Nuclear Magnetic Resonance as a Tool for Determining Protonation Constants of Natural Polyprotic Bases in Solution

The acid-base properties of the tetramine 1,5,10,14-tetraazatetradecane H 2 N(CH 2 ) 3 NH(CH 2 ) 4 NH(CH 2 ) 3 NH 2 (spermine) in deuterated water have been studied at 40°C at various p D values by means of NMR spectroscopy. Both one-dimensional 13 C{ 1 H} spectra and two-dimensional 1 H/ 13 C heterocorrelation spectra with inverse detection have been recorded. A calculation procedure of general validity has been developed to unravel the effect of rapid exchange between the various species in equilibrium as a function of p D of the solution. The method of calculation used in this part of the new computer program, HYPNMR, is independent of the equilibrium model. HYPNMR has been used to obtain the basicity constants of spermine with respect to the D + cation at 40°C. Calculations have been performed using either 13 C{ 1 H} or 1 H/ 13 C data individually, or using both sets of data simultaneously. The results of the latter calculations were practically the same as the results obtained with the single data sets; the calculated errors on the refined parameters were a little smaller. After appropriate empirical corrections for temperature effects and for the presence of D + in contrast to H + , the calculated constants are compared with spermine protonation constants which have been determined previously both from potentiometric and NMR data.