Conformational changes in β-lactoglobulin-surfactant interactions using circular dichroism

Includes bibliographical references.The interaction of surfactants and globular proteins often results in the disruption of the native secondary and tertiary structure of the protein and formation of protein surfactant complexes. This study attempts to clarify the mechanism of surfactant denaturation of proteins. β-lactoglobulin B was isolated from fresh whole milk according to the Fox isolation method. It was characterized with respect to electrophoretic mobility, sedimentation coefficient, and conformation. Circular dichroism spectra of the titration of β-lactoglobulin B with various detergents (sodium dodecylsulfate, sodium decylsulfate, sodium octylsulfate, sodium dodecylsarcosinate, dodecyltrimethyl- ammonium bromide, Tween 20, and Lubrol WX) showed that the protein undergoes conformational changes with increasing concentrations of ionic detergents. Nonionic detergents produce no detectable conformational change. As the concentration of the ionic detergent is increased, there is an increase in the amount of a helix and structure at the expense of random coil. Higher surfactant concentrations are required to obtain a comparable conformational change for shorter chain length detergents than is required for a similar detergent with a longer chain. This ability to unfold proteins may be related to the critical micelle concentration (CMC) rather than the chain length. It may be observed as a chain length effect, however, since the CMC decreases with increasing chain length. Perhaps all detergents are capable of cooperative association but detergents with low GMC's might not be able to reach a high enough monomer concentration to do so. Ionic detergents have higher GMC's than nonionic detergents of the same hydrocarbon length. This causes the cooperative mode of association more likely to be observed for ionic detergents than for nonionic detergents. This explains why no conformational change was observed for the nonionic detergents Tween 20 and Lubrol WX. In the presence of the cationic detergent, dodecyltrimethyl- ammonium bromide, the protein undergoes a conformational change comparable to that obtained with the anionic detergent, sodium dodecyl- sulfate, but a much higher detergent concentration is required for the cationic detergent. This advantage of anionic detergents is due to the fact that detergent ions tend to cluster about protein side chains with the opposite charge. The association of an anionic detergent is favored because arginyl and lysyl side chains project further from the protein surface and contribute more GH^ groups for incorporating into a cluster as compared to glutamyl and aspartyl side chains which would be involved in the binding of a cationic detergent. This would also agree with the fact that the weak binding process is thought to be primarily hydrophobic. Lower concentrations of complexed surfactants are organized like micelles in many respects. In the noncooperative region, clusters of surfactant molecules in a micellar environment are formed on or with the protein with the surfactant alkyl chains binding to the protein apolar groups. At high surfactant concentrations, the environment of the surfactant molecules is like that in simple surfactant micelles.M.S. (Master of Science