Fluorescence spectroscopic characterization of the interaction of human adult hemoglobin and two isatins, 1-methylisatin and 1-phenylisatin: a comparative study

In this report, steady state and time-resolved fluorescence along with circular dichroism (CD) spectroscopic, FTIR, and anisotropy investigations were made to reveal the nature of the interactions between human adult hemoglobin (Hb) and the isatins, 1-methylisatin (1-MI) and 1-phenylisatin (1-PI). From the analysis of the steady state and time-resolved fluorescence quenching of Hb in aqueous solution in the presence of an isatin, i.e., 1-MI, it seemed that the nature of the quenching was of static type and a mixture of both static and dynamic nature for 1-PI. The primary binding pattern between isatins and Hb has been interpreted as a combined effect of hydrophobic association and electrostatic interaction for 1-MI. For 1-PI, this was the combined effect of hydrophobic association and ionic interactions and salt bridges or/and proton transfer. The pretwisted structure of 1-PI facilitates ionic interactions with Hb. The binding constants, number of binding sites, and thermodynamic parameters had been computed. The binding average distances between the Hb-1-MI and Hb-1-PI determined from Forster's theory were found to be 4.02 and 5.28 nm, respectively. CD, steady state, and time-resolved anisotropy measurements had been done in support.