Comparison of observed pseudo-first order rate constants (k_obs) for pentasaccharide (H5) binding to antithrombin variants. Average k_obs values of fluorescence enhancement were measured after mixing pentasaccharide with antithrombin in a stopped flow fluorimeter using pentasaccharide concentrations at least fivefold higher than the antithrombin concentration (0.1 μmol/L) in buffers of I0.15 (main graph) or I0.3 (inset). The monitored fluorescence enhancements were fitted by a single exponential non-linear regression to yield the k_obs values. Each point represents the average of duplicate determinations of eight measurements per concentration. The lines are shown for (•) antithrombin Wibble, (▪) antithrombin Rouen VI, and (▴) -antithrombin.