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Primary Photoprocesses in a Fluoroquinolone Antibiotic Sarafloxacin

Authors

  • Fernando Lorenzo,

    1. Free Radical Research Laboratory, STFC Daresbury Laboratory, Warrington, UK
    2. School of Biology, Chemistry and Health Sciences, Manchester Metropolitan University, Manchester, UK
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  • Suppiah Navaratnam,

    Corresponding author
    1. Free Radical Research Laboratory, STFC Daresbury Laboratory, Warrington, UK
    2. Biomedical Sciences Research Institute, University of Salford, Manchester, UK
      *Corresponding author email: s.navaratnam1@salford.ac.uk (Suppiah Navaratnam)
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  • Ruth Edge,

    1. School of Chemistry, University of Manchester, Manchester, UK
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  • Norman S. Allen

    1. School of Biology, Chemistry and Health Sciences, Manchester Metropolitan University, Manchester, UK
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  • This paper is part of a Symposium-in-Print on Pharmaceutical Photochemistry.

*Corresponding author email: s.navaratnam1@salford.ac.uk (Suppiah Navaratnam)

Abstract

The photophysical properties of the fluoroquinolone antibiotic sarafloxacin (SFX) were investigated in aqueous media. SFX in water, at pH 7.4, shows intense absorption with peaks at 272, 322 and 335 nm, (ɛ = 36800 and 17000 dm3 mol−1 cm−1, respectively). Both the absorption and emission properties of SFX are pH-dependent; pKa values for the protonation equilibria of both the ground (5.8 and 9.1) and excited singlet states (5.7 and 9.0) of SFX were determined spectroscopically. SFX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Laser flash photolysis and pulse radiolysis studies have been carried out in order to characterize the transient species of SFX in aqueous solution. Triplet–triplet absorption has a maximum at 610 nm with a molar absorption coefficient of 17,000 ± 1000 dm3 mol−1 cm−1. The quantum yield of triplet formation has been determined to be 0.35 ± 0.05. In the presence of oxygen, the triplet reacts to form excited singlet oxygen with quantum yield of 0.10. The initial triplet (3A*) was found to react with phosphate buffer to form triplet 3B* with lower energy and longer lifetime and having an absorption band centered at 700 nm. SFX triplet was also found to oxidize tryptophan to its radical with concomitant formation of the anion radical of SFX. Hence the photosensitivity of SFX could be initiated by the oxygen radicals and/or by SFX radicals acting as haptens.

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