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. Author manuscript; available in PMC: 2011 Mar 9.
Published in final edited form as: Biochemistry. 2010 Mar 9;49(9):2018–2030. doi: 10.1021/bi902027z

Figure 3.

Figure 3

Titrations of the oligopeptide HWKK with polyU. Titrations at three HWKK concentrations (2 μM (○), 4 μM (●), and 6 μM (□)) observed as function of tryptophan fluorescence quenching (eq. 14) at 9.8 mM Na+, pH 6.0, and 25 °C. The concentration of polyU is given in moles of nucleotides per liter. Solid and dashed lines are the nonlinear least squares global fits across all three titrations using eq. 1, 3, and 4, with Qmax = 0.93 as determined by ligand binding density analysis (35). The dashed line reports the McGhee-von Hippel isotherm with logKobs = 5.11 ± 0.02 with a site size fixed at n = 4. The solid line isotherm shows the fit with the site size allowed to float (logKobs = 5.22 ± 0.02, n = 4.96 ± 0.11). If all parameters are allowed to float, logKobs = 5.23 ± 0.03, Qmax = 0.929 ± 0.007, and n = 4.97 ± 0.13 (data not shown in plot above).