A robust deamidation quantification method, called QuanDA, was developed to quantify the spontaneous nonenzymatic deamidation of peptides based on the isotopic distribution change of peptides in matrix-assisted laser desorption ionization (MALDI) mass spectra and non-negative least squares calculation. The predict model of QuanDA using theoretical spectra of pure un-deamidated and deamidated peptides for a series of simulated partial deamidated peptides is satisfying, with a coefficient of determination (R2) and root mean squared error (RMSE) of 0.9914 and 0.03356, respectively. It was applicable in the lack of reference standards of un-deamidated and deamidated peptides. The only requirements were the chemical formulas of un-deamidated and deamidated peptides for isotopic pattern calculation. QuanDA provided a rapid, low-cost and easy-accessible method for deamidation analysis in therapeutic drugs.

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