The chemistry of vitamin B12. Part 27. The cage phosphite ester 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane and the anionic dimethyl phosphite as ligands for cobalt(III) corrinoids

Abstract

The equilibrium constant for the substitution of co-ordinated H₂O 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 Co^III 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^– < SO₃^2–≈ L⁴≈ L⁵ < HCC^– < CH₂CH^– < CH₃^–, but a very different trans effect (based mainly on equilibrium constants), viz. H₂O < dbzm < L⁴ < CN^– < SO₃^2–≈ HCC^– < L⁵≈ CH₂CH^– < CH₃^–.