MorphDeCage (2-(4-methoxy-3-nitrophenyl)-2-morpholinoacetic acid) and PyrDeCage (2-(4-methoxy-3-nitrophenyl)-2-(methyl(pyridin-2-ylmethyl)amino)acetic) are Zn 2 + photocages that utilize photodecarboxylation of the methoxy derivative of metanitrophenylacetic acid as the release mechanism. Isothermal titration calorimetry (ITC) was used an alternative to usual approaches to measure the Zn 2 + binding affinities of these new compounds owing to unsuccessful measurement by competitive titration with 4-(2-pyridylazo)resorcinol (PAR). MorphDe-Cage forms a 1 : 1 ligand-metal complex with a 106 μM K d value.PyrDeCage forms both a 1 : 1 and 1 : 2 metal: ligand complexes with 3.2 and 21.7 μM K d 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 K d values of the SRPs measured by UV-vis titration and ITC were internally consistent and support the retraction of the original report (J.
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