Spectroscopic and interfacial investigation on the interaction of hemoglobin with conventional and ionic liquid surfactants

A comprehensive investigation on the interactions between equine heart hemoglobin and different surface active agents including ionic liquid surfactants (ILs) was performed in aqueous buffer (pH = 7.4) by ultraviolet-visible and fluorescence spectroscopic techniques coupled with tensiometry. The surface active agents include a cationic surfactant cetyltrimethylammonium bromide (CTAB), two cationic ILs 1-hexadecyl-3-methylimidazolium chloride (C16MeImCl), 1-butyl-3-methylimidazolium octyl sulfate (C4MeImOS), three anionic surfactants sodium N-dodecanoyl sarcosinate (SDDS), sodium dodecylbenzene sulfonate (SDBS), sodium cholate (NaC), and a non-ionic surfactant N-decanoyl-N-methylglucamine (Mega 10). In particular, surfactant-induced conversion of the iron coordination and spin states of hemoglobin was monitored by the shifts of the Soret peak. Fluorescence studies indicated exposure of the tryptophan and tyrosine residues of hemoglobin to polar environment when surface active agents were introduced, the relative contributions of these amino acids to the fluorescence intensity being different for different surfactants. Hemoglobin was found to be unfolded in surfactant solutions. Unfolding of hemoglobin was found to be more pronounced in presence of ionic surfactants than in presence of the non-ionic one.