NMR characterization of HO hydrogen exchange.

Quantification of HO concentration in aqueous solutions is of interest in many fields. It usually is based on indirect methods that rely on oxidation reactions that turn on/off fluorescent probes. Such methods can suffer from reaction incompleteness and interfering chemical species. We describe optimization of NMR detection that enables direct quantification of HO down to the nanomolar range. Taking advantage of fast hydrogen exchange (HX) between HO and water permits the use of very short interscan delays, greatly increasing sensitivity. The specific acid-, base- and water-catalyzed HX rates at 2 °C were measured to be 2.1 × 10, 6.1 × 10, and 1.4 × 10 Ms, respectively, which result in a minimum HX rate at pH 6.2. Furthermore, the exchange is accelerated by general acid/base catalysis. MES and phosphate buffers catalyze HX strongest in their unprotonated forms. For imidazole, only the unprotonated form catalyzes HX, which contrasts with acetic acid where only the protonated state catalyzes exchange. Inorganic salts such as sodium chloride and azide have negligible effect on HX. We present optimal conditions for accurate measurement of HO concentrations as low as 40 nM in aqueous samples in a few hours.