Single mantATP turnover fluorescence decays. Examples of single mantATP turnover reactions for WT and E525K 2HEP (a and b), and 25HEP (c and d) constructs as a function of ionic strength (20 and 100 mM KCl). Representative fluorescence decay fits (upper graph) using two-exponentials (thin solid black line) and their respective residuals (lower pair of graphs) are presented (the residuals from the single-exponential fits are shown in black for comparison). Relative amplitudes of the fast and slow phase-exponential fit rate constants determined the fraction of myosin in the DRX and SRX states, respectively. See Table 2 for summary of values.