Alternatively, sulfhydryl reactive groups can be employed that form disulfide bonds with sulfhydryl groups; such groups generally have the structure ???S???S???Ar where Ar is a substituted or unsubstituted nitrogen-containing heteroaromatic moiety or a non-heterocyclic aromatic group substituted with an electron-withdrawing moiety, such that Ar may be, for example, 4-pyridinyl, o-nitrophenyl, m-ni