Reactions of NO2 with chromium(III) complexes with histamine and pyridoxamine ligands studied by the stopped-flow technique

This study demonstrated the direct formation of the nitrogen dioxide (NO2) radical during the decomposition of 3-morpholinosydnonimine (SIN-1) in biological buffer 4-morpholinoethanosulfone acid solution. Consequently, at approximately pH 4, SIN-1 can be used successfully as a source of NO2. This conclusion is drawn from a comparison of the reactions of cis-[Cr(C2O4)(L– L)(OH2)2]+, where L–L denotes pyridoxamine (Hpm) or histamine (hm), with the gaseous NO2 radical obtained by two methods: from SIN-1 and from a simple redox reaction. These reactions were investigated using the stopped-flow technique. The measurements were carried out at temperatures ranging from 5 to 25 °C over a pH range from 6.52 to 9.11 for cis-[Cr(C2O4)(Hpm) (OH2)2]+ and from 6.03 to 8.15 for cis-[Cr(C2O4)(hm)(OH2)2] +. We also determined the thermodynamic activation parameter (Ea) and the uptake mechanism for each of the coordination compounds studied.