Thermodynamics of the Base-On/Base-Off Equilibrium of Alkyl-13-epi- and Alkyl-8-epicobalamins: Understanding the Thermodynamics of Axial Ligand Substitution in Alkylcobalt Corrinoids

Spectroscopic studies and determination of the formation constants and thermodynamic parameters for a new water-soluble Co(II) Schiff-base complex and some imidazole derivatives

“…The trend for variations of DS f is also consistent with this; the value and the sign of DS f are dependent on the nature of solvation which makes its values positive. The values of the obtained thermodynamic parameters are in reasonable agreement with the previously studied cobalamins and the complex is therefore a model for the study of the cobalamin system [35]. The values of DH f and DS f are both shifted to more negative values compared to previous report [19], confirming that the starting acceptor complex is more subjected to hydrogen bonding.…”

Thermodynamic and Kinetic Data for the Base‐On/Base‐Off Equilibration of Alkylcobalamins

“…In addition to the spectroscopic evidence for the existence of a five‐coordinate/six‐coordinate equilibrium for corrinoid alkyl(aqua)cobalt complexes, and the sensitivity of this equilibrium to the inductive effect of the alkyl ligand, there are thermodynamic results that suggest the same equilibrium occurs in the base‐off alkylcobalamins 11. It was found that this equilibrium is displaced toward the six‐coordinate aqua species as the organic ligand becomes more electron‐withdrawing, where there is an increasing compensation for the entropy loss from the entropy gain of H 2 O dissociation, and the entropy change no longer varies with the alkyl ligand.…”

“…However, for some compounds with the highest value of −Δ S (and the lowest −Δ G ), the loss of entropy upon coordination of the pendent axial nucleotide is largely uncompensated for by the entropic effect of the loss of an axial water ligand, i.e. these compounds are largely five‐coordinate 11. The UV/Vis spectrum of protonated base‐off AdoCbl (3 × 10 −5 M in 1 M HClO 4 ), exhibits a band at ca.…”

“…Wirt and Chance10 provided evidence for a five‐coordinate/six‐coordinate equilibrium in base‐off CH 3 Cbl and AdoCbl from the temperature dependence of the 1s‐3d preedge transition in the X‐ray absorption edge spectra of these complexes. In addition to the spectroscopic evidence, thermodynamic parameters for this equilibrium provide evidence that the base‐off RCbl’s exist as an equilibrium mixture of six‐coordinate aqua and five‐coordinate species, and that this equilibrium is sensitive to the nature of the organic ligand 11…”

Naturally occurring cobalamins have antimalarial activity

“…5. At pH 4.7, Ado-cbl and CH 3 -cbl have 8.5% (pK a 3.67 [36]) and 1.5% (pK a 2.90 [35]) of the protonated base-off form, respectively, but H 2 O-cbl and CN-cbl are 100% in the base-on form. At pH 4.6-4.8, 100% of H 2 O-cbl, 8.5% of CH 3 -cbl and 1.5% of Ado-cbl exist as positively charged species, but CN-cbl exists entirely as the neutral form.…”