tral, was also tested (Fig. 6C) to investigate possible electrostatic
effects of I or Cs. All experimental data fit linear Stern–
Volmer plots for collisional (dynamic) quenching, thereby excluding
a contribution from static quenching (46). The extent of
exposure of Trp-151 to a given quencher is related to the slope
of the plot, which yields the collisional KSV (see Experimental
Procedures). In the absence of TDG, the KSV values obtained for
all three quenchers reflect moderate accessibility of Trp-151 to
solvent (36). Moreover, the accessibility of the three quenchers
is proportional to their size, indicating that Trp-151 is in a
relatively neutral environment. Importantly, there is a significant
reduction of the KSV values for all three quenchers in the
presence of TDG (50%, 60%, and 30% for Cs, I, and
acrylamide, respectively), providing further evidence that Trp-
151 is shielded from the surrounding solvent by substrate.