Identification of glycated sites in ovalbumin under freeze-drying processing by liquid chromatography high-resolution mass spectrometry

- Lysine residues are the main glycated sites of ovalbumin lyophilized with glucose.
- Glycated sites located at the outer surface of the global protein.
- Unglycated sites located at the hydrophobic pocket and the six β-sheets.
- Few changes occurred in the main strands of the β-sheet to disturb the glycated sites.

The glycation reaction between ovalbumin and d-glucose during freeze-drying was investigated and the mechanism of protection of the protein structure was studied, the precise glycated sites and degree of substitution per peptide (DSP) of each site were determined using liquid chromatography high-resolution mass spectrometry. It was found that lysine residues are the main glycated sites under freeze-drying. K62 and K264 were the most reactive glycated sites in lyophilized ovalbumin, with a DSP close to 80%. The glycated sites were located at the outer surface of the global protein. The unglycated sites were located at the outer surface of the hydrophobic pocket and in the six main strands of the β-sheet. Therefore, the glycation reaction of the protein was occurred in the solvent accessible area. It was hypothesized that few changes occurred in the conformation to disturb the glycated sites under freeze-drying. In particular, the main strands of the β-sheet of ovalbumin were more stable. Freeze-drying was a mild process and protected the conformation without extensive denaturation.