VITAMIN-B12 COMPLEXES WITH PHOSPHITE LIGANDS .2. COORDINATION AND ACID-BASE EQUILIBRIA

The equilibrium constants for the substitution of coordinated H2O in aquocobalamin by P(OCH3)3 and P(OCH2CH3)3 have been determined. The decrease in equilibrium constants P(OCH2)3CCH2CH3 > P(OCH3)3 = P(OCH2CH3)3 is ascribed to the smaller steric size of P(OCH2)3CCH2CH3 in comparison to P(OCH3)3 and P(OCH2CH3)3. The pK's for the protonation of the nucleotide base (dbzm) in dbzm-Co-P(OCH3)3, dbzm-Co-P(OCH2CH3)3, dbzm-Co-P(OCH2CH3)2O-, and dbzm-Co-P[OCH(CH3)2]2O- have been determined. The general order of ligands for the thermodynamic trans-effect is: H2O < dbzm < P(OCH2)3CCH2CH3 < CN- < HC = C- < P(OCH3)3 < P(OCH2CH3)3 < P(OCH3)(F)O- < SO3(2-) < P(OCH3)2O- congruent-to CH2 = CH- congruent-to P(OCH2CH3)2O- < CH3- < P[OCH(CH3)2]2O- < CH3CH2-; this order is similar to the cis-effect order in cobalamins. The position of the phosphite ligands in this order are determined by their sigma-donor ability which, in turn, depends on their steric and inductive effects.