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Natural-abundance nitrogen-15 nuclear magnetic resonance spectroscopy has been used to determine the site of protonation in vitamin B_1. Selectively decoupling individual protons of the vitamin in water, D_2O, and ethylene glycol permitted assignment of the nitrogen resonances and thence the determination of the site of protonation, which was found to be N-1 of the pyrimidine ring.
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Cain et al. (1977) studied this question.
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