FIGURE 4.

PFL melting curves as measured by fluorescence spectroscopy. The fraction of PFL folded as a function of temperature is shown for PFL alone at 0.5 μm (■) or PFL in the presence of 1 (♦), 5 (▴), or 10 (●) molar equivalent of PFL-AE. Solid lines are the theoretical fits generated as described under “Experimental Procedures.” PFL-AE alone has a melting temperature of 53 °C. Fluorescence spectroscopy parameters: excitation wavelength, 294 nm; collecting wavelength, 290–410 nm; step, 3 nm; average, 200; photomultiplier tube setting, 400 V. The excitation wavelength was set at 294 nm to minimize contribution of tyrosine to the observed fluorescence.