The following procedure was adapted from the method of Waite et al. Fresh denaturing sodium acetate buffer (200 mM, 8 M urea (Sigma: BioUltra), pH = 4.5) was prepared. SyrB2-F121Y/DOPA protein was diluted and buffer exchanged into this denaturing buffer three times to remove HEPES. Another solution of 1.41 M NaNO2/0.41 M Na2MoO4 (nitration reagent, both Sigma: ACS Reagent) was prepared in MilliQ water. The denatured SyrB2-F121Y/DOPA protein (300 µL) was mixed with 1 equivalent (v/v) of the nitration reagent forming a yellowcolored solution. 1 M NaOH (400 µL) was immediately added to oxidize the nitrated species and form the pink-colored dinitro-quinone complex. The UV-Vis spectrum of the reaction was then recorded on a Cary 4000 instrument using the parameters described above.