Fluorescence spectroscopy studies showed that N TAILW518 has a maximum of emission at 356 nm, indicating that Trp 518 is fully exposed to the solvent (data not shown). Addition of gradually increasing XD concentrations triggers an increase in the fluorescence intensity in a dose-dependent manner, which indicates a modification in the pattern of interactions with neighboring groups. At the same time, addition of XD causes a progressive shift in the emission maximum from 356 nm to 352 nm (data not shown), thus indicating that Trp 518 becomes only slightly less exposed to the solvent.