Interactions between Different Classes of Surfactants and Metmyoglobin

Abstract

Complexes of horse metmyoglobin(metMb) with the anionic surfactants sodium bis(2-ethylhexyl) sulfosuccinate(AOT) and sodium dodecyl benzene sulfonate(SDBS),the cationic surfactants dodecyl trimethylammonium bromide(CTAB) and dodecyltrimethyl ammonium bromide(DTAB),and the zwitterionic surfactant 3-[(3-cholamidopropyl) dimethylammonio] propanesulfonate(CHAPS) were investigated by UV-Vis absorption,synchronous fluorescence emission,and circular dichroism(CD) spectroscopy.Experimental results show that the anionic and cationic surfactants can interact with metMb intensively depending on the surfactant concentration.The UV-Vis spectra indicate that AOT and SDBS interact with metMb at low concentrations.The addition of AOT(or SDBS) causes the formation of a six-coordinated low-spin heme(6-cLs) hemichrome as is evident from the red shift of the Soret band,the intensity decrease,concomitant appearance of two new Q bands,and the disappearance of ligand-to-metal charge transfer(LMCT).The surfactants disturb the Tyr and Trp microenvironment and change the second structure parameter of metMb while the α-helix content decreases.However,the interaction between metMb and CTAB(or DTAB) is different.They cannot disturb heme at very low concentrations but can disturb the Tyr and Trp microenvironment.CTAB and DTAB aggregates can convert metMb to a five-coordinated low-spin heme as shown by the blue shift of the Soret band and cause the heme monomer to leave the hydrophobic cavity of metMb through electrostatic attraction mainly.DTAB/metMb complexes behave in a slightly different way to CTAB/metMb because of their special structure.In contrast,no interaction is evident between the zwitterionic surfactant over a large range of concentrations because of the neutral charge of CHAPS,which precludes an effective electrostatic attraction between the ionic sites of CHAPS and a protein.The significant distance between the ionic sites with opposite charges in metMb precludes a double ionic interaction for each CHAPS surfactant molecule despite the presence of two oppositely charged ionic sites in the CHAPS molecule.Therefore,proteins interact with surfactants in multifarious ways and this depends on the surfactant species,concentration,and structure.