Irreversible thermal denaturation and formation of linear aggregates of ovalbumin

Changes in several physicochemical properties related to the thermal denaturation of ovalbumin have been investigated at neutral pH and low ionic strength. The far-UY circular dichroism (CD) spectrum at 80 degrees C indicated small secondary structural changes compared with those induced by addition of guanidine hydrochloride (GuHCI). The near-UV CD spectrum and difference absorption spectrum (250-320 nm) showed completely irreversible micro-environmental changes around the aromatic amino acid residues upon heat treatment (at >= 67 degrees C). In the sedimentation measurements of heated ovalbumin solutions a sharp, single peak, corresponding to soluble aggregates of low polydispersity, appeared, and these aggregates were ohserved as linear polymers by transmission electron microscopy. After 2 h of heating at 75 degrees C at pH 7.0, the intrinsic viscosity was similar to 20 times higher than the native one. We conclude that, under these conditions, although the globule form of ovalhumin molecule did not alter drastically upon thermal denaturation, partially denatured molecules which would expose the hydrophobic area(s) aggregate immediately and linear polymers (high-molecular-weight soluble aggregates) were formed.