Multifunctional Cross-Linking Reagents
Multifunctional cross-linkers contain more than two functional groups that will react with proteins and/or nucleic acids. The most common ones are trifunctional reagents, as an extension of bifunctional cross-linkers. The multifunctional groups could be all identical as in homobifunctional reagents (Chapter 5), all different as in heterobifunctional (Chapter 6), or mixed homo-and heterotypes. The functional groups will react with amino acid side chains according to their selectivity or speciŠcity as discussed in earlier chapters. Because most of the functional groups are not speciŠc toward a special target, almost all of the multifunctional reagents will react with different side chains of amino acids or nucleobases and can be considered heterofunctional even in the case of homomultifunctional reagents. Thus, multifunctional agents will anchor at multiple sites of complex proteins, cross-linking multiple components or locales. For trifunctional reagents, three target sites will be linked together. The most important usage of these multifunctional cross-linkers is for the investigations of protein structures and protein-protein interactions, particularly for multisubunit complex protein aggregates. Many of the reagents are used to prepare immunotoxins and antibody conjugates. There are also important applications in tissue engineering, cross-linking polymeric protein complexes, cells, and tissues for medical purposes. There are relatively few published multifunctional reagents and a few trifunctional reagents are used as bifunctional cross-linkers. The latter are mentioned in earlier chapters, but will be discussed below as trifunctional compounds. All other new multifunctional cross-linkers that appear in the literature are listed in Table 7.1 and will be further elaborated upon below.