Exposure of tryptophanyl residues in alpha-lactalbumin and lysozyme. Quantitative determination by fluorescence quenching studies.

The effect of iodide ion on the tryptophyl fluorescence of the homologous proteins lysozyme and alpha-lactalbumin in their native form, as well as in their modified structures and in fragments from these proteins was studied. By assessing the contribution to the total fluorescence of the exposed and buried Trp residues, and of the respective fluorescence quantum yields, the quantification of the number of Trp exposed to the solvent for all the species studied was possible. Both native proteins show an important increase in the number of Trp residues exposed to the solvent when treated with denaturing agents. The peptides L-II (aa 13-105) and alpha-I (aa 1-90) from lysozyme and alpha-lactalbumin, respectively, showed Trp residues with different degree of exposure, whereas the smaller fragments, L-III (aa 106-129) and alpha-II (aa 91-123), had all their Trp residues exposed to the solvent.