SUBSTITUENT EFFECT ANALYSIS IN ALKYLCOBALAMINS - CORRELATION OF THE N-15 NMR CHEMICAL-SHIFTS OF THE AXIAL NUCLEOTIDE AND CORRIN RING N22 AND N23 VIA A MULTIPARAMETER SUBSTITUENT EFFECT EQUATION

By use of the H-1, (SN)-S-15 HMQC experiment, optimized for a H-1-N-15 coupling of about 7 Hz, the N-15 NMR resonances of the axial nucleotide nitrogens (B1 and B3) and two of the corrin ring nitrogens (N22 and N23) have been observed for a series of 12 alkylcobalamins of the type YCH(2)Cbl. The B3, N22, and N23 chemical shifts are well correlated by a multiparameter substituent effect equation using traditional inductive, resonance, and steric substituent constants for the substituent, Y. The success of this approach suggests the importance of hyperconjugative delocalization of electron density between the metal center and the Y substituent and demonstrates the applicability of the classical Taft E(5) steric substituent constants to alkylcobalamins. Together with a similar correlation of the intramolecular equilibrium constant for the base-on/base-off reaction of these YCH(2)Cbl's, the data have been used to estimate the inductive, resonance, and steric substituent constants for the Y moiety of (5'-deoxyadenosyl)cobalamin (coenzyme B-12)