Introduction The application of surfactants in the field of biochem- istry has given importance to studies of the nature of the interaction between protein and surface active agents in biological phenomena such as biological membranes [1] and protein solubilization. It has also been suggested that surfactant-protein systems can be used as a model for biological membranes [2, 3]. Proteins and ionic surfactants show the property of having both charged groups and hydrophobic por- tions [4]. This implies that the interactions between surfactants and proteins are complex processes, in- volving different types of intermolecular forces. Therefore, the ionic head groups of surfactants may bind to oppositely charged groups on the protein sur- face by electrostatic forces, whereas non-polar tail groups of surfactants may bind to no-polar sites on the protein surface through hydrophobic forces [5]. The widespread use of the anionic surfactants has stimulated interest in the nature of the interactions between those surfactants and globular proteins. De- pending on the surfactant concentration, the protein conformation will adopt a specific structure. The use of anionic surfactants like sodium alkyl carboxylates at low concentrations usually induces the compaction of protein (folding), nevertheless at moderate concen- trations below the critical micelle concentration (cmc) are a potent denaturant for protein solution [6, 7]. The interaction of sodium alkyl carboxylates with lysozyme and myoglobin has been studied by UV spectroscopic methods [8–10], for whom, we have done the analysis of thermal unfolding curves. Lysozyme is a small protein, molecular mass 14.3 kDa, of 129 amino acids, containing 18 cationic amino acid residues and 12 anionic residues. Its struc- ture is stabilized by four disulfide bridges and the in- terior of the protein is almost hydrophobic while the surface is mostly polar. The isoelectric point is pI~11.0 and the protein is therefore positively charged in an aqueous solution. Myoglobin is a single-chain protein, molecular mass 16.7 kDa, of 153 amino acids, containing a heme group in the center. Its secondary structure is unusual in that it contains a very high propor- tion (75%) of α-helical secondary structure. The iso- electric point is pI~7.3. Work reported includes the analyse of the thermal denaturation curves for single protein in aqueous solu- tion and for protein plus surfactants at different con- centrations, determination of temperature dependence of ΔG [11], melting temperature (T m ) and others ther- modynamics parameters at T m like ΔH m , ΔS m and ΔC p . Experimental Materials Sodium octanoate (C 8 HONa) and sodium perfluoro- octanoate (C 8 FONa) of at least 97% purity were ob- tained from Lancaster Synthesis Ltd. Sodium dodecanoate (C 12 HONa) with purity greater than 99%, was obtained from Sigma Chemical Co. 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 87 (2007) 1, 211–215 THERMAL STABILITY OF LYSOZYME AND MYOGLOBIN IN THE PRESENCE OF ANIONIC SURFACTANTS Elena Blanco, J. M. Ruso * , J. Sabín, G. Prieto and F. Sarmiento Group of Biophysics and Interfaces, Department of Applied Physics, Faculty of Physics, University of Santiago de Compostela 15782 Santiago de Compostela, Spain The interactions of lysozyme and myoglobin with anionic surfactants (hydrogenated and fluorinated), at surfactant concentrations below the critical micelle concentration, in aqueous solution were studied using spectroscopic techniques. The temperature conformational transition of globular proteins by anionic surfactants was analysed as a function of denaturant concentration through absorbance measurements at 280 nm. Changes in absorbance of protein-surfactant system with temperature were used to determine the unfolding thermodynamics parameters, melting temperature, T m , enthalpy, ΔH m , entropy, ΔS m and the heat capacity change, ΔC p , between the native and denatured states. Keywords: lysozyme, myoglobin, thermal denaturation, thermodynamics, UV absorbance * Author for correspondence: faruso@usc.es