Abstract: Biotinylation reagent has a structure according to formula (I) With R1=alkyl, hydroxyalkyl or carboxyalkyl residue comprising 1 to 50 carbon atoms R2=alkyl, glycol, amine or peptide reside with 1 to 50 carbon atoms R3=Aryl, heteroaryl, with 3 to 50 carbon atoms R4, R5=Aryl, hetero-aryl, Alkyl with 3 to 50 carbon atoms Use of the agent in cell separation processes.

Abstract: The invention is directed to a method for preparing a detection reagent for CAR expressing cells according to general formula (I) (X-S)m-D ??(I) wherein D is a detection moiety, S a spacer unit and X a polypeptide comprising 10 to 80 amino acids and a N3 group, which binds to the antigen binding domain of a chimeric antigen receptor of a cell and m=4-200, characterized in reacting p detection moieties D with a spacer S according to general formula (II) wherein R is a leaving group, q=1 to 5, and n=4 to 200 and p is 4-250 thereby forming modified detection moiety Sp-D and reacting the modified detection moiety Sp-D with a plurality of polypeptides X to yield (X-S)m-D.