peptide-polymers via flow cytometry and confocal microscopy. The cell-penetrating peptides had a direct effect on the efficiency of polymer uptake for both high and low molecular weight polymers, demonstrating the versatility of the strategy. These "all-in-one", synthetically accessible RAFT agents allow highly controlled preparation of synthetic peptide-polymer conjugates and subsequent quantification of their delivery into cells.

Approved and investigational fluorescent optical imaging agents for disease detection in surgery.

Fluorescent optical imaging is becoming an increasingly attractive imaging tool that physicians can utilise as it can detect previously 'unseen' changes in tissue at a cellular level that are consistent with disease. This is possible using a range of fluorescently labelled imaging agents that, once excited by specific wavelengths of light, can illuminate damaged and diseased tissues. For surgeons, such agents can permit dynamic, intraoperative imaging providing a real-time guide as they resect diseased tissue.

Solid-Phase Synthesis of s-Tetrazines.

An efficient synthesis of s-tetrazines by solid-phase methods is described. This synthesis route was compatible with different solid-phase resins and linkers and did not require metal catalysts or high temperatures. Monosubstituted tetrazines were routinely synthesized using thiol-promoted chemistry, using dichloromethane as a carbon source, while disubstituted unsymmetrical aryl or alkyl tetrazines were synthesized using readily available nitriles. This efficient approach enabled the synthesis of s-tetrazines in high yields (70-94%), eliminating the classical solution-phase problems of mixtures of symmetrical and unsymmetrical tetrazines, with only a single final purification step required, and paves the way to the rapid synthesis of s-tetrazines with various applications in bioorthogonal chemistry and beyond.

A fluorogenic, peptide-based probe for the detection of Cathepsin D in macrophages.

Cathepsin D is a protease that is an effector in the immune response of macrophages, yet to date, only a limited number of probes have been developed for its detection. Herein, we report a water soluble, highly sensitive, pH insensitive fluorescent probe for the detection of Cathepsin D activity that provides a strong OFF/ON signal upon activation and with bright emission at 515 nm. The probe was synthesised using a combination of solid and solution-phase chemistries, with probe optimisation to increase its water solubility and activation kinetics by addition of a long PEG chain (5 kDa) at the C-terminus. A BODIPY fluorophore allowed detection of Cathepsin D across a wide pH range, important as the protease is active both at the low pH found in lysosomes and also in higher pH phagolysosomes, and in the cytosol. The probe was successfully used to detect Cathepsin D activity in macrophages challenged by exposure to bacteria.

Programmable and Flexible Fluorochromic Polymer Microarrays for Information Storage.

Photoresponsive fluorochromic materials are regarded as an effective means for information storage. Their reversible changes of color and fluorescence facilitate the storage process and increase the possible storage capacity. Here, we propose an optically reconfigurable Förster resonance energy transfer (FRET) process to realize tunable emissions based on photochromic spiropyrans and common fluorophores. The kinetics of the photoisomerization of the spiropyran and the FRET process of the composite were systematically investigated. Through tuning the ratios of the acceptor spiropyran and donor fluorophore and external light stimuli, a programmable FRET process was developed to obtain tunable outputs. More importantly, flexible microarrays were fabricated from such fluorochromic mixtures by inkjet printing (230 ppi) and the dynamic FRET process could also be applied to generate tunable fluorescence in ready-made microstructures. The flexible patterns created using the microarrays could be used as novel optically readable media for information storage by altering the composition and optical performance of every feature within the microarray. A key aspect of information storage such is anti-counterfeiting, and these colorful displays can be fabricated and integrated in a simple and straightforward system. The reliable fabrication and programmable optical performances of these large-scale flexible polymer microarrays represent a substantial step toward high-density and high-security information storage platforms.

Dyeing fungi: amphotericin B based fluorescent probes for multiplexed imaging.

The clinically used antifungal polyene amphotericin B was conjugated, via the mycosamine and the aglycon moieties, to fluorophores. The Cy5 conjugated probe showed selective labelling of fungi in the presence of bacteria, allowing multiplexed imaging and identification of microbial species in a co-culture of fungi and Gram-positive and Gram-negative bacteria.

Multifunctional, histidine-tagged polymers: antibody conjugation and signal amplification.

A polymer scaffold, with multiple reactive centres, was synthesised by RAFT polymerisation and conjugated to the antibody herceptin. A hexahistidine RAFT agent enabled the rapid and simple purification of polymer-protein conjugates, while the tetrazine conjugation strategy allows myriad cargos to be attached and amplified.

Radical polymerization inside living cells.

Polymerization reactions conducted inside cells must be compatible with the complex intracellular environment, which contains numerous molecules and functional groups that could potentially prevent or quench polymerization reactions. Here we report a strategy for directly synthesizing unnatural polymers in cells through free radical photopolymerization using a number of biocompatible acrylic and methacrylic monomers. This offers a platform to manipulate, track and control cellular behaviour by the in cellulo generation of macromolecules that have the ability to alter cellular motility, label cells by the generation of fluorescent polymers for long-term tracking studies, as well as generate a variety of nanostructures within cells. It is remarkable that free radical polymerization chemistry can take place within such complex cellular environments. This demonstration opens up a multitude of new possibilities for how chemists can modulate cellular function and behaviour and for understanding cellular behaviour in response to the generation of free radicals.

New substrates for stem cell control.

The capacity to culture stem cells in a controllable, robust and scalable manner is necessary in order to develop successful strategies for the generation of cellular and tissue platforms for drug screening, toxicity testing, tissue engineering and regenerative medicine. Creating substrates that support the expansion, maintenance or directional differentiation of stem cells would greatly aid these efforts. Optimally, the substrates used should be chemically defined and synthetically scalable, allowing growth under defined, serum-free culture conditions. To achieve this, the chemical and physical attributes of the substrates should mimic the natural tissue environment and allow control of their biological properties. Herein, recent advances in the development of materials to study/manipulate stem cells, both in vitro and in vivo, are described with a focus on the novelty of the substrates' properties, and on application of substrates to direct stem cells.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.