Figure 4.

1D STD monitored isotherm for the Aβ (12–28) self-association inhibition by Tf. Tf was titrated into a solution of salt-aggregated 650 μM Aβ (12–28) and 1D STD and STR spectra were acquired at 293 K and at 600 MHz using a Bruker TBI-Z probe. Experimental data points are reported as circles, whereas the solid line was obtained through nonlinear fitting of the equation f + (1-f)^∗K_d,app./(K_d,app. + [Tf]), where f is the fraction of Aβ (12–28) oligomers that are not Tf-binding competent, [Tf] is the concentration of free Tf and K_d,app. is an apparent average dissociation constant for the complexes between Tf and the other Aβ (12–28) oligomers (1). The dashed and dot-dashed lines were obtained using the same equation but by varying the K_d,app. value obtained through nonlinear curve fitting (i.e., 43 nM, assuming that the product of the total concentration of Tf-binding competent Aβ oligomers and of the ratio of the number of Tf molecules bound per Aβ oligomer versus the number of Aβ oligomers bound per Tf molecule is ≤ 5 nM. If this product is >5 nM, the K_d,app. values reported in the figure become just an upper limit to the best fit K_d,app. values). We also assumed that all Tf molecules bind to equivalent and independent binding sites (Scatchard-like model) (1). The latter assumption reflects the simplest model that fits the experimental data. The dotted line defines the IC₅₀ for the inhibition of Aβ (12–28) self-association inhibition by Tf.