Spectroscopic study on interaction of lomefloxacin with human serum albumin in the presence of copper ion

The interaction of lomefloxacin (LMF) with human serum albumin (HSA) in the presence of copper ions in a physiological medium and its thermodynamic characteristics were investigated by multi-spectroscopy. The experimental results showed that both LMF and LMF-Cu2+ could quench the fluorescence of HSA with a static quenching mechanism, indicating that LMF or LMF-Cu2+ could react with HSA. The apparent binding constants/numbers of binding sites were estimated as 4.924 X 10(5) L center dot mol(-1)/1.473 for LMF-HSA, 8.990 X 10(4) L center dot mol(-1)/1.785 for LMFCu2+ -HSA, 1.10 X 10(5) L center dot mol(-1)/1.21 for LMF-Cu2+ and 7.30X 10(2) L center dot mol(-1)/0.82 for HSA-Cu2+, respectively. Delta H and Delta S for LMF-HSA system were calculated to be -2.189 kJ center dot mol(-1) and 61.25 J center dot mol(-1)center dot K-1, while those for LMF-Cu2+-HSA system were -7.401 kJ center dot mol(-1) and 47.63 J center dot mol(-1)center dot K-1. Although the values of Delta H and Delta S in these two systems were different, the treads were similar, which indicated that electrostatic interactions in these two systems played a major role. According to Forster theory, the distances were given as 5.006 nm for HSA-LMF and 4.709 nm for HSA-LMF-Cu2+. Synchronous fluorescence and circular dichroism spectra confirmed further that the conformations of human serum albumin before and after interacting with LMF or LMF-Cu2+ were different. All the results revealed that copper ions promoted the interaction of lomefloxacin with human serum albumin.