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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 9
BKCSDE 32(9)
September 20, 2011 

Thermodynamic Investigation of the Formation of Complexes between Norfloxacin and Various Mononucleotides
Yong Jun Kwon, Hyun Mee Lee, Sung Wook Han, Dongjin Lee, Tae Sub Cho*
Nucleotides, Norfloxacin, Enthalpy, Entropy, Gibbs free energy
The fluorescence of norfloxacin was quenched by various nucleotides. The ratio of the fluorescence intensities in the absence and presence of nucleotide was linearly dependent on nucleotide concentration, suggesting that quenching occurred through the formation of nonfluorescent norfloxacin-nucleotide complexes. The gradient of the linear relationship represented the equilibrium constant of complex formation; it decreased with increasing temperature. The slopes of van’t Hoff plots constructed from the temperature-dependent equilibrium constants were positive in all cases, indicating that complex formation was energetically favorable – i.e., exothermic, with negative Gibb’s free energy. The equilibrium constant increased when triphosphate was used instead of monophosphate. It also increased when the oxygen at the C'2 position of the nucleotide was removed. Both enhancements were due to entropic effects: entropy decreased when complexes with AMP or GMP formed, while it increased when norfloxacin complexed with ATP, GTP, dAMP and dGMP.
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