Cerium (III) Chloride-induced conformational changes of human serum albumin, HSA, in phosphate buffer, 10 mM at pH 7.4 was investigated, using isothermal titration calorimetry (ITC), UV and fluorescence emission spectroscopic methods. The results indicate that CeCl3, Ce3+, induces irreversible denaturation of the HSA structure. The UV absorption intensity of HSA + Ce3+ shows a slight blueshift in the absorbance wavelength with increasing Ce3+ concentration. The fluorescence intensity was increased regularly and a slight redshift was observed in the emission wavelength. The HSA + Ce3+ complex quenches the fluorescence of HSA and changes the microenvironment of tryptophan residue. The emission intensity increases suggesting the loss of the tertiary structure of HSA. The results obtained from the ITC data are in agreement with the spectroscopic methods. The strong negative cooperativity of Ce3+ binding with HSA (Table 1) recovered from the extended solvation model, indicates that HSA has been denatured as a result of its interaction with Ce3+ ions.
The binding parameters between cyclodextrins (CDs) and human serum albumin (HSA) were investigated by isothermal titration calorimetry (ITC), fluorescence quenching, and UV-vis absorption spectroscopy at 300 K in 50 mM phosphate buffer solution. Among the various CDs investigated, b-CD has the greater ability to decrease the aggregation of HSA and the results indicated that the inhibition order is g-CD , a-CD , b-CD. The obtained heats for HSAþCDs interactions were reported and analysed in terms of the extended solvation model, which was used to reproduce the enthalpies of HSA interactions with CDs over a broad range of complex concentrations. The binding constant and thermodynamic parameters were obtained. These suggested that the binding reaction was driven by both enthalpy and entropy, and electrostatic interactions played a major role in the stabilising of HSA. The parameters d y A and d y B reflected the net effect of b-CD on the HSA stability at low and high cyclodextrin concentrations, respectively. The positive values for d y A indicated that b-CD stabilises the HSA structure at low concentrations. The UV absorption intensity of theses complexes increased and a slight red shift was observed in the absorbance wavelength with increasing the CD concentration. The fluorescence intensity of HSA decreased regularly and a slight blue shift was observed for the emission wavelength with increasing CD concentration. The results indicate that the CD complex could quench the fluorescence of HSA and changes the microenvironment of the tryptophan residue.
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