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Unusual behavior of cytosine amino group signal in 1H NMR spectra in DMSO depending on its concentration

By 1H NMR spectroscopy it was established that under increase ofcytosine concentration in anhydrous DMSO-d6 from 5 up to 35 mM the doublet signal of its amino group becomes gradually transformed into singlet one. This observation is interpreted as a result of lowering of the barrier of amino group turn due to central symmetrical self-association of the base through two equivalent intermolecular H-bonds N1H...O=C2, which is likely to weaken pπ-conjugation of lone electron pair ofthe amino nitrogen atom with the ring π-electron system. The cytosine N1-methylsubstitution was shown to impact amino group in the similar way as self-association.