We did not exhaustively explore the impact of the selection of the initial guesses on fitting the gradient data. The initial guesses selected here for S1 and S2 are somewhat biased because they used the average values for a subset of the present data set . The initial guess for the kref parameter is also based on the correlations shown in Eqs. ( ) and , which are built on the current data set, so these will be somewhat biased as well. Still, we did not observe any cases where the fits appeared to represent a local rather than a global minimum.