where δH and δN represent the change in the chemical shift of the amide proton and nitrogen, respectively. A weighting factor of 0.2 was employed to account for the difference in sensitivity of proton and nitrogen chemical shifts of the amide group (Rule & Hitchens, 2005; Williamson, 2013). The binding dissociation constant (KD) was calculated by fitting the changes in1H and 15N chemical shift as a function of ligand concentration using the single-site binding model (Rule & Hitchens, 2005) as described in equation 2.