Thermodynamics of the binding of nitrogenous ligands to β-trifluoromethylcobinamide and β-cyanomethylcobinamide

beta-trifluoromethylcobinamide was prepared from beta-trifluoromethylcobalamin using the trifluoromethanesulfonic acid method for cleavage of the phosphodiester bond. The equilibrium constants, K-L, for the coordination of beta-trifluoromethylcobinamide (beta-CF(3)Cbi) and beta-cyanomethylcobinamide (beta-NCCH(2)Cbi) by imidazole, pyridine, bromoethylamine and azide have been determined spectrophotometrically as a function of temperature. The values of K-L at 25 degrees C for the coordination of beta-CF(3)Cbi by imidazole, pyridine, bromoethylamine and azide were found to be 700 +/- 20, 108 +/- 8, 35 +/- 3 and 12.0 +/- 0.8 M-1 respectively. The values of K-L at 25 degrees C for coordination of beta-NCCH(2)Cbi by imidazole, pyridine, bromoethylamine and azide were 300 +/- 10, 52 +/- 3, 6.2 +/- 0.3, 11 +/- 1, respectively. The values of (Delta H-L and Delta S-L) in the case of beta-CF(3)Cbi were (- 0.84 +/- 0.04, - 0.46 +/- 0.14); (- 1.04 +/- 0.11, - 1.16 +/- 0.37); (- 1.67 +/- 0.07, - 3.77 +/- 0.25); (- 1.00 +/- 0.04 kcal mol(-1), - 2.13 +/- 0.15 cal mol(-1) K-1) for imidazole, pyridine, bromoethylamine and azide, respectively. The values of (Delta H-L and Delta S-L) in the case of beta-NCCH(2)Cbi were found to be (- 1.4 +/- 0.08, - 1.85 +/- 0.25); (- 1.56 +/- 0.06, - 3.25 +/- 0.20); (- 1.31 +/- 0.10, - 3.54 +/- 0.30); (- 0.94 +/- 0.08 kcal mol(-1), - 1.98 +/- 0.25 cal mol(-1) K-1) for imidazole, pyridine, bromoethylamine and azide, respectively. These binding constants and thermodynamic parameters allow the CF3 and NCCH2 ligands to be placed in the thermodynamic trans influence series. (C) 1998 Elsevier Science S.A. All rights reserved.