Scheme 2 CS–Michael addition reaction of probe C23 and SO2 (HSO3-1 in the solution)
When the probe is combined with HSA, the Michael addition reaction between the probe and SO2 is rapidly catalyzed by HSA, and this catalytic effect of HSA greatly improves the detection sensitivity of SO2. Studies have shown that the amino group of Lys199 in HSA contributes most of the catalytic activity.7 The results of liang et al showed good performance of this probe molecule for the simultaneous determination of HSA and SO2 molecules in urine and cells. And this probe was expected to be used in the diagnosis of liver cancer, drug evaluation, and the study of physiological and pathological functions of HSA and SO2. In this work, the structure and fluorescent character of this probe molecule were studied under density functional theory. The rotation of the unsaturated C-C bonds in the probe molecule led to several stable structures coexisted. The transfer between different stable structures provided the non-radiative energy loss pathway when the probe molecule recovered from the excited state to the ground state which led to no fluorescence in this process. The calculated results indicated the red fluorescence would be generated if the rotation of the unsaturated C-C bonds in the probe molecule was prohibited such as the probe molecule was stabilized in the hydrophobic cavity of HSA. The calculation also indicated that the Michael reaction between the probe molecule and SO2would be another effective way for the stabilization of the probe molecule and generating blue fluorescence which made this probe molecule is able of simultaneous determination of HSA and SO2molecules. All the figures were rendered by means of VMD 1.9.3 software8 and the analyses were finished by using the Multiwfn 3.7 code9.