Electronic and Nuclear Magnetic Resonance Spectroscopic Features of the 1’4’-Iminopyrimidine Tautomeric form of Thiamin Diphosphate- a Novel Intermediate on Enzymes Requiring this Coenzyme

摘要

Appropriate compounds were synthesized to create models for the 1’,4’-imino tautomer of the 4’-aminopyrimidine ring of thiamin diphosphate recently found to exist on the pathway of enzymatic reactions requiring this cofactor (Jordan, F., and Nemeria, N.S. (2005) Bioorganic Chemistry, 33, 190−215). The N1-methyl-4-aminopyrimidinium compounds synthesized on treatment with a strong base produce the 1,4-imino tautomer whose UV spectrum indicates a maximum between 300−320 nm, depending on the absence or presence of a methyl group at the 4-amino nitrogen. The λmax found is in the same wavelength range as the positive circular dichroism band observed on several enzymes and showed a very strong dependence on solvent dielectric constant. To help with the ¹⁵N chemical shift assignments, the model compounds were specifically labeled with ¹⁵N at the amino nitrogen atom. The chemical shift of the amino nitrogen was deshielded by N1-methylation, then dramatically further deshielded by more than 100 ppm on formation of the 1,4-iminopyrimidine tautomer. Both the UV spectroscopic values and the ¹⁵N chemical shift for the 1,4-iminopyrimidine tautomer should serve as useful guides to the assignment of enzyme-bound signals.