1H NMR Evidence for High Barriers to Amino Group Rotation in 4-Aminopyrimidines, Including Thiamin, at Low pH in Water

Article abstract of DOI:10.1021/jo00135a012

The 1H NMR spectrum of thiamin (1), 4-aminopyrimidine (2), 4-amino-5-(methoxymethyl)-2-methylpyrimidine (3), and thiothiamin (4) was recorded at a variety of concentrations, temperatures, and pH's in 80:20 (v/v) H2O-(2)H2O employing correlation spectroscopy and Redfield 2-1-4 sequences at 360 and 500 MHz.At pH's 1.5 to 3.0 units below the pK for N1 protonation, two amino hydrogen resonances were observed to persist at temperatures below about 10 deg C for 1, 26 deg C for 2, 40 deg C for 3, and 15 deg C for 4, indicative of hindered amino group rotation under these conditions.The concentration and temperature dependence of the behavior of the two NH resonances at low pH suggest that the increased barrier is not due to intermolecular interactions but simply to the favorable resonance interaction between the amino group nitrogen lone pair and the pyrimidine ring once the latter is N1 protonated.The barrier to amino group rotation in N1'-protonated thiamin was estimated to be 14.6 kcal/mol at pH 1.56, 12 deg C, in 1 M KCl solution.