Self-association of organocobalamins in aqueous solution

The 1H n.m.r. spectrum of base-on methylcobalamin (CH3–B12, base = terminal 5,6-dimethylbenz-imidazole of a side chain) in neutral aqueous solution shows concentration-dependent shifts which conform to a monomer–dimer equilibrium. The shift changes on dimerization are characteristic of ring-current effects and support a structure in which the corrin rings are adjacent and roughly parallel-planar. This is also supported by the apparent steric effect of the organo-group on the equilibrium constant: CH3– > CH3CH2–B12, and K= 0 for n-C3H7– and 5′-deoxyadenosyl–B12. The CH3–B12 dimer is favoured by increasing [NaCl](but not [NaClO4]), and disfavoured by added miscible organic solvents. The temperature dependence of K yields ΔH=–13 kcal mol–1 and ΔS=–33 cal K–1 mol–1 at [NaCl]= 0. The energy of association originates at least in part in π–π interactions between corrin rings. Base-off CH3–B12 in 1 mol dm–3 DCI gives a K value ca. 100-fold smaller than for the base-on form. The n.m.r. shifts indicate π–π bonding involving both 5,6-dimethylbenzimidazolium and corrin rings. The cobalamins H2O–B12+ and CN–B12 show no observable tendency to self-associate, and H2O–B12+, CN–B12, n-C3H7–B12, and 5′-deoxyadenosyl–B12 show no tendency to associate with base-on CH3–B12.