In this study,the binding mechanism of theophyline (TPL) and pepsin (PEP) was studied for the first time by ultraviolet-visible absorption spectroscopy,Fourier infrared spectroscopy,fluorescence spectroscopy,three-dimensional fluorescence spectroscopy,synchronous fluorescence spectroscopy,circular dichography andmolecular docking simulation method,and the mechanism of interaction between TPL and PEP was explored at themolecular level,which was helpful to conduct in-depth research on the pharmacotoxicity and efficacy of TPL.According to the Stern-Volmer equation,the dynamic fluorescence quenching rate constant Kq of TPL for PEP at three temperatures of 298,303,and 308 K is much greater than the maximum dynamic fluorescence quenching constant of 2.0×1010 L·(mol·s)-1,proving that TPL quenching PEP is static quenching.With the continuous increase of TPL concentration,the dynamic quenching constant Ksv of PEP shows a regular downward trend,and TPL can effectively quench the endogenous fluorescence of PEP and further infer that the quenching mechanism is static quenching.The three-dimensional fluorescence spectrum analysis showed that with the continuous increase of TPL concentration in the system,the peak fluorescence intensity representing tryptophan residues,tyrosine residues and peptide chain skeleton structure in PEP decreased significantly,and the peak position was redshifted,indicating that TPL a affected the secondary structure of PEP.Simultaneous fluorescence mapping analysis showed that when TPL binds to PEP,it is mainly concentrated on the tryptophan residues.Infrared spectroscopy showed is that TPL caused the functional groups in PEP to expand and vibrate,which changed the secondary structure of PEP.The ultraviolet absorption spectrum showed that the absorption peak and peak increased gradually with the increase of TPL concentration in the mixed system,indicating that TPL could change the secondary structure of PEP.Themolecular docking simulation method shows that the binding force between TPL and the amino acid residues GLU13,VAL30,TRP39,GLY76,GLY78 and PHE117 in PEP is van der Waals force,and hydrogen bonds are formed with amino acid residues THR77 and GLY217,and hydrophobic forces exist with amino acid residues TYR75,LEU112,ILE120 and PHE111,which proves that they are mainly bound to hydrogen bonds by van der Waals force.Further evidence that TPL changes the secondary structure of PEP.Circular dichromy chromatographic analysis showed that the proportion of β-folds in PEP decreased from 50.2% to 48.8%,and the proportion of α-helical structure increased from 8.1% to 8.4%.The proportion of β-corners increased from 18.3% to 18.7%;The proportion of random structures rose from 29.1% to 29.2%,indicating that TPL has changed the secondary structure of PEP.The results of this experiment are helpful to understand the binding mechanism of TPL and PEPand provide data basis for the use and research of TPL.