In the absence of protective reducing agents, Cys residues in purified proteins can be oxidized spontaneously by oxygen in the air, as frequently observed in protein crystal structures. However, the formation of an O-bridge via dehydration mechanism between a peroxidized Cys side chain and a primary amine of Lys side chain in proteins has not yet been reported. When an electron density feature was observed for an extra group or an extra atom between side chains of Cys-245 and Lys-158 in the crystal structure of histidinol phosphate phosphatase, mass spectrometric analysis was carried out for its chemical identification. That analysis led to a conclusion that this extra density corresponded to a methylene group. It was then proposed that these two residues were able to absorb CO and reduced it to CH spontaneously. Further examination of other protein structures in the PDB showed that the formation of this cross-linking species was a widespread phenomenon. This claim is examined in this study using methods recently developed for quantification of electrons around nucleus as the means for direct chemical identification. It is found that an O-bridge is actually formed between Cys and Lys side chains, instead of a CH -bridge.