Carbon‐13 nuclear magnetic resonance studies of creatine, creatinine and some of their analogs

Creatine (N-methyl-N-amidinoglycine), creatinine (1-methyl-2-aminoimidazolin-4-one) and a series of 38 of their close structural analogs have been examined using natural abundance 13C NMR spectroscopy at 25.16 MHz. Both proton-coupled and proton noise-decoupled spectra were recorded. Unequivocal assignments of the carbon resonances could be made in the vast majority of cases. Both 13C NMR chemical shifts and 1J(CH) values can be used to characterize and to differentiate readily between analogs of creatine and analogs of creatinine. For example, the 1J(CH) coupling constants for the α-carbons of the acyclic creatine analogs were all in the 140–142 Hz range, whereas the corresponding coupling constants for the related, cyclized creatinine analogs were all in the 150–152 Hz range.