Exploring the biophysical aspects and binding mechanism of thionine with bovine hemoglobin by optical spectroscopic and molecular docking methods

• The interaction between TH with BHb was investigated.
• TH quenches the intrinsic fluorescence of BHb by static quenching process.
• The distance (r) between BHb and TH was 3.64 nm as derived from FRET theory.
• The conformational changes of BHb induced by TH complexation were observed.
• The molecular docking study of TH with BHb also supports the experimental findings.

In the present investigation, we have elucidated the interaction between thionine (TH) and bovine hemoglobin (BHb) under physiological conditions by using absorption, emission, time resolved fluorescence, synchronous fluorescence, circular dichroism (CD) and three dimensional emission (3D) spectral studies. Molecular docking experiment was also carried out to establish the possible binding site of TH on BHb. The emission spectral studies revealed that, TH have the ability to bind with BHb and form a ground state complex via static quenching process. The calculated binding constant and the number of binding sites was found to be 3.65 × 104 dm3 mol−1 and 1.04, respectively. Förster Resonance Energy Transfer (FRET) theory was employed to calculate the distance (r) between donor (BHb) and acceptor (TH) as 3.64 nm. Furthermore, the conformational changes of BHb induced by TH complexation showed some degree of structural unfolding. In addition, molecular docking study confirmed that the most probable binding site of TH was located within the active cavity constituted by α1 and α2 subunits of BHb.

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