Binding of carmoisine, a food colorant, with hemoglobin: Spectroscopic and calorimetric studies

• The binding of the food colorant carmoisine with hemoglobin was studied.
• The binding caused significant quenching of the fluorescence of hemoglobin.
• FRET studies suggested that efficient energy transfer occurs from hemoglobin to the dye.
• The binding induced significant conformational changes in hemoglobin.
• The binding reaction was exothermic and favored by large positive entropy.

Interaction of the food colorant carmoisine with hemoglobin was studied using spectroscopic and calorimetric tools. The binding effected hypochromic changes in the Soret band of hemoglobin and induced remarkable quenching of the intrinsic fluorescence of hemoglobin. Synchronous fluorescence studies revealed that the polarity around the tryptophan residues of the protein was significantly increased in the presence of carmoisine whereas that around the tyrosine residues remained unchanged. Binding of carmoisine resulted in change of the secondary structure of hemoglobin reducing the helical composition to more than half the initial value. The binding was favored by large positive entropy changes and small but favorable enthalpy changes. The heat capacity change value and the occurrence of enthalpy–entropy compensation phenomena suggested the involvement of significant hydrophobic forces in the binding process. Detailed insights into the molecular interaction of carmoisine with hemoglobin are presented.