Assignments in the Carbon-13 Nuclear Magnetic Resonance Spectra of Vitamin B 12 , Coenzyme B 12 , and Other Corrinoids: Application of Partially-Relaxed Fourier Transform Spectroscopy

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Abstract
High-resolution Fourier transform NMR at 15.08 MHz was used to observe the proton-decoupled natural-abundance 13 C spectra of aqueous solutions of cobinamide dicyanide (0.067 M), cyanocobalamin (0.024 M), dicyanocobalamin (0.14 M), and coenzyme B 12 (0.038 M). Assignments were made with the aid of chemical shift comparisons, off-resonance single-frequency proton decoupling, partially-relaxed Fourier transform spectra, and splittings arising from 13 C- 31 P coupling. As expected, the 13 C spectra of the coronoids were appreciably more informative than the corresponding proton spectra. Nearly all the lines in the 13 C spectra of the corrinoids were well-resolved single-carbon resonances, in spite of the structural complexity. Partially relaxed 13 C Fourier transform NMR spectra, which yield spin-lattice relaxation times of each resolved resonance, were found to be a very useful addition to the arsenal of NMR techniques.