15 N‐NMR. Spectra of pterins and folic acid derivatives

15 N‐Chemical shifts of a series of 5, 6, 7, 8‐tetrahydropterins, 7, 8‐dihydropterins and pterins have been measured in acidic solution by means of a probe for 20 mm sample tubes. Included are the relevant data of folic acid (11) , 5, 6, 7, 8‐tetrahydrofolic acid (5) and N (5, 10)‐metheno‐5, 6, 7, 8‐tetrahydrofolic acid (6) . The different oxidation states are clearly relected in the chemical shifts of N (5) and N (8). Assignment of the nitrogen resonances was achieved by protonation effects (discrimination between N (1) and N (3)) and with the aid of alkyl substitution at C (6) and C(7), to distinguish between N (5) and N (8).