addition of a metal chelator results in the dissociation of the ligating group and the oligohistidine tag application_number US 201313962426 A description referring now to the drawings wherein like reference numerals refer to like parts throughout there is seen in fig1 a schematic representation of a reaction scheme for the synthesis of a selective derivatization of an oligohistidine sequence of a recombinant protein according to an embodiment according to an aspect of the invention are methods and systems for selective chemical reaction with an oligohistidine tag in a recombinant protein resulting in selective chemical modification of that protein according to another aspect are novel chemical reagents possessing at least two different functional groups the first functional group depicted as group u201c a u201d in fig1 comprises one two three or more ligating components depicted as u201c l u201d these ligating groups are adapted to be capable of coordinative binding through a divalent metal cation m 2 to an oligohistidine tag present in a recombinant protein the second functional group depicted as group u201c irg u201d in fig1 comprises an imidazole reactive group u201c irg u201d which is adapted to be capable of selective alkylation of an imidazole ring of a histidine residue of the oligohistidine tag through an addition elimination process according to an embodiment these two functional groups a and irg are connected by covalent bonds to a linker as represented by for example structure 1 in fig1 and structure 5 in fig2 because the.