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Plot of typical concentrations of glutamine, pyroglutamic acid and their sum as determined by 800 MHz 1H NMR spectroscopy in different portions of samples from the same pooled human serum obtained after protein precipitation using methanol (1:2 v/v). Although glutamine massively cyclizes to pyroglutamic acid, the sum of glutamine and pyroglutamic acid is the same in all the portions of serum samples (CV < 3%).
