Zugang zu metallorganischen Arylcobaltcorrinen durch radikalische Synthese: 4-Ethylphenylcobalamin, ein potenzielles “Antivitamin B12”

Surprising Homolytic Gas Phase Co-C Bond Dissociation Energies of Organometallic Aryl-Cobinamides Reveal Notable Non-Bonded Intramolecular Interactions

Aryl-cobalamins are a new class of organometallic structural mimics of vitamin B12 designed as potential 'antivitamins B12 '. Here, the first cationic aryl-cobinamides are described, which were synthesized using the newly developed diaryl-iodonium method. The aryl-cobinamides were obtained as pairs of organometallic coordination isomers, the stereo-structure of which was unambiguously assigned based on homo- and heteronuclear NMR spectra. The availability of isomers with axial attachment of the aryl group, either at the 'beta' or at the 'alpha' face of the cobalt-center allowed for an unprecedented comparison of the organometallic reactivity of such pairs. The homolytic gas-phase bond dissociation energies (BDEs) of the coordination-isomeric phenyl- and 4-ethylphenyl-cobinamides were determined by ESI-MS threshold CID experiments, furnishing (Co-Csp 2 )-BDEs of 38.4 and 40.6 kcal mol-1 , respectively, for the two β-isomers, and the larger BDEs of 46.6 and 43.8 kcal mol-1 for the corresponding α-isomers. Surprisingly, the observed (Co-Csp 2 )-BDEs of the Coβ -aryl-cobinamides were smaller than the (Co-Csp 3 )-BDE of Coβ -methyl-cobinamide. DFT studies and the magnitudes of the experimental (Co-Csp 2 )-BDEs revealed relevant contributions of non-bonded interactions in aryl-cobinamides, notably steric strain between the aryl and the cobalt-corrin moieties and non-bonded interactions with and among the peripheral sidechains.

Antivitamins B12 -Some Inaugural Milestones

The recently delineated structure- and reactivity-based concept of antivitamins B12 has begun to bear fruit by the generation, and study, of a range of such B12 -dummies, either vitamin B12 -derived, or transition metal analogues that also represent potential antivitamins B12 or specific B12 -antimetabolites. As reviewed here, this has opened up new research avenues in organometallic B12 -chemistry and bioinorganic coordination chemistry. Exploratory studies with antivitamins B12 have, furthermore, revealed some of their potential, as pharmacologically interesting compounds, for inducing B12 -deficiency in a range of organisms, from hospital resistant bacteria to laboratory mice. The derived capacity of antivitamins B12 to induce functional B12 -deficiency in mammalian cells and organs also suggest their valuable potential as growth inhibitors of cancerous human and animal cells.

Replacement of the Cobalt Center of Vitamin B12 by Nickel: Nibalamin and Nibyric Acid Prepared from Metal-Free B12 Ligands Hydrogenobalamin and Hydrogenobyric Acid

The (formal) replacement of Co in cobalamin (Cbl) by NiII generates nibalamin (Nibl), a new transition-metal analogue of vitamin B12 . Described here is Nibl, synthesized by incorporation of a NiII ion into the metal-free B12  ligand hydrogenobalamin (Hbl), itself prepared from hydrogenobyric acid (Hby). The related NiII  corrin nibyric acid (Niby) was similarly synthesized from Hby, the metal-free cobyric acid ligand. The solution structures of Hbl, and Niby and Nibl, were characterized by spectroscopic studies. Hbl features two inner protons bound at N2 and N4 of the corrin ligand, as discovered in Hby. X-ray analysis of Niby shows the structural adaptation of the corrin ligand to NiII ions and the coordination behavior of NiII . The diamagnetic Niby and Nibl, and corresponding isoelectronic CoI corrins, were deduced to be isostructural. Nibl is a structural mimic of four-coordinate base-off Cbls, as verified by its ability to act as a strong inhibitor of bacterial adenosyltransferase.

Does a Conjugation Site Affect Transport of Vitamin B12 -Peptide Nucleic Acid Conjugates into Bacterial Cells?

Gram-negative bacteria develop specific systems for the uptake of scarce nutrients, including vitamin B₁₂ . These uptake pathways may be utilized for the delivery of biologically relevant molecules into cells. Indeed, it was recently reported that vitamin B₁₂ transported an antisense peptide nucleic acid (PNA) into Escherichia coli and Salmonella Typhimurium cells. The present studies indicate that the conjugation site of PNA to vitamin B₁₂ has an impact on PNA transport into bacterial cells. Toward this end, a specifically designed PNA oligomer has been tethered at various positions of vitamin B₁₂ (central Co, R_5' -OH, c and e amide chains, meso position, and at the hydroxy group of cobinamide) by using known or newly developed methodologies and tested for the uptake of the synthesized conjugates by E. coli. Compounds in which the PNA oligonucleotide was anchored at the R_5' -OH position were transported more efficiently than that of other compounds tethered at the peripheral positions around the corrin ring. Of importance is the fact that, contrary to mammalian organisms, E. coli also takes up cobinamide, which is an incomplete corrinoid. This selectivity opens up ways to fight bacterial infections.

Organometallic DNA-B12 Conjugates as Potential Oligonucleotide Vectors: Synthesis and Structural and Binding Studies with Human Cobalamin-Transport Proteins

The synthesis and structural characterization of Co-(dN)₂₅ -Cbl (Cbl: cobalamin; dN: deoxynucleotide) and Co-(dN)₃₉ -Cbl, which are organometallic DNA-B₁₂ conjugates with single DNA strands consisting of 25 and 39 deoxynucleotides, respectively, and binding studies of these two DNA-Cbl conjugates to three homologous human Cbl transporting proteins, transcobalamin (TC), intrinsic factor (IF), and haptocorrin (HC), are reported. This investigation tests the suitability of such DNA-Cbls for the task of eventual in vivo oligonucleotide delivery. The binding of DNA-Cbl to TC, IF, and HC was investigated in competition with either a fluorescent Cbl derivative and Co-(dN)₂₅ -Cbl, or radiolabeled vitamin B₁₂ (⁵⁷ Co-CNCbl) and Co-(dN)₂₅ -Cbl or Co-(dN)₃₉ -Cbl. Binding of the new DNA-Cbl conjugates was fast and tight with TC, but poorer with HC and IF, which extends a similar original finding with the simpler DNA-Cbl, Co-(dN)₁₈ -Cbl. The contrasting affinities of TC versus IF and HC for the DNA-Cbl conjugates are rationalized herein by a stepwise mechanism of Cbl binding. Critical contributions to overall affinity result from gradual conformational adaptations of the Cbl-binding proteins to the DNA-Cbl, which is first bound to the respective β domains. This transition is fast with TC, but slow with IF and HC, with which weaker binding results. The invariably tight interaction of the DNA-Cbl conjugates with TC makes the Cbl moiety a potential natural vector for the specific delivery of oligonucleotide loads from the blood into cells.

Alpha- and Beta-Diastereoisomers of Phenylcobalamin from Cobalt-Arylation with Diphenyliodonium Chloride

Organometallic aryl-cobalamins are B₁₂ -derivatives featuring properties of potential 'B₁₂ antivitamins'. Herein, we describe a new method for the preparation of aryl-cobalamins using versatile diaryliodonium salts as arylation agents. Formate or sodium borohydride reduction of aquocobalamin in presence of diphenyliodonium chloride furnished Co_β -phenyl-cobalamin PhCbl in a roughly 3:1 to 1:1 ratio with its coordination isomer αPhCbl, a first representative 'base-off' Co_α -aryl-cobalamin. The new structures were secured by detailed spectroscopic analysis, supplemented by an X-ray crystal structure analysis of PhCbl. Both types of coordination isomers of the aryl-cobalamins promise to be useful molecular tools in biomedical and biological studies.

Antivitamin B12 Inhibition of the Human B12 -Processing Enzyme CblC: Crystal Structure of an Inactive Ternary Complex with Glutathione as the Cosubstrate

B₁₂ antivitamins are important and robust tools for investigating the biological roles of vitamin B₁₂ . Here, the potential antivitamin B₁₂ 2,4-difluorophenylethynylcobalamin (F2PhEtyCbl) was prepared, and its 3D structure was studied in solution and in the crystal. Chemically inert F2PhEtyCbl resisted thermolysis of its Co-C bond at 100 °C, was stable in bright daylight, and also remained intact upon prolonged storage in aqueous solution at room temperature. It binds to the human B₁₂ -processing enzyme CblC with high affinity (K_D =130 nm) in the presence of the cosubstrate glutathione (GSH). F2PhEtyCbl withstood tailoring by CblC, and it also stabilized the ternary complex with GSH. The crystal structure of this inactivated assembly provides first insight into the binding interactions between an antivitamin B₁₂ and CblC, as well as into the organization of GSH and a base-off cobalamin in the active site of this enzyme.

Antivitamins B12--A Structure- and Reactivity-Based Concept

B12 -antimetabolites are compounds that counteract the physiological effects of vitamin B12 and related natural cobalamins. Presented here is a structure- and reactivity-based concept of the specific 'antivitamins B12 ': it refers to analogues of vitamin B12 that display high structural similarity to the vitamin and are 'locked chemically' to prevent their metabolic conversion into the crucial organometallic B12 -cofactors. Application of antivitamins B12 to healthy laboratory animals is, thus, expected to induce symptoms of B12 -deficiency. Antivitamins B12 may, hence, be helpful in elucidating still largely puzzling pathophysiological phenomena associated with B12 -deficiency, and also in recognizing physiological roles of B12 that probably still remain to be discovered.

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 .

Organometallic B12-DNA conjugate: synthesis, structure analysis, and studies of binding to human B12-transporter proteins

Design, synthesis, and structural characterization of a B12-octadecanucleotide are presented herein, a new organometallic B12-DNA conjugate. In such covalent conjugates, the natural B12 moiety may be a versatile vector for controlled in vivo delivery of oligonucleotides to cellular targets in humans and animals, through the endogenous B12 transport systems. Binding of the organometallic B12 octadecanucleotide to the three important human proteins of B12 transport was studied, to examine its structural suitability for the task of eventual in vivo oligonucleotide delivery. Binding was efficient with transcobalamin (TC), but not so efficient with the homologous glycoproteins intrinsic factor and haptocorrin. Binding of the B12 octadecanucleotide to TC suggests the capacity of the B12 moiety to serve as a natural vector for specific transport of single stranded, organometallic oligonucleotide loads from the blood stream into cells.