Photoexcitation of tryptophan groups induced reduction of disulfide bonds in hen egg white lysozyme

Prolonged exposure of hen egg white lysozyme to near-UV light results in an unusual fluctuating and red shift of fluorescence spectra. By chemical detection of free thiol groups with sulfhydryl reagent and spectroscopic measurement, it is found that the fluorescence changes are accompanied with the cleavage of disulfide bonds upon irradiation. However, tryptophan and the new generated free thiol groups are not stable within the protein and will further react during prolonged irradiation. The existence of photo-induced dimers is uncovered based on the fact that some intermediately formed thiol radicals in different lysozyme molecules can further form intermolecular S–S bonds. In addition, the decrease of UV absorption ability of lysozyme indicates that tryptophan will be reoxidated after the reduction of disulfide bonds. Then a conclusion is drawn that the disulfide bonds in lysozyme are only partially reduced and the reduction of Cys30-Cys115 may be preferential based on the structural analysis. Meanwhile, it is found that the tertiary structure of lysozyme is not very compact, and the secondary structure is also gradually disturbed during illumination.