Fig. 8. Fluorescence spectra of YFP-MetQ-R189CouA-H88F titrated with FBS for determination of l-Met concentration in FBS. Fluorescence spectra were recorded at each concentration of FBS with excitation at 360 nm. The concentration of l-Met in each FBS sample was determined using the fitting equation from the titration experiment in Fig. 6A. Assay conditions: 100 nM sensor protein, 50 mM phosphate buffer (pH 9.0), 50 mM NaCl and FBS (indicated concentration).