PROTONATION OF PHOSPHORAMIDE MUSTARD AND OTHER PHOSPHORAMIDES

The chemistry of the bifunctional alkylating agent phosphoramide mustard and model phosphoramides was probed by multinuclear NMR spectroscopy as a function of pH. Between pH 1 and 11, both the P-31 and N-15 resonances for phosphoramide mustard displayed a single monobasic titration curve with a pK(a) of 4.9. The protonation below pH 4.9 correlates with the loss in reactivity of the mustard. The O-17 NMR spectrum of O-17-enriched phosphoramide mustard shows little change with pH. The data on the mustard was compared to N-15 and P-31 NMR data on N-15-enriched phosphoramidic acid, phosphorodiamidic acid, and phosphoric triamide. Contrary to the conclusions of previous studies, our combined P-31, N-15, and O-17 NMR results are more consistent with N-protonation of phosphoramide mustard rather than an O-protonation. Theoretical calculations on the phosphoramidic acid, phosphorodiamidic acid, and phosphoric triamide show O-protonation to be more stable in the gas phase. For the latter two compounds, the calculations suggest that N-protonation may be the most stable protonated form in the aqueous phase. These findings influence our understanding of the structure-activity relationships of phosphoramide mustards.