LIU Cui-ge1, 2, XU Yi-zhuang1*, WEI Yong-ju2, QI Jian1, XU Zhen-hua1, YE Fang3, WU Jin-guang1
1. College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China 2. College of Chemistry, Hebei Normal University, Shijiazhuang 050016, China 3. Guangzhou Baiyunshan Pharmaceutical Factory, Guangzhou 510515, China
Abstract:Fluorescence spectra, ultraviolet spectra and protonation of Ofloxacin (OFL) at different pH values have been studied. In strong acidic solutions, OFL molecule might accept two protons to exist as ternary acid H3L2+ with a maximum emission wavelength (λmax) at 505 nm. Along with the increase in pH value, fluorescence spectrum of OFL changed, an iso-fluorescence point at 352 nm was found in the fluorescence excitation spectra, and at the same time, isosbestic points were found in its UV absorption spectra. This spectral feature reveals that H3L2+ gradually lost hydrogen ion combined on C-4 keto oxygen. In the range of pH 2.5 to pH 4, OFL exists as H2L+ with λmax at 499 nm. When pH>4, the fluorescence emission peak at 499 nm gradually blue-shifted to 455 nm as pH was increasing, and an iso-fluorescence emission point was formed at 484 nm,indicating the dissociation of hydrogen ion of carboxylic acid at C-3. At about pH 7.0, OFL exists in the dipole ion form HL with λmax at 455 nm, which is the strongest fluorescence form. When pH>8, λmax red-shift from 455 to 475 nm as pH increases, meanwhile, fluorescence intensity decreases, indicating that HL lost hydrogen ion combined on N-4 of piperazinyl group. When pH>10, OFL exists in anion ion form L-,and fluorescence intensity decreases with pH increasing, but λmax remains essentially constant, indicating that medium environment influences the fluorescence property of OFL.