Purpose: Light is known to induce histidine (His) oxidation and His-His crosslinking in proteins. The crosslinking is resulted from the nucleophilic attack of a His to a photooxidized His from another protein. The goal of this work is to understand if covalent buffer adducts on His residues can be generated by light through similar mechanisms in nucleophilic buffers such as Tris and His.
Methods: A model protein (DNase) was buffer exchanged into nucleophilic buffers before light exposure. Photogenerated products were characterized by tryptic peptide mapping with mass spectrometry (MS) analysis. Several buffer adductions on His residues were identified after light exposure. To understand the influencing factors of such reactions, the levels of adducts were measured for six nucleophilic buffers on all His residues in DNase.
Results: The levels of adducts were found to correlate with the solvent accessibility of the His residue. The levels of adducts also correlate with the structure of the nucleophile, especially the steric restrictions of the nucleophile. The levels of adducts can be higher than that of other His photoreaction products, including photooxidation and crosslinking.
Conclusions: In nucleophilic buffers, light can induce covalently-linked adducts to His residues.
Keywords: histidine; light; oxidation; singlet oxygen.