Hydrogen carbonate (bicarbonate, HCO(3)(-)) has been proposed to accelerate the decomposition of N(2)O(3) because N-nitrosation of morpholine via a nitric oxide/oxygen mixture ((*)NO/O(2)) was inhibited by the addition of HCO(3)(-) at pH 8.9 [Caulfield, J. L., Singh, S. P., Wishnok, J. S., Deen, W. M., and Tannenbaum, S. R. (1996) J. Biol. Chem. 271, 25859-25863]. In the study presented here, it is shown that carbon dioxide (CO(2)) is responsible for this kind of protective effect because of formation of amine carbamates, whereas an inhibitory function of HCO(3)(-) is excluded. N-Nitrosation of morpholine (1-10 mM) at pH 7.4-7.5 by the (*)NO-donor compounds PAPA NONOate and MAMA NONOate (0.5 mM each) was not affected by the presence of large amounts of HCO(3)(-) (up to 100 mM) in aerated aqueous solution. Similar results were obtained by replacing the (*)NO-donor compounds with authentic (*)NO (900 microM). In agreement with data from the study cited above, (*)NO/O(2)-mediated formation of N-nitrosomorpholine (NO-Mor) was indeed inhibited by about 45% in the presence of 50 mM HCO(3)(-) at pH 8.9. However, 500 MHz (13)C NMR analysis with (13)C-enriched bicarbonate revealed that significant amounts of morpholine carbamate are formed from reaction of equilibrated CO(2) with morpholine (1-100 mM) at pH 8.9, but only to a minor extent at pH 7. 5. The protective effect of morpholine carbamate formation is explained by a significantly reduced charge density at nitrogen. This view is supported by the results of density functional theory/natural population analysis, i.e., quantumchemical calculations for morpholine and morpholine carbamate. In agreement with its lower pK(a), another secondary amine, piperazine, had already produced significant amounts of piperazine carbamate at pH 7. 4 as shown by (13)C NMR spectrometry. Consequently, and in contrast to morpholine, N-nitrosation of piperazine (2 mM) by both (*)NO/O(2) (PAPA NONOate, 0.5 mM) and the (*)NO/O(2)(-)(*)-releasing compound SIN-1 (1 mM) was inhibited by about 66% in the presence of 200 mM HCO(3)(-).