Minimization of side reactions during Lys Tag derivatization of C-terminal lysine peptides

► In this study we found that Lys Tag derivatization of lysine-containing peptides leads to side reactions such as methylation, deamidation and Lys Tag labeling of the N-terminus. ► We found temperature and pH to be the main variables to control side-reactions. ► We show that side-reactions during Lys Tag derivatization can be avoided or minimized by simply lowering the reaction temperature from 55 °C to room temperature.