1. U.v. difference spectra show that the anionic surfactant sodium n-dodecyl sulphate unfolds ribonuclease A at pH7.3 and 10.3, but that the cationic surfactant n-dodecyltrimethylammonium bromide does not affect the conformation of the enzyme. 2. Equilibrium-dialysis experiments show that sodium n-dodecyl sulphate binds to ribonuclease A, but no binding of n-dodecyltrimethylammonium bromide could be detected at pH7.3. 3. The enzymic activity of ribonuclease A is unaffected by n-dodecyltrimethylammonium bromide up to a concentration of 0.03m at 25°C. 4. Ultracentrifuge studies support the conclusion that n-dodecyltrimethylammonium bromide does not interact significantly with ribonuclease A. 5. The enthalpy change as measured by microcalorimetry on binding of sodium n-dodecyl sulphate to ribonuclease A is consistent with an exothermic enthalpy of binding occurring simultaneously with an endothermic enthalpy of chain unfolding.

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