A kinetic study on the nitric oxide trapping by the Fe(III)- dithiocarbamate-nitroxyl complex: Influence of the ligand structure and external pressure on the trapping rate

The Fe(III)-dithiocarbamate-nitroxyl (Fe-dtc-nitroxyl) complex, where nitroxyl represents nitroxyl free radical (R-N(O)-R'), has been shown to trap nitric oxide (NO) in aqueous solution, replacing the nitroxyl ligand. As a result, non-complexed nitroxyl radical was released to the solution. Fe(III)-dithiocarbamate (Fe-dtc) complexes without the nitroxyl ligand also traps NO, producing NO-Fe-dtc complex. These two reactions have been used for the purpose of NO detection. We investigated how the ligand structure and external pressure influence NO trapping rates in these complexes. The ratios of NO trapping rates (k₁/k₂) between various Fe-dtc-nitroxyl complexes and the Fe-dtc complex were determined by using a competitive NO trapping method. The ratio k₁/k₂ was 29.8±2.8 for Fe-dtc-nitroxyl and Fe-dtc, where dtc = N-(dithiocarboxy) sarcosine (DTCS) and nitroxyl = 2,2,6,6-tetramethyl- piperidine-1-oxyl (TEMPO). For another combination, dtc = N-methyl-D-glucamine dithiocarbamate (MGD)/nitroxyl = TEMPO, k₁/k₂ was 7.19±0.25. Overall, NO trapping rate of the Fe-dtc-nitroxyl complex was faster than that of the Fe-dtc complex, and k₁/k₂ for Fe-MGD complex was dependent on the electron-withdrawing or-repelling nature of the functional group in the ligand. Based on pressure dependence experiments for the competitive reaction, we obtained large negative activation volumes for NO trapping of the Fe-dtc-nitroxyl complex as well as the difference in activation volumes (∼15 to ∼26 cm³ mol^-1) between the NO trapping reactions by the two Fe complexes. These data sets with different nature allowed us to speculate the reaction mechanism for NO trapping of the Fe-dtc-nitroxyl complex.