ulfhydryl-reactive groups can be employed that form disulfide bonds with sulfhydryl groups; in one example, such groups 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-pyridyl, o-nitrophenyl, m-nitrophenyl, p