The unfolding of a-lactalbumin has been studied intensively because it adopts a 'molten globule' state under mild denaturing conditions. Near-UV circular dichroism and tryptophan-fluorescence spectroscopy have been widely used to evaluate the destabilization of the native state of the protein. Both spectroscopic techniques involve radiation of the protein by light of 280 nm.

In this abstract we demonstrate that this irradiation of goat a-lactalbumin induces reduction of S-S bonds. This reduction may trouble the studies on the protein folding-unfolding equilibria.

Irradiation of goat a-lactalbumin, at 3°C and neutral pH, with light of 280 nm, induces a shift in the fluorescence spectrum of the native protein, referring to the exposure of fluorescent Trp-groups to the aqueous medium. The near-UV CD-spectra indicate that the protein conserves the rigidity of the native tertiary structure.

Irradiation of goat a-lactalbumin also induces disulfide reduction. The exposure of Trp in the native protein seems directly related to this reduction.

These results have been registered for both the apo-protein and the Ca²⁺-loaded protein. By increasing the temperature to 37°C, the amount of disulfide bridges in the Ca²⁺-loaded protein remains unaltered while it slightly decreases for the apo-a-lactalbumin. The different thermal effect is probably related to the conformational change of the apo-protein.