Fig. 4. UV-vis absorbance changes at 420 nm—from which the k_obs was calculated—for the reaction of nitrite (20 mM) with NP2 (≈5 μM) and NP4 (≈5 μM) at 37 °C in 0.1 M phosphate buffer (pH 6.5). Insert: typical UV-vis absorption spectra for the reaction of NPs with nitrite.