The fraction of T33C that reacted with PegSSP after photoreaction with INA. The data are from the gel shown in Fig. 3. The integrated densities of the KcsA monomer band and of the KcsA-PegSSP adduct band were determined and added, and the fraction (y) of the total in the KscA-PegSSP adduct band was calculated. The means of the duplicate lanes (and average errors) for each of the duplicate reactions are shown. The errors in all cases were smaller than the symbol. A nonlinear least-squares fit to the data of the equation y = ae−b[INA] yielded b, which was divided by the time of photolysis, 60 s, to yield an apparent second-order rate constant, kINA (M−1⋅s−1).