Recent Developments in Flavin-based Catalysis

Expanding Reaction Horizons: Evidence of the 5-Deazaflavin Radical Through Photochemically Induced Dynamic Nuclear Polarization

Deazaflavins are important analogues of the naturally occurring flavins: riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). The use of 5-deazaflavin as a replacement coenzyme in a number of flavoproteins has proven particularly valuable in unraveling and manipulating their reaction mechanisms. It was frequently reported that one-electron-transfer reactions in flavoproteins are impeded with 5-deazaflavin as the cofactor. Based on these findings, it was concluded that the 5-deazaflavin radical is significantly less stable compared to the respective flavin semiquinone and quickly re-oxidizes or undergoes disproportionation. The long-standing paradigm of 5-deazaflavin being solely a two-electron/hydride acceptor/donor-"a nicotinamide in flavin clothing"-needs to be re-evaluated now with the indirect observation of a one-electron-reduced (paramagnetic) species using photochemically induced dynamic nuclear polarization (photo-CIDNP) 1 H nuclear magnetic resonance (NMR) under biologically relevant conditions.

Going Full Circle with Organocatalysis and Biocatalysis: The Latent Potential of Cofactor Mimics in Asymmetric Synthesis

Many enzymes work in tandem with small molecule cofactors, which have inspired organocatalyst designs. Chemical modification of cofactor scaffolds has increased organocatalytic reactivity and reaction scope. This synopsis presents a selection of recent advances in the use of cofactors (native and mimics) in organocatalysis and biocatalysis. We aim to highlight the benefits of combining fundamental knowledge gained in both bio- and organo-catalysis for asymmetric biocatalysis.

Formation of Fungal 2,18-Dioxo-2,18-seco Indole Diterpenes by Nonenzymatic Flavin-Catalyzed Oxidative Ring Expansion and Oxygen Incorporation from Solvent Water

Most naturally occurring indole diterpenes share a 6/5/5/6/6/6 hexacyclic ring system, while a 6/8/6/6/6 pentacyclic skeleton is occasionally observed. In this study, we demonstrate the formation of an eight-membered C-N heteroring via nonenzymatic flavin-catalyzed oxidative indole ring opening. More interestingly, 18O-labeled experiments proved that the two incorporated oxygen atoms are predominantly originated from water instead of molecular oxygen. In this process, the oxidized form of flavin catalyzes two successive oxidations of amines to imines with involvement of hydrolysis for the ring expansion. The reduced flavin is then regenerated by oxidation with molecular oxygen to form H2O2.

Contemporary photoelectrochemical strategies and reactions in organic synthesis

In recent years, with the development of organic synthetic chemistry, a variety of organic synthetic methods have been discovered and applied in practical production. Photochemistry and electrochemistry have been widely used in organic synthesis recently due to their advantages such as mild conditions and green and environmental protection and have now been developed into two of the most massive synthetic strategies in the field of organic synthesis. In order to further enhance the potential of photochemistry and electrochemistry and to overcome the limitations of each, organic synthetic chemists have worked to combine the two synthetic strategies together to develop photoelectrochemistry as a new synthetic method. Photoelectrochemistry achieves the complementary advantages and disadvantages of photochemistry and electrochemistry, avoids the problem of using stoichiometric oxidants or reductants in photochemistry and easy dimerization in electrochemistry, generates highly reactive reaction intermediates under mild conditions, and achieves reactions that are difficult to accomplish by single photochemistry or electrochemistry. This review summarizes the research progress in the field of photoelectrochemistry from the perspective of photoelectro-chemical catalysts in recent years, analyzes the catalytic mechanism of various catalysts in detail, and gives a brief outlook on the research direction and development prospects in this field.

Multistimuli-Responsive Chromism of Vinylene-Linked Bisflavin Based on the Aggregation and Redox Properties

Multistimuli-responsive chromism was observed for vinylene-linked bisflavin 1 a with an extended π-conjugated platform. The yellow emission of a dilute solution of 1 a in CHCl₃ (0.2 mM) observed at 298 K under UV excitation was changed to orange or red emission upon (1) an increase of concentration, (2) a decrease of temperature, and (3) variation of the solvent. This is in contrast to the almost non stimuli-responsive chromism of the N-methylated bisflavin analogue 1 b and monoflavin 2 a. Mechanistic investigation by ¹ H NMR analysis under various conditions revealed that the extended π-conjugation platform and imide moiety of 1 a generate controllability in the formation of lower- and higher-ordered aggregates, which induce variation of the emission color upon change. Bisflavin 1 a also exhibited redox-induced chromism, where the orange emission of 1 a was quenched by the addition of hydrazine under anaerobic conditions, and changed back to the original emission upon subsequent bubbling of O₂ gas.

Immobilization of a flavin molecule onto poly(methacrylic acid)s and its application in aerobic oxidation catalysis: effect of polymer stereoregularity

The isoalloxazine ring system, called flavin, was successfully immobilized on poly(methacrylic acid)s, PMAAs, with different tacticity via post-polymerization modification under suitable conditions. The resulting flavin-containing polymers showed catalytic activity for aerobic oxidation reactions, in which the polymer stereoregularity clearly influenced their catalytic activity.

Electrophotocatalysis: Combining Light and Electricity to Catalyze Reactions

Visible-light photocatalysis and electrocatalysis are two powerful strategies for the promotion of chemical reactions that have received tremendous attention in recent years. In contrast, processes that combine these two modalities, an area termed electrophotocatalysis, have until recently remained quite rare. However, over the past several years a number of reports in this area have shown the potential of combining the power of light and electrical energy to realize new catalytic transformations. Electrophotocatalysis offers the ability to perform photoredox reactions without the need for large quantities of stoichiometric or superstoichiometric chemical oxidants or reductants by making use of an electrochemical potential as the electron source or sink. In addition, electrophotocatalysis is readily amenable to the generation of open-shell photocatalysts, which tend to have exceptionally strong redox potentials. In this way, potent yet selective redox reactions have been realized under relatively mild conditions. This Perspective highlights recent advances in the area of electrophotocatalysis and provides some possible avenues for future work in this growing area.

Colorimetric high-throughput screening method for directed evolution of prazole sulfide monooxygenase

Baeyer-Villiger monooxygenases (BVMOs) are important biocatalysts for the enzymatic synthesis of chiral sulfoxides, including chiral sulfoxide-type drugs proton pump inhibitors for the treatment of gastrointestinal diseases. However, native BVMOs are not yet suitable for practical application due to their unsatisfactory activity and thermostability. Although protein engineering approaches can help address these issues, few feasible high-throughput methods are available for the engineering of such enzymes. Herein, a colorimetric detection method to distinguish sulfoxides from sulfides and sulfones was developed for prazole sulfide monooxygenases . Directed evolution enabled by this method has identified a prazole sulfide monooxygenase CbBVMO variant with improved activity and thermostability in catalyzing the asymmetric oxidation of lansoprazole sulfide. A 71.3% increase in conversion and 6°C enhancement in the melting point were achieved compared with the wild-type enzyme. This new method is feasible for high-throughput screening of prazole sulfide monooxygenases variants with improved activity, thermostability, and/or substrate specificity.

Derivatives of natural organocatalytic cofactors and artificial organocatalytic cofactors as catalysts in enzymes

Catalytically active non-metal cofactors in enzymes carry out a variety of different reactions. The efforts to develop derivatives of natural occurring cofactors such as flavins or pyridoxal phosphate and the advances to design new, non-natural cofactors are reviewed here. We report the status quo for enzymes harboring organocatalysts as derivatives of natural cofactors or as artificial ones and their application in the asymmetric synthesis of various compounds.

Reduced Molecular Flavins as Single-Electron Reductants after Photoexcitation

Flavoenzymes mediate a multitude of chemical reactions and are catalytically active both in different oxidation states and in covalent adducts with reagents. The transfer of such reactivity to the organic laboratory using simplified molecular flavins is highly desirable, and such applications in (photo)oxidation reactions are already established. However, molecular flavins have not been used for the reduction of organic substrates yet, although this activity is known and well-studied for DNA photolyase enzymes. We report a catalytic method using reduced molecular flavins as photoreductants and γ-terpinene as a sacrificial reductant. Additionally, we present our design for air-stable, reduced flavin catalysts, which is based on a conformational bias strategy and circumvents the otherwise rapid reduction of O₂ from air. Using our catalytic strategy, we were able to replace superstoichiometric amounts of the rare-earth reductant SmI₂ in a 5-exo-trig cyclization of substituted barbituric acid derivatives. Such flavin-catalyzed reductions are anticipated to be beneficial for other transformations as well and their straightforward synthesis indicates future use in stereo- as well as site-selective transformations.

Modulating catalytic activity of a modified flavin analogue via judicially positioned metal ion toward aerobic sulphoxidation

This manuscript describes the synthesis, spectroscopic and crystallographic characterization of a cadmium complex of 10-propoylisoalloxazine-7-carboxylic acid (Flc-Cd). Catalytic activity of Flc-Cd towards aerobic sulphoxidation reaction was investigated in the presence of l-ascorbic acid as the reducing agent. Notably the neutral un-metalated flavin analogue did not show any significant catalytic activity. The design strategy for Flc provides a close proximity of the metal centre to the flavin core without compromising the catalytic site thereby assisting the product formation when compared to unmetallated Flc. Minor enantioselectivity is also observed in cases where unsymmetrical sulphides were used; indicative of the possible involvement of chiral l-ascorbic acid in the intermediate formation.

Design and synthesis of a catalytically efficient metal-flavin complex toward aerobic sulphoxidation was achieved via judicially positioning the metal ion near the catalytic site thereby assisting the intermediate formation.

Efficient Aerobic Oxidation of Organic Molecules by Multistep Electron Transfer

This Minireview presents recent important homogenous aerobic oxidative reactions which are assisted by electron transfer mediators (ETMs). Compared with direct oxidation by molecular oxygen (O₂), the use of a coupled catalyst system with ETMs leads to a lower overall energy barrier via stepwise electron transfer. This cooperative catalytic process significantly facilitates the transport of electrons from the reduced form of the substrate‐selective redox catalyst (SSRC^red) to O₂, thereby increasing the efficiency of the aerobic oxidation. In this Minireview, we have summarized the advances accomplished in recent years in transition‐metal‐catalyzed as well as metal‐free aerobic oxidations of organic molecules in the presence of ETMs. In addition, the recent progress of photochemical and electrochemical oxidative functionalization using ETMs and O₂ as the terminal oxidant is also highlighted. Furthermore, the mechanisms of these transformations are showcased.

5,11-Diazadibenzo[hi,qr]tetracene: Synthesis, Properties, and Reactivity toward Nucleophilic Reagents

5,11-Diazadibenzo[hi,qr]tetracene was synthesized as a new nitrogen-substituted polycyclic heteroaromatic compound by Pd-catalyzed cycloisomerization of an alkyne precursor followed by oxidative cyclization with bis(trifluoroacetoxy)iodobenzene. The substitution of imine-type nitrogen atoms significantly enhanced its electron-accepting character and facilitated the direct nucleophilic addition of arylamines under strongly basic conditions to afford the desired amino-substituted products. The introduction of amino groups induced a remarkable red-shift in their absorption spectra; the tetrasubstituted product exhibited intense near-infrared absorbing property. Furthermore, the π-electronic system, which includes a redox-active 1,4-diazabutadiene moiety, underwent reversible interconversion to its corresponding reduced form upon reduction with NaBH₄ and aerobic oxidation.

Facile access to foldable redox-active flavin-peptide conjugates

A convenient approach for the synthesis of foldable redox-active flavin peptide conjugates was established. A model β-hairpin oligopeptide motif was utilized to demonstrate that azidolysine side-chains are readily functionalised with an alkyne-bearing flavine derivative. The folding equilibrium of the peptide backbone as well as the redox behaviour of the flavin moieties remains intact after the conjugation.

Aerobic Oxidations in Asymmetric Synthesis: Catalytic Strategies and Recent Developments

Despite the remarkable advances in the area of asymmetric catalytic oxidations over the past decades, the development of sustainable and environmentally benign enantioselective oxidation techniques, especially with the efficiency level similar to natural enzymes, still represents a challenge. The growing demand for enantiopure compounds and high interest to industry-relevant green technological advances continue to encourage the research pursuits in this field. Among various oxidants, molecular oxygen is ubiquitous, being available at low cost, environmentally benign and easy-to-handle material. This review highlights recent achievements in catalytic enantioselective oxidations utilizing molecular oxygen as the sole oxidant, with focus on the mechanisms of dioxygen activation and chirogenesis in these transformations.

Synthesis of Xanthones, Thioxanthones and Acridones by a Metal-Free Photocatalytic Oxidation Using Visible Light and Molecular Oxygen

9H-Xanthenes, 9H-thioxanthenes and 9,10-dihydroacridines can be easily oxidized to the corresponding xanthones, thioxanthones and acridones, respectively, by a simple photo-oxidation procedure carried out using molecular oxygen as oxidant under the irradiation of visible blue light and in the presence of riboflavin tetraacetate as a metal-free photocatalyst. The obtained yields are high or quantitative.

Synthetic applications of flavin photocatalysis: a review

Encouraging developments in the field of photocatalysis in last decades, biomolecules namely flavins have been observed to act as a catalyst in several photoredox-catalysed synthetic methodologies.

Flow Photochemistry as a Tool in Organic Synthesis

Photochemical transformations of molecular building blocks have become an important and widely recognized research field in the past decade. Detailed and deep understanding of novel photochemical catalysts and reaction concepts with visible light as the energy source has enabled a broad application portfolio for synthetic organic chemistry. In parallel, continuous-flow chemistry and microreaction technology have become the basis for thinking and doing chemistry in a novel fashion with clear focus on improved process control for higher conversion and selectivity. As can be seen by the large number of scientific publications on flow photochemistry in the recent past, both research topics have found each other as exceptionally well-suited counterparts with high synergy by combining chemistry and technology. This review will give an overview on selected reaction classes, which represent important photochemical transformations in synthetic organic chemistry, and which benefit from mild and defined process conditions by the transfer from batch to continuous-flow mode.

Ferulic Acid Decarboxylase Controls Oxidative Maturation of the Prenylated Flavin Mononucleotide Cofactor

Prenylated flavin mononucleotide (prFMN) is a recently discovered modified flavin cofactor containing an additional nonaromatic ring, connected to the N5 and C6 atoms. This cofactor underpins reversible decarboxylation catalyzed by members of the widespread UbiD enzyme family and is produced by the flavin prenyltransferase UbiX. Oxidative maturation of the UbiX product prFMNH₂ to the corresponding oxidized prFMN^iminium is required for ferulic acid decarboxylase (Fdc1; a UbiD-type enzyme) activity. However, it is unclear what role the Fdc1 enzyme plays in this process. Here, we demonstrate that, in the absence of Fdc1, prFMNH₂ oxidation by O₂ proceeds via a transient semiquinone prFMN^radical species and culminates in a remarkably stable prFMN-hydroperoxide species. Neither forms of prFMN are able to support Fdc1 activity. Instead, enzyme activation using O₂-mediated oxidation requires prFMNH₂ binding prior to oxygen exposure, confirming that UbiD enzymes play a role in O₂-mediated oxidative maturation. In marked contrast, alternative oxidants such as potassium ferricyanide support prFMN^iminium formation both in solution and in Fdc1.

H2 as a fuel for flavin- and H2O2-dependent biocatalytic reactions

The soluble hydrogenase from Ralstonia eutropha provides an atom efficient regeneration system for reduced flavin cofactors using H₂ as an electron source. We demonstrated this system for highly selective ene-reductase-catalyzed C[double bond, length as m-dash]C-double bond reductions and monooxygenase-catalyzed epoxidation. Reactions were expanded to aerobic conditions to supply H₂O₂ for peroxygenase-catalyzed hydroxylations.

Biochemistry of prenylated-FMN enzymes

The reversible (de)carboxylation of unsaturated carboxylic acids is carried out by the UbiX-UbiD system, ubiquitously present in microbes. The biochemical basis of this challenging reaction has recently been uncovered by the discovery of the UbiD cofactor, prenylated FMN (prFMN). This heavily modified flavin is synthesized by the flavin prenyltransferase UbiX, which catalyzes the non-metal dependent prenyl transfer from dimethylallyl(pyro)phosphate (DMAP(P)) to the flavin N5 and C6 positions, creating a fourth non-aromatic ring. Following prenylation, prFMN undergoes oxidative maturation to form the iminium species required for UbiD activity. prFMN^iminium acts as a prostethic group and is bound via metal ion mediated interactions between UbiD and the prFMN^iminium phosphate moiety. The modified isoalloxazine ring is place adjacent to the E(D)-R-E UbiD signature sequent motif. The fungal ferulic acid decarboxylase Fdc from Aspergillus niger has emerged as a UbiD-model system, and has yielded atomic level insight into the prFMN^iminium mediated (de)carboxylation. A wealth of data now supports a mechanism reliant on reversible 1,3 dipolar cycloaddition between substrate and cofactor for this enzyme. This poses the intriguing question whether a similar mechanism is used by all UbiD enzymes, especially those that act as carboxylases on inherently more difficult substrates such as phenylphosphate or benzene/naphthalene. Indeed, considerable variability in terms of oligomerization, domain motion and active site structure is now reported for the UbiD family.

Investigating the Photochemistry of C7 and C8 Functionalized N(5)-Ethyl-flavinium Cation: A Computational Study

Flavins are a diverse set of compounds with a wide variety of biological and nonbiological applications. Applications of flavins receiving attention recently consist of electro- and photocatalytic oxidation of substrates for organic synthesis, bioengineered nanotechnology, and water splitting catalysts, among others. While there is vast knowledge regarding the structure-property relationships of flavins and their electrochemistry, there is much less work elucidating the structure property relationships as they pertain to flavinium photochemistry. Herein, we report the effect of molecular tailoring on the molecular properties of N(5)-ethyl-flavinium cation (Et-Fl⁺), a derivative of the biocatalytic coenzyme riboflavin, by incorporating electron withdrawing and donating groups at the C7 and C8 position of the isoalloxazine ring. The presence of electron withdrawing groups at the C8 position caused a red shift in the absorption spectrum, while the electron donating groups caused a blue shift. Functionalization at the C7 position had the opposite effect on the absorption spectrum. The effects of single substitution were relatively negated with simultaneous functionalization at both the C8 and C7 positions. Difference density plots indicate no change in the nature of the S₁ excited state, which was confirmed by optimization of the excited state geometries. The results presented in this study indicate that functionalization of the isoalloxazine unit affects the photophysical properties of N(5)-ethyl-flavinium cations.

Modern Stereoselective Synthesis of Chiral Sulfinyl Compounds

Chiral sulfinyl compounds, sulfoxides, sulfoximines, sulfinamides, and other derivatives, play an important role in asymmetric synthesis as versatile auxiliaries, ligands, and catalysts. They are also recognized as pharmacophores found in already marketed and well-sold drugs (e.g., esomeprazole) and used in drug design. This review is devoted to the modern methods of preparation of sulfinyl derivatives in enantiopure or enantiomerically enriched form. Selected new approaches leading to racemic products for which the asymmetric variant can be developed in the future are mentioned as well.

Electrochemistry Broadens the Scope of Flavin Photocatalysis: Photoelectrocatalytic Oxidation of Unactivated Alcohols

Riboflavin-derived photocatalysts have been extensively studied in the context of alcohol oxidation. However, to date, the scope of this catalytic methodology has been limited to benzyl alcohols. In this work, mechanistic understanding of flavin-catalyzed oxidation reactions, in either the absence or presence of thiourea as a cocatalyst, was obtained. The mechanistic insights enabled development of an electrochemically driven photochemical oxidation of primary and secondary aliphatic alcohols using a pair of flavin and dialkylthiourea catalysts. Electrochemistry makes it possible to avoid using O2 and an oxidant and generating H2 O2 as a byproduct, both of which oxidatively degrade thiourea under the reaction conditions. This modification unlocks a new mechanistic pathway in which the oxidation of unactivated alcohols is achieved by thiyl radical mediated hydrogen-atom abstraction.

Flavin Conjugated Polydopamine Nanoparticles Displaying Light-Driven Monooxygenase Activity

A hybrid of flavin and polydopamine (PDA) has been explored as a photocatalyst, drawing inspiration from natural flavoenzymes. Light-driven monoxygenase activity has been demonstrated through the oxidation of indole under blue light irradiation in ambient conditions, to afford indigo and indirubin dyes. Compared to riboflavin, a flavin-polydopamine hybrid is shown to be more resistant to photobleaching and more selective toward dye production. In addition, it has been demonstrated that it can be recycled from the solution and used for up to four cycles without a marked loss of activity, which is a significant improvement compared to other heterogenous flavin catalysts. The mechanism of action has been explored, indicating that the PDA shell plays an important role in the stabilization of the intermediate flavin-peroxy species, an active component of the catalytic system rather than acting only as a passive nanocarrier of active centers.

Combining Flavin Photocatalysis and Organocatalysis: Metal-Free Aerobic Oxidation of Unactivated Benzylic Substrates

We report a system with ethylene-bridged flavinium salt 2b which catalyzes the aerobic oxidation of toluenes and benzyl alcohols with high oxidation potential ( E_ox > +2.5 V vs SCE) to give the corresponding benzoic acids under visible light irradiation. This is caused by the high oxidizing power of excited 2b ( E(2b*) = +2.67 V vs SCE) involved in photooxidation and by the accompanying dark organocatalytic oxygenation provided by the in situ formed flavin hydroperoxide 2b-OOH.

Comparison of riboflavin-derived flavinium salts applied to catalytic H2O2 oxidations

A series of flavinium salts, 5-ethylisoalloxazinium, 5-ethylalloxazinium, and 1,10-ethylene-bridged alloxazinium triflates, were prepared from commercially available riboflavin. This study presents a comparison between their optical and redox properties, and their catalytic activity in H2O2 oxidations of sulfide, tertiary amine, and cyclobutanone. Reflecting the difference between the π-conjugated ring structures, the flavinium salts displayed very different redox properties, with reduction potentials in the order of: 5-ethylisoalloxazinium > 5-ethylalloxazinium > 1,10-ethylene-bridged alloxazinium. A comparison of their catalytic activity revealed that 5-ethylisoalloxazinium triflate specifically oxidises sulfide and cyclobutanone, and 5-ethylalloxazinium triflate smoothly oxidises tertiary amine. 1,10-Bridged alloxazinium triflate, which can be readily obtained from riboflavin in large quantities, showed moderate catalytic activity for the H2O2 oxidation of sulfide and cyclobutanone.

Nickel(ii) riboflavin complex as an efficient nanobiocatalyst for heterogeneous and sustainable oxidation of benzylic alcohols and sulfides

A novel Ni(ii)Rf₂ nanocomplex was prepared, which efficiently oxidizes a wide range of benzyl alcohols and sulfides by employing ideal oxidants in green reaction media.

Design of peptide-containing N5-unmodified neutral flavins that catalyze aerobic oxygenations

Simulation of the monooxygenation function of flavoenzyme (Fl-Enz) has been long-studied with N5-modified cationic flavins (FlEt+ ), but never with N5-unmodified neutral flavins (Fl) despite the fact that Fl is genuinely equal to the active center of Fl-Enz. This is because of the greater lability of 4a-hydroperoxy adduct of Fl, FlOOH , compared to those of FlEt+ , FlEtOOH , and Fl-Enz, FlOOH-Enz. In this study, Fl incorporated into a short peptide, flavopeptide (Fl-Pep), was designed by a rational top-down approach using a computational method, which could stabilize the corresponding 4a-hydroperoxy adduct (FlOOH-Pep) through intramolecular hydrogen bonds. We report catalytic chemoselective sulfoxidation as well as Baeyer-Villiger oxidation by means of Fl-Pep under light-shielding and aerobic conditions, which are the first Fl-Enz-mimetic aerobic oxygenation reactions catalyzed by Fl under non-enzymatic conditions.

Riboflavin as a bioorthogonal photocatalyst for the activation of a PtIV prodrug

Encouraging developments demonstrate that few transition metal and organometallic catalysts can operate in a bioorthogonal fashion and promote non-natural chemistry in living systems by minimizing undesired side reactions with cellular components. These catalytic processes have potential for applications in medicinal chemistry and chemical biology. However, the stringent conditions of the cell environment severely limit the number of accessible metal catalysts and exogenous reactions. Herein, we report an unorthodox approach and a new type of bioorthogonal catalytic reaction, in which a metal complex is an unconventional substrate and an exogenous biological molecule acts as a catalyst. In this reaction, riboflavin photocatalytically converts a PtIV prodrug into cisplatin within the biological environment. Due to the catalytic activity of riboflavin, cisplatin-like apoptosis is induced in cancer cells under extremely low doses of light, potentially preventing systemic off-target reactions. Photocatalytic and bioorthogonal turnover of PtIV into PtII species is an attractive strategy to amplify the antineoplastic action of metal-based chemotherapeutics with spatio-temporal control.