Formation of Nitrile Imine in the Ground and Excited States from Photolysis of 2-<i>N</i>-Methylpyrrole-5-formylamino-tetrazole

Photo-cross-linking reactions have been widely used for exploring the network of dynamic protein-protein interaction, and photolysis of cross-linker plays an extremely important role in the formation of the reactive intermediate and subsequent linking reaction. However, the detailed mechanism of the relevant reactions has been rarely reported up to date. In this work, 2-<i>N</i>-methylpyrrole-5-formylamino-tetrazole (<b>MPFT</b>) was chosen as a representative of the photo-cross-linkers developed recently. Structures and properties of <b>MPFT</b> in the three lowest-lying electronic states have been determined at the level of multistate complete active space second-order perturbation theory (MS-CASPT2). Meanwhile, the direct <i>ab initio</i> quantum trajectory mean-field (QTMF) method has been used to simulate nonadiabatic dynamics initiated from the first excited singlet state of <b>MPFT</b>. The combination of the MS-CASPT2 calculation and the QTMF simulation not only provided new insights into the mechanism of the <b>MPFT</b> photolysis but also revealed that the singlet excited-state nitrile imine is likely to be the reactive intermediate that is responsible for the cross-linking reaction with the site of target protein. Our conclusions obtained from MS-CASPT2 calculations and QTMF dynamical simulations are supported by the experimental findings.