Cu-catalyzed click conjugation of cobalamin to a BODIPY-based fluorophore: A versatile tool to explore the cellular biology of vitamin B12.

The Cu-catalyzed click conjugation of an azide-functionalized vitamin B12 (cobalamin) and an alkyne-labeled 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) led to the formation of a highly stable fluorescent BODIPY-labeled vitamin B12 (λex/λem = 495/508 nm). The formation of what has been identified as an iodine adduct of the conjugate was also observed as a side-product during this reaction and could be removed using HPLC. BODIPY-labeled vitamin B12 was characterized by NMR and HR-ESI-MS. In vitro studies on wild-type human fibroblasts indicated that BODIPY-labeled vitamin B12 could internalize in a manner similar to that of untagged vitamin B12. ATP-binding cassette sub-family D member 4 (ABCD4) is a lysosomal localized transporter required to export vitamin B12 from the lysosomal lumen to the cytosol. Mutations in this transporter result in the accumulation of vitamin B12 in lysosomes. In human fibroblasts harbouring a mutation in ABCD4, BODIPY-labeled vitamin B12 accumulated in the lumen of lysosomes. Our data suggests the potential use of BODIPY-labeled vitamin B12 to investigate the intracellular behavior of the vitamin in the context of disorders related to the abnormal cellular utilization of the vitamin. Moreover, results presented here demonstrate that click chemistry could be exploited for the conjugation of vitamin B12 to various other fluorophores.

Exceptional Photochemical Stability of the Co-C Bond of Alkynyl Cobalamins, Potential Antivitamins B12 and Core Elements of B12-Based Biological Vectors.

Alkynylcorrinoids are a class of organometallic B12 derivatives, recently rediscovered for use as antivitamins B12 and as core components of B12-based biological vectors. They feature exceptional photochemical and thermal stability of their characteristic extra-short Co-C bond. We describe here the synthesis and structure of 3-hydroxypropynylcobalamin (HOPryCbl) and photochemical experiments with HOPryCbl, as well as of the related alkynylcobalamins: phenylethynylcobalamin and difluoro-phenylethynylcobalamin. Ultrafast spectroscopic studies of the excited state dynamics and mechanism for ground state recovery demonstrate that the Co-C bond of alkynylcobalamins is stable, with the Co-N bond and ring deformations mediating internal conversion and ground state recovery within 100 ps. These studies provide insights required for the rational design of photostable or photolabile B12-based cellular vectors.

Investigation Of Vitamin B12-Modified Amphiphilic Sodium Alginate Derivatives For Enhancing The Oral Delivery Efficacy Of Peptide Drugs.

PURPOSE: Peptide drugs have been used in therapy various diseases. However, the poor bioavailability of peptide drugs for oral administration has limited their clinical applications, on account of the acidic environment and digestive enzymes inside the human gastrointestinal tract. To enhance stability in the human gastrointestinal tract, bioavailability, and targeted drug delivery of peptide drugs through oral administration, a vitamin B12-modified amphiphilic sodium alginate derivative (CSAD-VB12) was synthesized. MATERIALS AND METHODS: A vitamin B12-modified amphiphilic sodium alginate derivative (CSAD-VB12) was synthesized via the N,N'-dicyclohexylcarbodiimide active method at room temperature, and then characterized using FTIR and 1H NMR spectroscopy. Insulin was used as a model peptide drug and the insulin-loaded CSAD-VB12 (CSAD-VB12/insulin) nanoparticles with negative zeta potentials were prepared in PBS (pH=7.4). Scanning electron microscopy was used to observe CSAD-VB12/insulin as spherical nanoparticles. The CSAD-VB12 derivatives and CSAD-VB12/insulin nanoparticles displayed nontoxicity towards the human colon adenocarcinoma (Caco-2) cells by CCK-8 test. Caco-2 cell model was used to measure the apparent permeability (Papp) of insulin, CSAD/insulin and CSAD-VB12/insulin. Furthermore, confocal was used to confirm the endocytosis of intestinal enterocytes. Type 1 diabetes mice were used to evaluate the intestinal absorption and retention effect of test nanoparticles. RESULTS: They were observed as spherical nanoparticles in the size of 30-50 nm. The CSAD-VB12 derivatives and CSAD-VB12/insulin nanoparticles displayed nontoxicity towards the human colon adenocarcinoma (Caco-2) cells. Comparing with insulin and the CSAD/insulin nanoparticles, the CSAD-VB12/insulin nanoparticles exhibited higher permeation ability through intestinal enterocytes in the Caco-2 cell model. Oral administration of the CSAD-VB12/insulin nanoparticles to Type 1 diabetic mice yields higher intestinal retention effect, targeted absorption, and outstanding efficacy. CONCLUSION: CSAD-VB12 derivatives enhance the small intestinal absorption efficacy and retention of peptide by oral administration, which indicated that it could be a promising candidate for oral peptide delivery in the prospective clinical application.

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.

Bioavailability of Vitamin B12 from Dairy Products Using a Pig Model.

The present study compares the bioavailability of vitamin B12 (B12) of dairy products or synthetic B12, using the pig as an experimental model for humans. Eleven pigs were used in a cross-over design to assess the net portal drained viscera (PDV) flux of blood plasma B12 after ingestion of tofu (TF; devoid of B12), Swiss cheese (SC), Cheddar cheese (CC), yogurt (YG), and synthetic B12 (TB12; TF supplemented with cyanocobalamin), providing a total of 25 µg of B12 each. PDV blood plasma flow for SC and CC were higher than for TF and TB12 (p ≤ 0.04) whereas YG was higher than TF (p = 0.05). Porto-arterial difference of blood plasma B12 concentrations were higher for CC and TB12 than for TF and YG (p ≤ 0.04) but not different from SC (p ≥ 0.15). Net PDV flux of B12 was only different from zero for CC. However, the net PDV flux of B12 for CC was not different from SC or TB12. Cumulative net PDV flux of B12 for SC, TB12, and CC were 2.9, 4.4, and 8.3 µg 23 h post-meal, corresponding to a bioavailability of 11.6%, 17.5%, and 33.0%, respectively. In conclusion, CC had the best bioavailability of B12 among the tested dairy products or compared to synthetic B12.

A Cobalamin Activity-Based Probe Enables Microbial Cell Growth and Finds New Cobalamin-Protein Interactions across Domains.

Understanding the factors that regulate microbe function and microbial community assembly, function, and fitness is a grand challenge. A critical factor and an important enzyme cofactor and regulator of gene expression is cobalamin (vitamin B12). Our knowledge of the roles of vitamin B12 is limited, because technologies that enable in situ characterization of microbial metabolism and gene regulation with minimal impact on cell physiology are needed. To meet this need, we show that a synthetic probe mimic of B12 supports the growth of B12-auxotrophic bacteria and archaea. We demonstrate that a B12 activity-based probe (B12-ABP) is actively transported into Escherichia coli cells and converted to adenosyl-B12-ABP akin to native B12 Identification of the proteins that bind the B12-ABP in vivo in E. coli, a Rhodobacteraceae sp. and Haloferax volcanii, demonstrate the specificity for known and novel B12 protein targets. The B12-ABP also regulates the B12 dependent RNA riboswitch btuB and the transcription factor EutR. Our results demonstrate a new approach to gain knowledge about the role of B12 in microbe functions. Our approach provides a powerful nondisruptive tool to analyze B12 interactions in living cells and can be used to discover the role of B12 in diverse microbial systems.IMPORTANCE We demonstrate that a cobalamin chemical probe can be used to investigate in vivo roles of vitamin B12 in microbial growth and regulation by supporting the growth of B12 auxotrophic bacteria and archaea, enabling biological activity with three different cell macromolecules (RNA, DNA, and proteins), and facilitating functional proteomics to characterize B12-protein interactions. The B12-ABP is both transcriptionally and translationally able to regulate gene expression analogous to natural vitamin B12 The application of the B12-ABP at biologically relevant concentrations facilitates a unique way to measure B12 microbial dynamics and identify new B12 protein targets in bacteria and archaea. We demonstrate that the B12-ABP can be used to identify in vivo protein interactions across diverse microbes, from E. coli to microbes isolated from naturally occurring phototrophic biofilms to the salt-tolerant archaea Haloferax volcanii.

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.

5'-Vitamin B12 derivatives suitable for bioconjugation via the amide bond.

Vitamin B12 is an attractive candidate for a drug or an imaging-agent carrier into cells, due to its dietary uptake and well established transport through glycoproteins. Utilization of this system requires an appropriate functionalization of vitamin B12 that both allows for the conjugation of therapeutics and does not interrupt its recognition by transport proteins. Modifications at the 5'-position on the ribose moiety are among a few approaches which meet the criteria. In this article we present vitamin B12 derivatives bearing either the amino or the carboxylic group at the 5'-position. The presence of these functional groups enables conjugation of biologically important molecules to vitamin B12via the amide bond. The established method is not only limited to organic media but also works in an aqueous environment, giving the desired products in very good yields.

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 Coß -phenyl-cobalamin PhCbl in a roughly 3:1 to 1:1 ratio with its coordination isomer αPhCbl, a first representative 'base-off' Coα -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.

Vitamin†B12 Phosphate Conjugation and Its Effect on Binding to the Human B12 -Binding Proteins Intrinsic Factor and Haptocorrin.

The binding of vitamin B12 derivatives to human B12 transporter proteins is strongly influenced by the type and site of modification of the cobalamin original structure. We have prepared the first cobalamin derivative modified at the phosphate moiety. The reaction conditions were fully optimized and its limitations examined. The resulting derivatives, particularly those bearing terminal alkyne and azide groups, were isolated and used in copper-catalyzed alkyne-azide cycloaddition reactions (CuAAC). Their sensitivity towards light revealed their potential as photocleavable molecules. The binding abilities of selected derivatives were examined and compared with cyanocobalamin. The interaction of the alkylated derivatives with haptocorrin was less affected than the interaction with intrinsic factor. Furthermore, the configuration of the phosphate moiety was irrelevant to the binding process.

Cellular uptake of metallated cobalamins.

Cellular uptake of vitamin B12-cisplatin conjugates was estimated via detection of their metal constituents (Co, Pt, and Re) by inductively coupled plasma mass spectrometry (ICP-MS). Vitamin B12 (cyano-cob(iii)alamin) and aquo-cob(iii)alamin [Cbl-OH2](+), which differ in the ß-axial ligands (CN(-) and H2O, respectively), were included as control samples. The results indicated that B12 derivatives delivered cisplatin to both cellular cytosol and nuclei with an efficiency of one third compared to the uptake of free cisplatin cis-[Pt(II)Cl2(NH3)2]. In addition, uptake of charged B12 derivatives including [Cbl-OH2](+), [{Co}-CN-{cis-PtCl(NH3)2}](+), [{Re}-{Co}-CN-{cis-PtCl(NH3)2}](+), and [{Co}-CN-{trans-Pt(Cyt)(NH3)2}](2+) (Cyt = cytarabin) was high compared to neutral B12, which implied the existence of an additional internalization pathway for charged B12 vitamin analogs. The affinities of the charged B12 derivatives to the B12 transporters HC, IF and TC were similar to that of native vitamin B12.

Synthesis, solution and crystal structure of the coenzyme B(12) analogue Co(ß)-2'-fluoro-2',5'-dideoxyadenosylcobalamin.

Crystal structure analyses have helped to decipher the mode of binding of coenzyme B12 (AdoCbl) in the active site of AdoCbl-dependent enzymes. However, the question of how such enzymes perform their radical reactions is still incompletely answered. A pioneering study by Gruber and Kratky of AdoCbl-dependent glutamate mutase (GLM) laid out a path for the movement of the catalytically active 5'-deoxyadenosyl radical, in which H-bonds between the protein and the 2'- and 3'-OH groups of the protein bound AdoCbl would play a decisive role. Studies with correspondingly modified coenzyme B12-analogues are of interest to gain insights into cofactor binding and enzyme mechanism. Here we report the preparation of Coß-2'-fluoro-2',5'-dideoxyadenosylcobalamin (2'FAdoCbl), which lacks the 2'-OH group critical for the interaction in enzymes. 2'FAdoCbl was prepared by alkylation of cob(I)alamin, obtained from the electrochemical reduction of aquocobalamin. Spectroscopic data and a single crystal X-ray analysis of 2'FAdoCbl established its structure, which was very similar to that one of coenzyme B12. 2'FAdoCbl is a (19)F NMR active mimic of coenzyme B12 that may help to gain insights into binding interactions of coenzyme B12 with AdoCbl-dependent enzymes, proteins of B12 transport and of AdoCbl-biosynthesis, as well as with B12-riboswitches.

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.

Vitamin B12 derivatives for orthogonal functionalization.

The synthesis of vitamin B12 derivatives for selective orthogonal conjugation at both the Co center and 5'-OH is reported. Newly developed, reduction-free, direct alkynylation of vitamin B12 at the central cobalt ion proved to be versatile, with the formed acetylides, unlike other metalloorganic derivatives, showing remarkable heat and light stability, thus making them promising candidates as a drug carrier. Subsequently, high-yielding functionalization can be achieved via a sequence of selective [1,3] dipolar azide-alkyne cycloadditions (AACs) or carbamate formation followed by AAC.

The synthesis of a corrole analogue of aquacobalamin (vitamin B12a) and its ligand substitution reactions.

The synthesis of a Co(III) corrole, [10-(2-[[4-(1H-imidazol-1-ylmethyl)benzoyl]amino]phenyl)-5,15-diphenylcorrolato]cobalt(III), DPTC-Co, bearing a tail motif terminating in an imidazole ligand that coordinates Co(III), is described. The corrole therefore places Co(III) in a similar environment to that in aquacobalamin (vitamin B12a, H2OCbl(+)) but with a different equatorial ligand. In coordinating solvents, DPTC-Co is a mixture of five- and six-coordinate species, with a solvent molecule occupying the axial coordination site trans to the proximal imidazole ligand. In an 80:20 MeOH/H2O solution, allowed to age for about 1 h, the predominant species is the six-coordinate aqua species [H2O-DPTC-Co]. It is monomeric at least up to concentrations of 60 µM. The coordinated H2O has a pKa = 9.76(6). Under the same conditions H2OCbl(+) has a pKa = 7.40(2). Equilibrium constants for the substitution of coordinated H2O by exogenous ligands are reported as log K values for neutral N-, P-, and S-donor ligands, and CN(-), NO2(-), N3(-), SCN(-), I(-), and Cys in 80:20 MeOH/H2O solution at low ionic strength. The log K values for [H2O-DPTC-Co] correlate reasonably well with those for H2OCbl(+); therefore, Co(III) displays a similar behavior toward these ligands irrespective of whether the equatorial ligand is a corrole or a corrin. Pyridine is an exception; it is poorly coordinated by H2OCbl(+) because of the sterically hindered coordination site of the corrin. With few exceptions, [H2O-DPTC-Co] has a higher affinity for neutral ligands than H2OCbl(+), but the converse is true for anionic ligands. Density functional theory (DFT) models (BP86/TZVP) show that the Co-ligand bonds tend to be longer in corrin than in corrole complexes, explaining the higher affinity of the latter for neutral ligands. It is argued that the residual charge at the metal center (+2 in corrin, 0 in corrole) increases the affinity of H2OCbl(+) for anionic ligands through an electrostatic attraction. The topological properties of the electron density in the DFT-modeled compounds are used to explore the nature of the bonding between the metal and the ligands.

Reduction-free synthesis of stable acetylide cobalamins.

A straightforward, reduction-free method for the synthesis of organometallic cobalamins has been developed. Stable phenylacetylide derivatives were characterized by X-ray analysis, showing a pronounced influence of the electronic nature of substituents on their structure.

Phenylethynylcobalamin: a light-stable and thermolysis-resistant organometallic vitamin B(12) derivative prepared by radical synthesis.

Don't take this antivitamin! 2-Phenylethynylcobalamin was prepared in a newly developed radical reaction using cob(II)alamin and 1-iodo-2-phenylethyne. It has an exceptionally short organometallic bond and is a remarkably light-stable and heat-resistant organometallic cobalamin. It is bound well by two important proteins of the human B12 transport system and has properties that are as expected for a new type of an "antivitamin B12 ".

Synthesis of a B ring opened 7,8-seco-vitamin B12 derivative with Grob fragmentation.

A synthetic route toward B ring opened 7,8-seco-cyanocobalamins is described. Hydrolysis of a novel c-lactone vitamin B12 (B12) derivative generates a cobalamin (Cbl) with a ß-bromo alcoholate subunit that reacts in situ via Grob fragmentation to the secocorrin.

Synthesis, characterization, Co-S bond reactivity of a vitamin B12 model complex having pentafluorophenylthiolate as an axial ligand.

Heptamethyl (aquo)(pentafluorophenylthiolate)cobyrinate perchlorate, [(H2O)(C6F5S)Cob(III)7C1ester]ClO4, was synthesized as a B12 model complex having a thiolate ligand in the axial position. The axial ligand change in heptamethyl (diaquo)cobyrinate diperchlorate, [(H2O)2Cob(III)7C1ester](ClO4)2, from H2O to C6F5S(-) afforded the B12-thiolate complex. The B12-thiolate model complex was characterized by UV-vis, NMR and ESI-mass spectroscopies. The coordination of C6F5S(-) to the cobalt center affected the spectroscopic properties of the corrin ring through the electronic interaction between the axial ligand (C6F5S(-)) and the equatorial ligand (corrin). The photolysis of the B12-thiolate model complex led to the homolytic cleavage of the Co(iii)-S bond to form the Co(II) complex and the phenyl thiyl radical. The thermolysis of the B12-thiolate model complex also led to the homolytic cleavage of the Co(III)-S bond. Furthermore, the reactivity of the Co(III)-S bond of the B12-thiolate model complex was applied to the catalytic oxidation of C6F5SH to C6F5S-SC6F5.

Access to organometallic arylcobaltcorrins through radical synthesis: 4-ethylphenylcobalamin, a potential "antivitamin B(12)".

Locked B(12): 4-Ethylphenylcobalamin, a novel organometallic arylcobalamin, has been synthesized in a radical reaction. This vitamin B(12) antimetabolite features a strong Co-C bond, and represents a "locked" form of vitamin B(12) . It may be used in animal studies to induce functional vitamin B(12) deficiency artificially to help clarify still controversial issues related to the pathophysiology of vitamin B(12) deficiency.