Fig. 4.
Fluorescence extinction coefficient (FEC) determination of four FRET-peptide substrates. The concentration of FRET-peptide was varied and plotted against the total arbitrary fluorescence values (AFUs) released upon complete cleavage of the substrate by an excess of SARS-CoV 3CLpro. Data were fit to a line to a line and the slope is the FEC constant in units of AFUs/μM. The FEC values and their errors are listed in Table 3.