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Enantioselectivity in the dynamic quenching of the luminescence of the Δ and Λ enantiomers of racemic Tb(III)(pyridine-2,6-dicarboxylate=DPA) 3 3− and Eu(DPA) 3 3− by a series of corrinoids is demonstrated by time resolved luminescence and circular-polarization-of-luminescence (CPL) spectroscopy. Studied are cyanocobalamin (vitamin B 12 ), aquacobalamin (B 12a ) and its conjugated base hydroxocobalamin (HOCbl), dicyanocobinamide ((CN) 2 Cbi) and the heptamethyl ester of dicyanocobyrinic acid ((CN) 2 Cby(OMe) 7 ). For this set of quenchers (Q), the diastereomeric quenching rate constants ( k q Δ and k q Λ ) are reported together with the degree of enantioselectivity E q =( k q Δ − k q Λ )/( k q Δ + k q Λ ). In the systems with Tb, values of the average rate constant k q avg (=( k q Δ + k q Λ )/2) are 1.0, 2.9 and 0.53 10 8 M −1 s −1 for CNCbl, (CN) 2 Cbi, (CN) 2 Cby(OMe) 7 with E q =−0.24, −0.20, +0.01 (standard error of E q is 0.01). The quenching by B 12a is strongly dependent on pH and ionic strength ( I ); when I =12 mM we find k q avg =5.3, E q =−0.23 at pH 6.7 and k q avg =1.3, E q =−0.27 at pH 8.9. Corresponding rates for Eu are 0.41, 27, 3.4 10 7 M −1 s −1 and for B 12a , 7.3 and 1.2 10 7 M −1 s −1 , corresponding values for E q −0.27, −0.29, +0.02, −0.21 and −0.29. The quenching reaction is modeled as a pre-equilibrium involving the formation of an encounter complex (association constant K ) followed by the actual electronic energy transfer step (rate k et ). By relating the quenching data with molecular structure it is argued that the binding in the encounter complex involves two hydrogen bonds between the uncoordinated carboxylate oxygen atom of two DPA ligands of Ln(DPA) 3 3− and two amide groups of the corrinoid, presumably involving the a and g, the a and b, or the b and g side chains. For some corrinoid/Ln(DPA) 3 3− complexes the association constants and enantioselectivities in the ground state are known (Spectrochimica Acta 55A (1999) 1837–1855), which allows for an estimate of the average rate of energy transfer, k et avg (i.e. ( k et Δ + k et Λ )/2). The enantioselectivity in the quenching reaction is lower than in the ground state association which is interpreted in terms of different values of k et in the two diastereomeric Ln-corrinoid complexes; for both Tb and Eu we find k et Δ / k et Λ ∼0.3 with CNCbl, B 12a at pH 6.7 and (CN) 2 Cbi. These data imply that the chiral discrimination in the energy transfer is considerable and counteracts that in the binding but does not dominate it. |
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