The binding of three corrinoids, cyanocobalarnin (vitamin B-12a, CNCbl), aquacobalamin (vitamin B-12a, H(2)OCbl(+)) and dicyanocobinamide ((CN),Cbi), collectively denoted by Q, to the Delta- and Lambda-enantiomer of Ln(DPA):- complexes (Ln = lanthanide, DPA = 2,6-pyridine dicarboxylate) in aqueous solution at room temperature is investigated by NMR and Circular Dichroism (CD) spectroscopy. Tb(DPA):- induces shifts of the H-1-NMR resonances of Q. A formula, describing the dipolar interaction between the proton spin and the paramagnetic moment of the lanthanide complex, is fitted to the lanthanide induced shifts, yielding the position of Tb(DPA):- and the orientation of its magnetic axis with respect to the corrinoid. The magnitude of the shifts provide the value of K-avg average of the association constants of the two diastereomeric complexes (Delta-Tb(DPA)(3)(3-)Q) and Lambda-Tb(DPA)(3)(3-)Q). We find K-avg = 4 +/- 0.5 (B-12) and 0.8 +/- 0.2 M (-1) (CN),Cbi). The value for H-2,OCbl (+) depends strongly on the solution's ionic strength I: K-avg = (7 +/- 1) x 10(1), 22 + 5 and 16 + 7 M (-1) at 1= 0.007, 0.12 and 0.22 M, respectively. The addition of the corrinoids to a (racemic) solution of Nd(DPA):-, induces CD in the electronic Nd(III) transitions in the 700-800 nm region (ICD). This Pfeiffer effect is attributed to an excess of A-Nd(DPA)(3)(3-) - in the enantioselective, outer-sphere association with the corrinoids. From the magnitude of the ICD as a function of [Q], in conjunction with the (indirectly) determined CD effect of A-Nd(DPA):-, values are determined for K-Delta - K-Lambda. We find Kh - Kh = - 24 +/- 2 M - for H(2)OCbl(+) at pH 6.7 and 1= 0.12 h/I and - 12 +/- 2 M - ' at I = 0.22 M; - 4.4 1 0.5 M - ' for CNCbl and - 0.9 +/- 0.3 M (-1) for (CN),Cbi. Adopting the view that in the lanthanide series Ln(DPA):- chelates are isostructural, the results of the NMR and CD experiments are combined to estimate the enantioselectivity in the binding E-b defined as (k(Delta) - k(Lambda))/2K(avg). We find E-b,= - 0.5 + 0.2 for H(2)OCbl(+), - 0.7 +/- 0.2 for CNCbl and - 0.7 +/- 0.3 for (CN)(2)Cbi. On the basis of the NMR data, a molecular structure is proposed for the two diasteromeric{Ln(DPA)(3)(3-)Q} complexes. The bonding involves hydrogen bonds between the amide proton on the a and g side chain of the corrinoid and the carboxylate oxygen of two DPA ligands. (C) 1999 Elsevier Science B.V. All rights reserved.
