We computed the emission maximum at several fixed points along the PT reaction coordinate, which is defined by the transfer of a proton from the OH group of the phenolic fragment to the oxygen atom of the acetate (See Fig. ). A significant red shift was observed as the neutral emitter is converted to the phenolate form, indicating that the deprotonation process has a considerable impact on the emission properties. Subsequently, we focused on evaluating the primary influence of the protein on the emission energy and the PT process between the neutral emitter and Asp162 by employing the PDLD/S-LRA method. This required a meticulous analysis of the PT process across both the ground and excited state free energy surfaces (∆𝐺), and the integration of the free energy change (∆∆𝐺) between these surfaces into the QCFF/PI emission energy calculations, as illustrated in Figure . To delineate these free energy surfaces, we estimated the deprotonation free energy using the calculated p𝐾 𝑎 values of the neutral emitter and Asp162 in the ground state (S0) and the first excited singlet state (S1), applying Equation ( ):