Functionalisation of vitamin B12 derivatives with a cobalt ß-phenyl ligand boosters antimetabolite activity in bacteria.

This study describes the syntheses of four singly- and two doubly-modified vitamin B12 derivatives for generating antimetabolites of Lactobacillus delbrueckii (L. delbrueckii). The two most potent antagonists, a Coß-phenyl-cobalamin-c,8-lactam and a 10-bromo-Coß-phenylcobalamin combine a c-lactam or 10-bromo modification at the "eastern" site of the corrin ring with an artificial organometallic phenyl group instead of a cyano ligand at the ß-site of the cobalt center. These two doubly-modified B12 antagonists (10 nM) inhibit fully B12-dependent (0.1 nM) growth of L. delbrueckii. In contrast to potent 10-bromo-Coß-phenylcobalamin, single modified 10-bromo-Coß-cyanocobalamin lacking the artificial organometallic phenyl ligand does not show any inhibitory effect. These results suggest, that the organometallic ß-phenyl ligand at the Co center ultimately steers the metabolic effect of the 10-bromo-analogue.

A CuII -Salicylidene Glycinato Complex for the Selective Fluorometric Detection of Homocysteine over 20 Proteinogenic Amino Acids.

Homocysteine (Hcy) is a sulfur-containing α-amino acid that differs by one methylene (CH2 ) subunit from homologous cysteine (Cys). Elevated levels of Hcy are diagnostic markers of cardiovascular disease and other medical conditions. We present a new CuII -salicylidene glycinato complex 1 for the selective fluorometric detection of Hcy in water. In the presence of this analyte, the non-fluorescent copper-complex demetallates and disassembles into its building blocks. This process liberates a 3-chloro-5-sulfosalicylaldehyde signaling unit and is accompanied by a 51-fold turn-on fluorescence at 485 nm (λex =350 nm). Out of twenty proteinogenic amino acids, only histidine (12-fold turn-on fluorescence) and Cys (8-fold turn-on fluorescence) trigger some disassembly of probe 1. In comparison with important pioneering work on the detection of biothiols, this study strikingly demonstrates that structural modifications of chelate core structures steer substrate selectivity of metal-based probes. Importantly, probe 1 has proven suitable for the detection of Hcy in artificial urine.

Induced fit activity-based sensing: a mechanistic study of pyrophosphate detection with a "flexible" Fe-salen complex.

Activity-based sensing of biological targets is attracting increasing attention. In this work, we report detailed UV-Vis and fluorescence mechanistic studies on an Fe-salen based probe, [FeIII{salenMeCl2(SO3)2}OH2]- for pyrophosphate (PPi) detection. In the presence of PPi as an analyte, the probe disassembles into its molecular subunits and releases a fluorescent signal. Our studies illustrate that the aqua form of the complex (1-OH2) is the active species and that upon substitution of Fe-coordinated H2O and an initial end-on coordination of HP2O7 3-, the "trapped" pyrophosphate species switches from a monodentate to a bidentate coordination mode (i.e. linkage isomerism) via a probable equilibrium process. The elusive intermediate is further stabilized by a hydrogen bonding interaction that activates the probe for the subsequent final irreversible rate-limiting step, and allows selective discrimination between the other pyrophosphate (H2P2O7 2- and P2O7 4-) species in favour of the HP2O7 3-. The flexible mode of molecular recognition and binding of HP2O7 3- by the tetradentate probe 1-OH2 is unexpected and most effective at physiological pH, and has precedence in enzymatic catalysis (i.e. induced fit principle). These binding properties explain the previously observed outstanding selectivity of 1-OH2 for pyrophosphate over other (poly)oxophosphates and potentially competing analytes.

Detection of glyphosate with a copper(II)-pyrocatechol violet based GlyPKit.

This paper describes the development of a test kit for the selective detection of glyphosate (GlyP). A copper(II)-pyrocatechol violet complex was selected by a screening approach from a pool of 96 combinations of metal ions and commercially available indicators and subsequently incorporated as a detection zone into a hydrophobic C18 solid support. With this kit, detection of 20 µM GlyP in tap water by the "naked eye" is possible and quantifications by smartphone analysis with a limit of detection as low as 2.66 µM (450 µg L-1) have been demonstrated in a proof-of-principle study.

Reversible metal-centered reduction empowers a Ni-Corrin to mimic F430.

This communication presents a novel truncated NiII-containing metbalamin and describes its reversible one electron reduction to a catalytically active NiI species, that features cofactor F430 model character. Our results strikingly demonstrate that stabilization of NiI is not restricted to the related hydroporhyrinoid ligands and is of relevance to the application of metallocorrins in (biomimetic) catalysis.

Metal-Salen-based Probes for the Selective Detection of Phosphates via a Disassembly Approach.

Polyoxophosphates are ubiquitous in nature and play important roles in biological systems and in the environment. Pyrophosphate (PPi), an important member of this family, is produced in over 200 biochemical reactions and is a marker in clinical diagnosis. In addition to its endogenous role, PPi alias E450 is currently used as preservative, emulsifier or taste intensifier in foodstuff. Despite this widespread occurrence and biological importance, it is rather surprising that robust chemical systems that detect selectively and sensitively PPi in challenging matrices are still lacking. This mini review focuses on metal-salen complexes as reaction-based fluorescent sensors for the selective detection of PPi and other phosphates. The mode of detection is based on a novel disassembly approach in which the metal ion is sequestered by the target analyte from the complex and the metal-free ligand hydrolyses spontaneously into its fluorescent subunits. Optimizations of the probe and applications for PPi detection in cells and foodstuff are described.

FeIII -Salen-Based Probes for the Selective and Sensitive Detection of E450 in Foodstuff.

Inorganic pyrophosphate (PPi) is considered as a diagnostic marker for various diseases such as cancer and vascular calcification. PPi also plays an important preservative role as an additive E450 in foodstuff. In this work, a selective FeIII -salen-based probe for PPi is described; this probe disassembles in the presence of the target analyte into its molecular blocks, 1,2-propanediamine and 3-chloro-5-formyl-4-hydroxybenzenesulfonic acid. The latter signaling unit leads to a fluorometric response. Compared with a related prototype, the new complex shows a 2.3-times stronger emission at 500 nm and a 155-times better selectivity of PPi over adenosine triphosphate (ATP). Importantly, the new probe was successfully applied for detecting E450 in foodstuff.

Quantifying cyanide in water and foodstuff using corrin-based CyanoKit technologies and a smartphone.

This paper describes the detection of endogenous cyanide using corrin-based CyanoKit technologies in combination with a smartphone readout device. When applied to the detection of cyanide in water, this method demonstrates high repeatability and discriminative power with a limit of blank of 0.074 ppm and an instrument limit of detection of 0.13 ppm. Quantification of endogenous cyanide in cassava and bitter almond extracts with the smartphone readout is in excellent agreement with independent analyses using traditional spectrophotometric detection. The prototype system objectively detects levels of cyanide with a high granularity at the point-of-need and does not depend on large, heavy and expensive instrumentation. The methodology has the potential to be easily adopted in resource limited situations and low-income countries.

A Nickel(II)-Containing Vitamin†B12 Derivative with a Cofactor-F430-type π-System.

F430 is a unique enzymatic cofactor in the production and oxidation of methane by strictly anaerobic bacteria. The key enzyme methyl coenzyme M reductase (MCR) contains a hydroporphinoid nickel complex with a characteristic absorption maximum at around 430 nm in its active site. Herein, the three-step semisynthesis of a hybrid NiII -containing corrinoid that partly resembles F430 in its structural and spectroscopic features from vitamin B12 is presented. A key step of the route is the simultaneous demetalation and ring closure reaction of a 5,6-secocobalamin to metal-free 5,6-dihydroxy-5,6-dihydrohydrogenobalamin with cobaltocene and KCN under reductive conditions. Studies on the coordination chemistry of the novel compound support an earlier hypothesis why nature carefully selected a corphin over a corrin ligand in F430 for challenging nickel-catalyzed biochemical reactions.

Stabilizing intramolecular cobalt-imidazole coordination with a remote methyl group in the backbone of a cofactor B12-protein model.

This communication describes the stabilizing effect (ΔΔG° = -4 kJ mol-1) of a remote methyl group in the backbone of a cobalamin-enzyme mimic on intramolecular imidazole-cobalt coordination. For this purpose, two B12 derivatives with an appended imidazole base were synthesized and analysed with spectrophotometric pH titrations. Qualitative conformation analysis of the backbone structure suggests that a thermodynamically unfavoured gauche interaction in the base-off form of a model containing an (R)-configured CH3 group at position C176 of the linker between the corrin ring and the terminal imidazole ligand steers the base toward cobalt coordination.

Determination of the formal redox potentials of the cyanhaemoglobin/cyanmethaemoglobin and the myoglobin/metmyoglobin couples at neutral pH.

Determination of a representative formal redox potential of the Fe(II)/Fe(III) redox couple in cyanhaemoglobin, at pH=7 and related to the state in solution, was the objective of this work. It was achieved at low concentrations of the protein (5µM) to circumvent undesired adsorption. Square-wave voltammetry instead of classical cyclic voltammetry was applied because this method is more sensitive and provides information on the formal redox potential and reversibility, even for rapid processes. We obtained E°'=-0.12±0.01V for cyanhaemoglobin and E°'=-0.10±0.01V, vs. SHE, for myoglobin in comparison. These values differ by only 20mV because the two Fe(II)/Fe(III) redox centres are embedded in closely resembling chemical environments. The small difference is probably owed to the additional axially coordinating cyanide ligand in cyanmethaemoglobin which slightly favours the Fe(III) state in the haem macrocycle.

Modified biovectors for the tuneable activation of anti-platelet carbon monoxide release.

This communication describes the anti-platelet effects of a new class of cis-rhenium(ii)-dicarbonyl-vitamin B12 complexes (B12-ReCORMs) with tuneable CO releasing properties.

Inorganic Cyanide as Protecting Group in the Stereospecific Reconstitution of Vitamin B12 from an Artificial Green Secocorrinoid.

The synthesis of vitamin B12 in four steps from an artificial green secocorrinoid is presented. The stereospecific reconstitution of the B-ring of the cobalamin involves a quantitative and rapid ligand-centered radical ring closure reaction leading first to a new B12 derivative with antivitamin activity that is subsequently converted to the natural product. Chemoselectivity in the one-electron reduction of the macrocycle was achieved by introducing inorganic cyanide as an axially coordinating protecting group of the otherwise reduction-sensitive CoIII-ion. The integrity of structure and function of the reconstituted natural product was unequivocally proven by single crystal structural analysis and a microbiological assay using Lactobacillus leichmannii.

A Disassembly Strategy for Imaging Endogenous Pyrophosphate in Mitochondria by Using an Fe(III) -salen Complex.

Inorganic pyrophosphate (PPi) is produced from nucleoside triphosphates in important biosynthetic reactions and is considered a diagnostic marker for various diseases, such as cancer, crystal deposition disease, and arthritis. Traditional methods for biological PPi detection rely on off-line analytics after sample destruction. Molecular probes for imaging this biologically important analyte with temporal and spatial control in living cells are currently in demand. Herein, we report an Fe(III) -salen complex as the first small reaction-based probe for endogenous mitochondrial PPi following a disassembly approach. Significantly, we successfully applied this complex for the detection of increased cellular PPi levels, and its performance was not affected by the presence of mitochondrial ATP in living cells.

Modulating the cobalt redox potential through imidazole hydrogen bonding interactions in a supramolecular biomimetic protein-cofactor model.

A realistic model for the active site of histidine-on cobalamin@protein complexes is reported and studied under homogeneous and immobilized conditions. Analysis of lower ligand modulation and its influence on the properties of the biomimetic compound are presented. The cofactor attachment by a protein's histidine residue was imitated by covalently linking an artificial imidazole-containing linker to cobyric acid. The resulting intramolecular coordination complex is an excellent structural model of its natural archetype, according to 2D 1H-NMR studies and molecular modeling. The effect of deprotonation of the axially coordinating imidazole ligand - as proposed for natural cofactor complexes - tunes significantly the position of the cathodic peak (ΔV = -203 mV) and stabilizes thereby the CoIII form. Partial deprotonation of the imidazole moiety through hydrogen bonding interactions was then achieved by immobilizing the biomimetic model on hydrophobic C18 silica, which yielded an unprecedented insight on how this class of Cbl-dependent proteins may fine-tune their properties in biological systems.

Detecting biologically relevant phosphates with locked salicylaldehyde probes in water.

This communication describes a disassembly based approach for the detection of biologically relevant di- and triphosphates in water using locked fluorescent salicylaldehyde probes.

Recent trends in the development of vitamin B12 derivatives for medicinal applications.

This Feature Article highlights recent developments in the field of vitamin B12 derivatives for medicinal applications. The following topics are emphasized: (1) the development of aquacorrinoids for cyanide detection and detoxification, (2) the use of vitamin B12 conjugates and (3) antivitamins B12 for therapy and diagnosis, and (4) the design of corrinoids as activators of soluble guanylyl cyclase (sGC).

Antivitamins for Medicinal Applications.

Antivitamins represent a broad class of compounds that counteract the essential effects of vitamins. The symptoms triggered by such antinutritional factors resemble those of vitamin deficiencies, but can be successfully reversed by treating patients with the intact vitamin. Despite being undesirable for healthy organisms, the toxicities of these compounds present considerable interest for biological and medicinal purposes. Indeed, antivitamins played fundamental roles in the development of pioneering antibiotic and antiproliferative drugs, such as prontosil and aminopterin. Their development and optimisation were made possible by the study, throughout the 20th century, of the vitamins' and antivitamins' functions in metabolic processes. However, even with this thorough knowledge, commercialised antivitamin-based drugs are still nowadays limited to antagonists of vitamins B9 and K. The antivitamin field thus still needs to be explored more intensely, in view of the outstanding therapeutic success exhibited by several antivitamin-based medicines. Here we summarise historical achievements and discuss critically recent developments, opportunities and potential limitations of the antivitamin approach, with a special focus on antivitamins K, B9 and B12 .

Corrin-based chemosensors for the ASSURED detection of endogenous cyanide.

Cassava (Manihot esculenta Crantz) is a staple food for more than 500 million people, especially in Africa and South America. However, its consumption bears risks as it contains cyanogenic glycosides that convert enzymatically to toxic cyanide during cell damage. To avoid serious health problems by unintentional cyanide intake, this dangerous product of decomposition must be removed before consumption. For monitoring such food processing procedures and for controlling the quality and safety of cassava products on the market, a convenient and reliable analytical method for routine applications without laboratory equipment is required. This Perspective summarizes the authors' work on corrin-based chemosensors for the ('naked-eye') detection of endogenous cyanide in cassava samples. Considering selectivity, sensitivity, handling and speed of detection, these systems are superior to currently applied methods. Based on these properties, the development of a test kit for application by rural farmers in remote locations is proposed.

Straightforward rapid spectrophotometric quantification of total cyanogenic glycosides in fresh and processed cassava products.

In this study, we extend pioneering studies and demonstrate straightforward applicability of the corrin-based chemosensor, aquacyanocobyrinic acid (ACCA), for the instantaneous detection and rapid quantification of endogenous cyanide in fresh and processed cassava roots. Hydrolytically liberated endogenous cyanide from cyanogenic glycosides (CNp) reacts with ACCA to form dicyanocobyrinic acid (DCCA), accompanied by a change of colour from orange to violet. The method was successfully tested on various cassava samples containing between 6 and 200 mg equiv. HCN/kg as verified with isonicotinate/1,3-dimethylbarbiturate as an independent method. The affinity of ACCA sensor to cyanide is high, coordination occurs fast and the colorimetric response can therefore be instantaneously monitored with spectrophotometric methods. Direct applications of the sensor without need of extensive and laborious extraction processes are demonstrated in water-extracted samples, in acid-extracted samples, and directly on juice drops. ACCA showed high precision with a standard deviation (STDV) between 0.03 and 0.06 and high accuracy (93-96%). Overall, the ACCA procedure is straightforward, safe and easily performed. In a proof-of-concept study, rapid screening of ten samples within 20 min has been tested.