The equilibrium constant for the substitution of co-ordinated H 2O in aquocobalamin by 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2] octane (L4) has been determined in aqueous solution at 25 °C by u.v.-visible spectrophotometry to be log K = 7.2; comparison with the estimated pK of 1.5 for the protonation of L4 shows the marked preference of L4 for CoIII over the proton. The cobinamide and acidified cobalamin (with protonated dbzm, the nucleotide base) containing anionic dimethyl phosphite (L5) show a reversible temperature-dependent change in spectrum, ascribed to an equilibrium between five- and six-co-ordinate forms with axial ligands L5-Co and L5-Co-OH2 respectively. Axial ligands L4 and L5 exert a similar cis effect (based on wavelength of the γ band), viz. H2O < dbzm < CN- < SO32- ≈ L4 ≈ L5 < HC≡C- < CH2CH- < CH3-, but a very different trans effect (based mainly on equilibrium constants), viz. H2O < dbzm < L4 < CN- < SO32- ≈ HC≡C- < L5 ≈ CH2=CH- < CH3 -.
|Original language||English (US)|
|Number of pages||7|
|Journal||Journal of the Chemical Society, Dalton Transactions|
|State||Published - 1987|
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