Isothermal Titration Calorimetry for Characterizing the Zinc(II)‐Binding Properties of Photocaged Complexes with Micromolar Affinity

Abstract

AbstractMorphDeCage (2‐(4‐methoxy‐3‐nitrophenyl)‐2‐morpholinoacetic acid) and PyrDeCage (2‐(4‐methoxy‐3‐nitrophenyl)‐2‐(methyl(pyridin‐2‐ylmethyl)amino)acetic) are Zn2+ photocages that utilize photodecarboxylation of the methoxy derivative of meta‐nitrophenylacetic acid as the release mechanism. Isothermal titration calorimetry (ITC) was used an alternative to usual approaches to measure the Zn2+ binding affinities of these new compounds owing to unsuccessful measurement by competitive titration with 4‐(2‐pyridylazo)resorcinol (PAR). MorphDeCage forms a 1 : 1 ligand‐metal complex with a 106 μM Kd value. PyrDeCage forms both a 1 : 1 and 1 : 2 metal: ligand complexes with 3.2 and 21.7 μM Kd values respectively. To further demonstrate the efficacy of the ITC methodology and provide a comparison to direct UV‐vis titrations data, two photocages based on Sanger's reagent (SRPs) were prepared. The Kd values of the SRPs measured by UV‐vis titration and ITC were internally consistent and support the retraction of the original report (J. Am. Chem. Soc. 2020, 142, 3806–3813), which was withdrawn due to errors in binding affinity measurements.