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In this study, the fluorescence spectra of sarafloxacin (SAR) under different pH conditions were investigated to determine the structural changes due to protonation that result from change in pH. At pH < 1.02, SAR exists in the H L form for which the maximum fluorescence emission wavelength was about 455 nm. At pH 1.87-4.94, SAR exists in the H L form in which H L loses one proton in the nitrogen molecule at the 1-position in the quinoline ring. Fluorescence intensity was strong and steady and the maximum emission wavelength was 458 nm. At pH 7.14-9.30, the maximum emission wavelengths were gradually blue shifted to 430 nm with increase in pH, here SAR exists in the form of a bipolar ion HL in which H L loses a carboxyl group proton. At pH > 11.6, HL transforms into anionic L in which HL loses one proton from the piperazine ring, leading to a decrease in fluorescence intensity, and the maximum emission wavelength was red shifted to approximately 466 nm. The two-step dissociation constant pKa for SAR was calculated, pK was 6.06 ± 0.37 and pK for SAR was 10.53 ± 0.19. In a pH 3.62 buffer solution with quinine sulfate as the reference, the fluorescence quantum yield of SAR at the maximum excitation wavelength of 276 nm was 0.09.

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The interaction between Cefpirome and Trypsin was studied using multi-spectroscopy and molecular docking methods. The results showed that quenching mechanism of the Trypsin-Cefpirome system was probably static. The main interaction force was electrostatic force, and the number of binding sites in this system was approximately equal to 1. The quenching curves indicated that the tyrosine and tryptophan residues were both involved in the reaction. Synchronous fluorescence spectroscopy further confirmed that the main binding site was closer to tryptophan residues. The distance (r) between Trypsin and Cefpirome was less than 7 nm, which indicated that the energy transfer from Trypsin to Cefpirome was non-radiative energy transfer. The values of the Hill's coefficients showed that there was negative cooperativity in the interaction between Cefpirome and Trypsin.

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