Spectroscopic and Interfacial Properties of Myoglobin/Surfactant Complexes

Abstract

The complexes of horse myoglobin (Mb) with the anionic surfactant sodium dodecyl sulfate (SDS), and with the cationic surfactants cetyltrimethylammonium chloride (CTAC) and decyltrimethylammonium bromide (DeTAB), have been studied by a combination of surface tension measurements and optical spectroscopy, including heme absorption and aromatic amino acid fluorescence. SDS interacts in a monomeric form with Mb, which suggests the existence of a specific binding site for SDS, and induces the formation of a hexacoordinated Mb heme, possibly involving the distal histidine. Fluorescence spectra display an increase of tryptophan emission. Both effects point to an increased protein flexibility. SDS micelles induce both the appearance of two more heme species, one of which has the features of free heme, and protein unfolding. Mb/CTAC complexes display a very different behavior. CTAC monomers have no effect on the absorption spectra, and only a slight effect on the fluorescence spectra, whereas the formation of CTAC aggregates on the protein strongly affects both absorption and fluorescence. Mb/DeTAB complexes behave in a very similar way as Mb/CTAC complexes. The surface activity of the different Mb/surfactant complexes, as well as the interactions between the surfactants and Mb, are discussed on the basis of their structural properties.

Giant Extracellular Hemoglobin of Glossoscolex paulistus: An excellent Prototype of Biosensor and Blood Substitute

“…These authors argue that alterations of the heme structure are common to both modes of interaction, implying that the sites of electrostatic and hydrophobic contacts should be located in the vicinity of the cytochrome c heme pocket. In agreement with this discussion, Tofani and co-workers (Tofani et al, 2004) observed for Horse myoglobin (MbH), that only in a SDS/MbH ratio higher than 400, would occur a more significant protein unfolding and, for consequence, a more exposed and accessible heme pocket. Studies focused on the SDS-cytochrome c interaction at neutral pH have demonstrated that the unfolded state is stabilized when occur the bis-histidine (hemichrome) species formation, being that subsequent modifications of the secondary structure are rate-limited by the histidine dissociation rate (Das et al, 1999).…”

“…Otzen and Oliveberg (Otzen et al, 2002), studying a small protein S6 in the presence of SDS, argue that monomeric SDS binds to the native state, but global unfolding would occur only above the critical micelle concentration (cmc). Indeed, this verification is corroborated for various works about interaction between surfactants and hemoproteins (Oellerich et al, 2003;Tofani et al, 2004;Das et al, 1999). Oellerich and coworkers (Oellerich et al, 2003), analyzing the interaction between SDS and cytochrome c, explain that the differences observed below and above the cmc are due to the different modes of binding of SDS monomers and micelles.…”

“…Many approaches have been focused such as the kind of protein-surfactant interaction, the influence of the aggregation state of the surfactant (monomer, pre-micellar aggregate and micelle) on the protein structure, the properties of the surfactant-protein system, the characterization of the interaction sites on the protein surface, the identification of the intermediate protein conformations, etc (Liu et al, 2007;Liu et al, 2005;Ajloo et al, 2002). Research involving interaction of cationic and anionic surfactants with macromolecules, especially proteins, have been developed (Maulik et al, 1998;Tofani et al, 2004), being the interaction of charged headgroups the main focus of these works. Furthermore, depending of the concentration, surfactants can act as unfolding and denaturant agents, which can be used to evaluate the structural properties of the different proteins and their active sites.…”

“…Above the cmc, at milimolar surfactant concentrations, the protein-surfactant interaction is an extremely complicated phenomenon. Generally, surfactants induce a decrease of the -helix content, and this effect is smaller for cationic than for anionic ones (Tofani et al, 2004). However, in the case of the giant extracellular hemoglobins, which are also called erytrocruorins, such as Glossoscolex paulistus hemoglobin, this is not true (Santiago et al, 2007).…”

An electroreflectance approach to study out the puzzling state of myoglobin in a DDAB film on a pyrolytic graphite electrode surface

“…Upon reduction, the Soret band peak shifted to a longer wavelength with absorbance enhancement, and sharp α-and β-bands appeared in the Q-band region at 559 and 529 nm, respectively. These absorption spectral features indicate that DDAB does not prevent C 18 Im from its coordination to iron center of hemin.…”