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We investigated the relationship between thermodynamic stability and amyloid aggregation propensity for a set of single mutants of the alpha-spectrin SH3 domain (Spc-SH3). Whilst mutations destabilizing the domain at position 56 did not enhance fibrillation, the N47A mutation increased the rate of amyloid fibril formation by 10-fold. Even under conditions of identical thermodynamic stability, the aggregation rate was much higher for the N47A mutant than for the WT domain. We conclude that the N47A mutation does not change the apparent mechanism of fibrillation or the morphology of the amyloid fibrils, and that its amyloidogenic property is due to its effect upon the rate of the conformational events leading to nucleation and not to its overall destabilizing effect.

The thermal denaturation of bovine beta-lactoglobulin B was investigated by high-sensitivity differential scanning microcalorimetry between pH 1.5 and 3.0 in 20 mM phosphate buffer. The process was found to be a reversible, two-state transition. Progressive addition of guanidine hydrochloride at pH 3.0 leads to the appearance of a low-temperature calorimetric endotherm, corresponding to the cold renaturation of the protein. Circular dichroism experiments have confirmed the low and high temperature denaturation processes, and have shown some structural differences between both denatured states of beta-lactoglobulin B.