Nitric oxide formation by ultrasound in aqueous solutions

In this study we demonstrate formation of nitric oxide in aqueous nitrogen-containing solutions exposed to 50 kHz cavitation-producing ultrasound (standard bath sonicator) using electron paramagnetic resonance detection of (NO)-N-. by trapping with the sodium N-methyl-D-glucamine dithiocarbamate iron(II) complex ((MGDFe2+) or by measuring the conversion of the nitronyl nitroxide, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-3- oxide-1-oxyl (carboxy-PTIO), to the imino nitroxide, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl. The (MGDFe2+ complex which was used in most experiments was suitable for (NO)-N-. detection over a wide pH range (pH 3-7.8: the working range for carboxy-PTIO was pH 6-8.5), and the measured rate constant of (MGDFe2+ reaction with (NO)-N-. was 2.3 times higher than for carboxy-PTIO. In air-saturated water the rate of (NO)-N-. production by ultrasound was similar to 0.5 mu M/min. The presence of dissolved oxygen was not essential for production of (NO)-N-.; the highest yields of (NO)-N-. (similar to 1.2 mu M (NO)-N-./min) were found under an atmosphere of 40% N-2 and 60% argon. The formation of (NO)-N-. by ultrasound in aqueous solutions can be understood in terms of combustion chemistry-type reactions occurring inside the ''hot'' collapsing cavitation bubbles. We also show that other N-containing molecules can serve as a source of nitrogen for (NO)-N-. production. The possibility of ultrasound-mediated (NO)-N-. formation to alleviate hypoxia of tumors should be explored.