Reinvestigation of the thermodynamics of spectrin self-association

The thermodynamics of the self-association reactions of human spectrin have been reinvestigated by means of sedimentation equilibrium over the temperature range 18-40 degrees C. The experimental data were analysed in terms of a cooperative isodesmic model of association. The van't Hoff plot showed that the standard change in enthalpy for the heterodimer-tetramer step was temperature-dependent, leading to an estimate of -8.5 kJ mol-1 K-1 for the change in molar heat capacity, delta Cp. Curvature in the van't Hoff plots, not detected in previous studies, was revealed through the increased precision of the data and the wider temperature range examined. On the assumption that delta Cp reflects hydrophobic interactions in the tetramer that cannot be formed in the heterodimer, it can be estimated that approximately 50 CH2 groups per heterodimer participate in hydrophobic interactions in the tetramer that cannot be formed in the heterodimer.