COMPARISON OF THE PROTONATION OF ISOPHOSPHORAMIDE MUSTARD AND PHOSPHORAMIDE MUSTARD

The alkylating agent isophosphoramide mustard (IPM) spontaneously form s a relatively stable aziridine derivative which can be directly obser ved using NMR spectroscopy. The protonations of LPM and its aziridine were probed using H-1, P-31, N-15, and O-17 NMR spectroscopy. The posi tions of the P-31, N-15, and O-17 resonances of IPM between pH 2 and 1 0 each exhibit a single monobasic titration curve with the same pK(a) of 4.31 +/- 0.02. On the basis of a comparison with other compounds an d our earlier work with phosphoramide mustard, the NMR results for IPM indicate that protonation occurs at nitrogen and not oxygen. Over thi s same pH range, each of the H-1, P-31, and N-15 resonances of IPM-azi ridine also show a single monobasic titration with a pK(a) of 5.30 +/- 0.09. The magnitude of the change in chemical shifts suggests that th e protonation of the IPM-aziridine occurs at the ring nitrogen. Theore tical gas-phase calculations of PM, IPM, and IPM-aziridine suggest O-p rotonation to be more likely; however, aqueous phase calculations pred ict the N-protonated forms to be most stable. Furthermore, for PM and IPM-aziridine, which contain nonequivalent nitrogens, the theoretical calculations and experimental data both agree as to which nitrogen und ergoes protonation. These results suggest that the IPM-aziridine remai ns unprotonated under physiological conditions and may, in part, expla in the lower alkylating activity of IPM as compared to PM.