Fluorescence spectra, fluorescence quantum yield and dissociation constant of sarafloxacin.

Abstract

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 H3 L2+ form for which the maximum fluorescence emission wavelength was about 455nm. At pH1.87-4.94, SAR exists in the H2 L+ form in which H3 L2+ 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 458nm. At pH7.14-9.30, the maximum emission wavelengths were gradually blue shifted to 430nm with increase in pH, here SAR exists in the form of a bipolar ion HL in which H2 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 466nm. The two-step dissociation constant pKa for SAR was calculated, pKa1 was 6.06±0.37 and pKa2 for SAR was 10.53±0.19. In a pH3.62 buffer solution with quinine sulfate as the reference, the fluorescence quantum yield of SAR at the maximum excitation wavelength of 276nm was 0.09.