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This study aims to investigate the interaction between cefonicid sodium (CFS) and papain (PAPA) using fluorescence spectroscopy, synchronous fluorescence spectroscopy and molecular docking methods. The results indicated that the fluorescence intensity of PAPA was decreased considerably upon the addition of CFS through a static quenching mechanism. Synchronous fluorescence spectroscopy studies showed that the combination of CFS and PAPA could change the conformation of PAPA. At the temperature of 293 K, there was a good linear relationship between the fluorescence intensity of the system and the concentration of CFS in the range of 6.0×10-6 to 1.0×10-4 mol/L and the detection limit of the method was 3.05×10-6 mol/L (n=10). From the results of the thermodynamic constant and molecular model analysis, it could be inferred that the CFS and PAPA molecules mainly combine by electrostatic attraction and hydrogen bonding. The binding model was established based on the experimental data, and the binding rate data of CFS and PAPA was obtained. The results showed that taking PAPA while taking CFS was safe.

The reaction mechanism of cefepime (CEF) with papain (PAPA) was investigated by both fluorescence quenching and synchronous fluorescence spectroscopy in different temperature (293, 308 and 318 K). The results demonstrated that the interaction between CEF and PAPA was taking place via static quenching. Thermodynamic parameters revealed that electrostatic interaction played major roles in the interaction. The order of magnitudes of binding constant is 104, and the number of binding site in the system was closed to 1. The binding rates of CEF to PAPA (λex = 280 nm) were 81.46%~95.95% (293K), 80.50%~95.49% (308K) and 79.77%~95.12% (318K). There was a linear relationship between ΔF and the concentrations of CEF in the range of 6.0×10-6~7.0×10-5 mol/L. And the correlation coefficient of the linear curve was 0.9973.

In this paper, the fluorescence spectra of Enrofloxacin (ENR) in different pH conditions was studied in order to determine its structural changes due to protonation with pH changes. The ENR two-step dissociation constant is calculated and the fluorescence quantum yield under acidic conditions is measured. In the strong acidic conditions, ENR exists of H3L 2+ form of which maximum emission wavelength is at 450 nm. At the condition of pH 2.45 to 4.23, ENR exists of H 2 L + form with strong and steady fluorescence. The maximum emission wavelength is still 450 nm. At the condition of pH more than 4.23, the maximum emission wavelengths are gradually blue shifted to 445 nm and the fluorescence intensity decrease with the increase of pH which shows that H 2 L + loses one proton with the increase of pH and exists in the form of bipolar ion HL. When the pH is more than 12.28, the fluorescence intensity are weakened to nearly disappear with the increase of pH value, indicating that HL gradually loses the proton with the conversion to the anion of L - which is weaker fluorescence. In the buffer solution of pH 3.00, with quinine sulfate as reference, the fluorescence quantum yield of ENR at excitation wavelength of 274 nm is 0.125.

The reaction mechanism of rhodamine B (RHB) with bovine serum albumin (BSA) was investigated using fluorescence spectroscopy and synchronous fluorescence spectroscopy at different temperatures (298 K, 310 K and 318 K). The results showed that electrostatic force played a major role on the conjugation reaction between BSA and RHB, and the type of quenching was static quenching. Primary binding site for RHB was sub-hydrophobic domain IIA, and the number of binding sites was 1. The order of magnitude of binding constants ( K a ) was 10 4 . The value of Hill’s coefficients ( n H ) was approximately equal to 1, which suggested no cooperativity in BSA-RHB system. The donor-to-acceptor distance r < 7 nm indicated that the static fluorescence quenching of BSA by RHB was also a non-radiation energy transfer process. The results of two methods were consistent that showed the synchronous fluorescence spectroscopy could be used to study the reaction mechanism between drug and protein, and was a useful supplement to the conventional fluorescence quenching method.