Protein-Detergent Interactions.

Interactions of sperm whale ferrimyoglobin with the cationic detergent laurylpyridinium chloride in aqueous medium were studied by measurement of spectral changes in the Soret band of the prosthetic group. Spectrophotometric titrations were carried out in 0.02 M sodium phosphate buffer pH 7.1 at 25°C in the concentration ranges of 1 nnM-1 μM myoglobin and up to 10 mM laurylpyridinium chloride and also at various NaCl concentrations, up to 0.5 M. A thermodynamic analysis was carried out assuming in all-or-none interaction between the hemoprotein and the detergent. The apparent stoichiometric coefficients for the association of the detergent with the protein and the average standard free energy changes of the interactions were evaluated. The thermodynamic data obtained correspond to values characteristic for hydrophobic interactions. The dependence of the spectral transition curves on total myoglobin concentration found in the present system enabled an evaluation of equilibria involving dissociation of the prosthetic group from the ferrimyoglobin by the ac~ien of the cationic detergent. Direct experimental evidence obtained for this process by chromatography has been presented previously. This leads to interpretation of the spectral effects observed as a result of association between ferriheme and laurylpyridinium chloride molecules. [ABSTRACT FROM AUTHOR]