A view on phosphate ester photochemistry by time-resolved solid state NMR. Intramolecular redox reaction of caged ATP

The light-driven intramolecular redox reaction of adenosine-5'-triphosphate-[P3-(1-(2-nitrophenyl)-ethyl)]ester (caged ATP) has been studied in frozen aqueous solution using time-resolved solid state NMR spectroscopy under continuous illumination conditions. Cleavage of the phosphate ester bond leads to 0.3, 1.36, and 6.06 ppm downfield shifts of the alpha-, beta-, and gamma-phosphorus resonances of caged ATP, respectively. The observed rate of ATP formation is 2.4 +/- 0.2 h(-1) at 245 K. The proton released in the reaction binds to the triphosphate moiety of the nascent ATP, causing the upfield shifts of the 31P resonances. Analyses of the reaction kinetics indicate that bond cleavage and proton release are two sequential processes in the solid state, suggesting that the 1-hydroxy,1-(2-nitrosophenyl)-ethyl carbocation intermediate is involved in the reaction. The beta-phosphate oxygen atom of ATP is protonated first, indicating its proximity to the reaction center, possibly within hydrogen bonding distance. The residual linewidth kinetics are interpreted in terms of chemical exchange processes, hydrogen bonding of the beta-phosphate oxygen atom and evolution of the hydrolytic equilibrium at the triphosphate moiety of the nascent ATP. Photoreaction of caged ATP in situ gives an opportunity to study structural kinetics and catalysis of ATP-dependent enzymes by NMR spectroscopy in rotating solids.