Fluorescence intensity variations of the single tryptophan in N TAILW518 was measured by using a Cary Eclipse (Varian) equipped with a front-face fluorescence accessory at 20 C, by using 2.5 nm excitation and 10 nm emission bandwidths. The excitation wavelength was 290 nm and the emission spectra were recorded between 300 nm and 540 nm. Titrations were performed in a 1-mL quartz fluorescence cuvette containing 1 mM N TAILW518 in 10 mM sodium phosphate buffer at pH 7, and by gradually increasing the XD concentration from 0.05 mM to 3 mM. Experimental fluorescence intensities were corrected by subtracting the spectrum obtained with XD alone (note that XD is devoid of tryptophan residues). Data were analyzed by plotting the relative fluorescence intensities at the maximum of emission at increasing XD concentrations. The dissociation equilibrium constant (K Dapp ) value was determined from data fitted to a single exponential equation, by using the PRISM 3.02 nonlinear regression tool (GraphPad).