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Abstract
The structural effect of the three low molecular weight cationic amphiphiles, dodecylammonium chloride (DAC), hexadecylpyridinium chloride (HPC) and hexadecyllysinester dihydrochloride (HLDC) on lysozyme at different temperatures was investigated by means of calorimetrical and optical methods. These amphiphiles are characterized by high (DAC) and low (HPC) critical micelle concentration in electrolyte-free solution, respectively, while HLDC forms liposomes in water. None of the amphiphiles at any concentration investigated (0.05-0.6 g amphiphile/g protein) significantly changed the native characteristics of the protein at 25°C. The results from the structural methods employed were confirmed by enzymatic activity measurements. At increased temperatures the effect on lysozyme was dependent on the association state of the amphiphile. Complete inhibition of thermal aggregation was the only effect registered at amphiphile concentration below cmc, while concentrations near and above the critical micelle concentration, also influenced the thermally induced unfolding of lysozyme. The amplitude of the major thermal transition was significantly lowered, while only minor structural changes not affecting the enzymatic activity were observed below this temperature. The postdenaturation structure of lysozyme in presence of the cationic amphiphiles used thus seems to exhibit a higher degree of order compared to lysozyme without amphiphile.