A fresh solution of 5 mM ammonium acetate, pH = 6.5 (AA, Mallinckrodt: Analytical Reagent) was prepared in MilliQ water. Both the WT SyrB2 and F121Y/DOPA SyrB2 were buffer exchanged four times into AA using prewashed (100 mM NaOH then AA) 0.5 mL Amicon™ Ultra 30 kDa MWCO centrifugal filter units (Millipore Sigma). Mass spectral analysis of intact SyrB2 protein(s) in their native state was achieved by direct infusion of aqueous buffer-