The invention provides for a method of crosslinking a hetero-bifunctional photo crosslinking compound to an immunoglobulin having at least one heterocyclic photo reactive group and at least one non-photo reactive group where the non-photo reactive group is coupled to an effector molecule and the photo reactive group is coupled to the nucleotide binding site of an immunoglobulin. Alternatively, the photo crosslinker contains an orthogonal reactive group such as a thiol, which can be coupled to an effector molecule or functionalized ligand.
