Vitamin B12 as an allosteric cofactor; dual fluorescence, hysteresis, oscillations and the selection of corrin over porphyrin

Studies of the emission spectra of four Co(III) cobinamides (diaquo-, aquohydroxo-, dihydroxo- and dicyano-) show (1) that the excited states corresponding to the alphabeta and epsilon absorption bands behave like the S(1) and S(2) levels in the non-alternant hydrocarbon azulene (with emission from S(2)> S(1) in violation of Kasha's rule) and (2) that the excited states include a TICT (twisted intramolecular charge transfer) mechanism, as in the simpler cyanines, but where the TICT state gives rise to dual fluorescence instead of cis-trans isomerisation. Combined with the previously reported dual fluorescence from the S(1) level in synthetic metal corrinoids and in the naturally-occurring metal-free corrin, this provides evidence that the existence of an additional (metastable) ground state with a significantly different vibronic splitting and nuclear configuration is an intrinsic property of the basic corrin ligand (irrespective of the nature of the side-chains and the metal ion or even the absence of a metal) which distinguishes it from porphyrin. The occurrence of hysteresis (and its associated oscillations) in redox reactions of the cobinamides involving both the Co(III/II) and Co(II/I) couples indicates that the corrin ligand also has an intrinsic ability to exist in different conformations or "allosteric" forms with differing redox potential, which further distinguishes it from the porphyrin ligand. Possible links between the existence of an additional metastable ground state and of allosteric changes and the likely reasons for the selection of corrin over a porphyrin for the vitamin B(12)-dependent enzymes are discussed.     

Probing the nature of the Co(III) ion in corrins: the structural and electronic properties of dicyano- and aquacyanocobyrinic acid heptamethyl ester and a stable yellow dicyano- and aquacyanocobyrinic acid heptamethyl ester

A stable yellow derivative of cobyrinic acid heptamethyl ester, (5R,6R)-Coα,Coβ-dicyano-5,6-dihydro-5-hydroxy-heptamethylcob(III)yrinate-c,6-lactone (DCSYCbs), was prepared from dicyanocobyrinic acid heptamethyl ester (DCCbs). The C5 carbon is oxidized and the c side chain cyclized to form a lactone at C6; the 13 atom, 14 π-e(-) delocalized system of corrins is interrupted, giving a triazamethine system with four conjugated double bonds between N22 and N24 and an isolated double bond between N21 and C4. Stable yellow aquacyanocobyrinic acid heptamethyl ester (ACSYCbs) was prepared by driving off HCN with N(2) in a methanol/acetic acid solution. The electronic spectra of DCCbs and DCSYCbs appear similar except that the bands in DCSYCbs are shifted to shorter wavelengths and the γ-band is much less intense. The experimental spectra were adequately modeled using TD-DFT at the PBE1PBE/6-311G(d,p) level of theory. DCSYCbs crystallizes in the space group P2(1)2(1)2(1) (R(1) = 6.08%) with Z = 4, including one methanol solvent molecule and one water molecule per cobester. The addition of a hydroxyl group at C5 causes loss of the double bond between C5 and C6 and elongation of the C5-C6 bond. From a combination of two-dimensional (1)H TOCSY and ROESY NMR spectra and (1)H/(13)C HSQC and HMBC data, the complete (1)H and (13)C NMR assignments of DCSYCbs were possible, except for two of the ester methyl groups and the (13)C resonances of the two axial cyanide ligands. The latter were assigned using relative chemical shifts calculated by GIAO-DFT methods. The (59)Co resonance of DCCbs was observed at 4074 ppm while that of DCSYCbs is shifted downfield to 4298 ppm. Comparison with available (59)Co data of analogous systems suggests that the more π-conjugated corrin of DCCbs interacts more strongly with the metal than the less extensively conjugated macrocycle of DCSYCbs. As the strength of the interaction between Co(III) and an equatorial macrocycle increases, ν(CN) of axially coordinated CN(-) shifts to lower frequency; in DCSYCbs and DCCbs ν(CN) occurs at 2138 and 2123 cm(-1), respectively. Hence the corrin ligand in DCCbs interacts more strongly with the metal than the stable yellow corrin ligand, with its diminished conjugation. The UV-vis spectral data and DFT-calculated MOs are consistent with greater overlap between the corrin and the metal orbitals in DCCbs relative to DCSYCbs, which gives the metal in the former a softer, more covalent character.     

A sensitive high-throughput HPLC assay for simultaneous determination of everolimus and clobetasol propionate

A novel sensitive high-throughput high-performance liquid chromatography assay is developed and validated for the simultaneous determination of everolimus and clobetasol propionate in pharmaceutical formulations. The chromatographic separation is achieved on a Zorbax Eclipse XDB-C18 reversed-phase column using a gradient elution, with solvent A: ammonium acetate (pH 6.8; 0.01 M) and solvent B: acetonitrile. The mean recovery ranges from 95.1% to 100.0% for clobetasol propionate and from 97.9% to 103.7% for everolimus. The limit of quantitation for each analyte is 0.02 microg/mL. The percent relative standard deviations are less than 3% for intra- and inter-day analyses. The proposed method can be used for the routine quality control of everolimus and clobetasol propionate in complex pharmaceutical formulations, especially the drug-delivery systems with a low total drug-load.     

Probing the nature of the Co(III) ion in corrins: a comparison of the thermodynamics and kinetics of the ligand substitution reactions of aquacyanocobyrinic acid heptamethyl ester and stable yellow aquacyanocobyrinic acid heptamethyl ester

Equilibrium constants (log K) for the substitution of coordinated H(2)O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and stable yellow aquacyanocobyrinic acid heptamethyl ester (stable yellow aquacyanocobester, ACSYCbs), in which oxidation of the C5 carbon of the corrin interrupts the normal delocalized system of corrins, by ligands with soft (CN(-), SO(3)(2-), and S(2)O(3)(2-)) and hard (NO(2)(-) and N(3)(-)) donors have been determined. The ligands with a harder donor atom (N in N(3)(-) and NO(2)(-)) produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs. If the donor atom is softer (C in CN(-) and S in SO(3)(2-)), then ΔH is less positive, or more negative, for reactions with ACCbs than with ACSYCbs. The softer metal in ACCbs has a preference for softer ligands and the harder metal in ACSYCbs for the harder ligands. A kinetics study in which CN(-) substitutes H(2)O on Co(III) shows that ACCbs is more labile than ACSYCbs; the second-order rate constant k(II) is between 4.6 (at 5 °C) and 2.6 (at 35 °C) times larger. ΔH(?) for the reaction of CN(-) with ACCbs is smaller by some 12 kJ mol(-1) than that for the reaction with ACSYCbs, consistent with an earlier transition state in which bonding between the softer metal of ACCbs and the ligand is greater than that of ACSYCbs with its harder metal. This difference in ΔH(?) makes ACCbs over 100 times more labile, although the effect is masked by a ΔS(?) value that is over 30 J K(-1) mol(-1) more negative. There is a significant increase in the inertness of Co(III) upon a decrease in the extent of conjugation of the corrin ligand. Modifying the electronic structure of the equatorial ligand in the cobalt corrins can modify the thermodynamics and kinetics of its reactions with exogenous ligands.     

Association of Molybdenum Ionic Species with Alumina Surface

The adsorption isotherms at 25, 45, and 65 degrees C of molybdenum solutions of concentration ranges between 10(-3) and 3x10(-2) M(Mo) (pH 4-5) on different alumina samples are investigated. The analysis is conducted using a modified Frumkin isotherm which takes a more realistic account of the lateral interaction between adsorbed species and considers that the adsorption takes place on the most basic OH groups on the surface of alumina. The results are discussed in view of the difference in solutions speciation, and the changes in the pH of the remaining supernatant solutions. The solution temperature, PZC of the used aluminas, the configuration of the basic OH groups on their surface, and the pore structure have been shown to intervene effectively. Copyright 2000 Academic Press.     

Cobalamins and the spectrochemical series

UV-visible-NIR spectra of a variety of cobalamins were run in water and methanol. A broad absorption band (band A) with extinction coefficients of about an order of magnitude less than those of the alphabeta bands was found in the red and NIR regions for Cl-cobalamin (Cl-cbl), Br-cbl, I-cbl, SC(NH(2))(2)-cbl(+) and SeCN-cbl. OCrO(3)-cbl(-), which also has a broad absorption band in the NIR was prepared for the first time. After deconvolution, similar broad bands were seen in the visible region for many other cobalamins. The wavelengths for band A placed the cobalamins in an order similar to the spectrochemical series but different from that of the alphabeta and gamma bands (pi-pi* transitions), which follow the nephelauxetic series. Band A was ascribed to a ligand-to-metal charge transfer (LMCT) transition from a pi orbital in the corrin ring to Co(iii). This is the first systematic study of LMCT bands in cobalamins.     

Diiso­propyl­phosphitocobalamin–acetone–water (1/3.48/7.56)

The di­iso­propyl­phosphite ligand in the title diiso­propyl­phosphitocobalamin compound, [Co(C₆₈H₁₀₂N₁₃O₁₇P₂)]·3.48C₃H₆O·7.56H₂O, coordinates to the Co^III atom via its P atom. The crystal structure is isomorphous with that of other cobalamins that adopt packing type II [Gruber, Jogl, Klintschar & Kratky (1998). Vitamin B₁₂ and B₁₂ Proteins, edited by Kräutler, Arigoni & Golding, pp. 335–347. New York: Wiley–VCH], with a Co—P bond length [2.227 (1) Å] similar to that found in other phosphitocobalamins. The structural trans influence in cobalamins is discussed.