test: for chloride: 50 mM sodium phosphate, pH 8.3, 1 mM FAD, 20% (v/v) glycerol (>99.5%, Thermo Fisher Scientific), and 300 mM NaCl, whereas the final buffer was 60 mM potassium phosphate, pH 8.3, 1% (v/v) glycerol, and 20 mM NaCl; for bromide: buffer contained 60 mM sodium phosphate, pH 8.3, 1 mM FAD, 20% (v/v) glycerol, and 300 mM NaBr (99.99%, Thermo Fisher Scientific) for reconsitution and 20 mM NaBr in the final buffer with 60 mM potassium phosphate (pH=8.3)