Homobifunctional Cross-Linking Reagents
Homobifunctional reagents are compounds that contain two reactive functional groups that have equal reactivity toward the same amino acid side chain or DNA base. In all cases, these two reactive groups are identical. There may, however, be exceptions where the reaction mechanism indicates that the two identical reactive groups react differently. For example, in the case of nitrosourea derivatives, it is thought that one of the two reactive groups spontaneously forms a carbonium intermediate which elicits the cross-linking reaction. In such cases, these reagents are referred to as heterobifunctional if two different groups are cross-linked. A reagent, although classiŠed as a homobifunctional cross-linker, may cross-link two different groups when the second identical group is not available. For example, a sulfhydryl-targeted reagent may cross-link a thiol and an amino group when a second sulfhydryl group is not available for reaction. This situation is particularly so for protein-DNA cross-linking and for DNA strand cross-linking where two different bases are cross-linked due to the complementary nature of the helical structure. Similarly, nonspeciŠc photoactive compounds containing two identical photosensitive groups are regarded as homobifunctional compounds. In addition, a compound with obscure functional groups is classiŠed as homobifunctional if it cross-links two identical groups in proteins.