KB005038: Is the High Sensitivity Protein 250 labeling charge-matched? 

No, the labeling is based on N-hydroxy-succinimidyl ester chemistry. The fluorescent dye is negatively charged and binds to the positively charged lysine affecting the charge of the labeled protein and therefore the pI. Hence, the labeling should be performed in the presence of abundant protein and a relatively low dye concentration to keep the numbers of introduced labels per protein to a minimum. The size of the dye is approximately 850 g/mol.