Certain maleimide-N-alkylenecarboxylate-ortho-nitrobenzenesulfonic acid esters and derivatives useful for coupling biological materials

Coupling of biological materials is disclosed via 4-hydroxy-3-nitrobenzene sulfonic acid sodium salt (HNSA) esters. The novel activated esters are structured so as to react at one end with an amine group of a selected biological or non-biological material, thereby releasing the spectroscopically monitorable HNSA dianion, and at the other end with, typically, a sulfhydryl group of a second material which may or may not be biological. The esters provide for a cross-linking reaction that may be easily controlled and monitored.