Room temperature fluorescence and phosphorescence study on the interactions of iodide ions with single tryptophan containing serum albumins

• The effect of KI on the decay parameters of HSA, ESA and LSA has been examined.
• The crystallographic structure of the albumins studied has been analysed.
• Two distinct conformations of the albumins studied have been indicated.
• The fluorescence decay kinetics of the albumins studied were diexponential.
• The phosphorescence lifetimes of the albumins studied are increased by iodide.

In this study, the influence of heavy-atom perturbation, induced by the addition of iodide ions, on the fluorescence and phosphorescence decay parameters of some single tryptophan containing serum albumins isolated from: human (HSA), equine (ESA) and leporine (LSA) has been studied. The obtained results indicated that, there exist two distinct conformations of the proteins with different exposure to the quencher. In addition, the Stern–Volmer plots indicated saturation of iodide ions in the binding region. Therefore, to determine quenching parameter, we proposed alternative quenching model and we have performed a global analysis of each conformer to define the effect of iodide ions in the cavity by determining the value of the association constant. The possible quenching mechanism may be based on long-range through-space interactions between the buried chromophore and quencher in the aqueous phase. The discrepancies of the decay parameters between the albumins studied may be related with the accumulation of positive charge at the main and the back entrance to the Drug Site 1 where tryptophan residue is located.

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