Terminal groups in Starburst dendrimers: activation and reactions with proteins

Starburst dendrimers are novel nanoscopic synthetic polymers of defined molecular mass and geometry. These macromolecules, available in commercial quantities, contain methyl carboxylate and primary amino-terminal groups. The presence of these groups on any macromolecule limits its usefulness especially in cases where, for specific modulation of the properties of biologically active molecules, covalent bond formation is desirable between the biologically active molecules and the macromolecule. This paper describes activation of the surface groups of Starburst dendrimers for incorporation of a number of reactive electrophilic and nucleophilic groups and utilization of these reactive groups in formation of covalent bonds between dendrimers and alkaline phosphatase. The protein-dendrimer complexes have been reacted further with the Fab' fragment of an anti-creatine kinase MB isoenzyme antibody to form multifunctional dendrimer reagents. The enzymatic and immunochemical properties of these protein-dendrimer reagents have been evaluated by an immunoassay system. Nucleophilic thiols and electrophilic phenyliodoacetamido, iodoacetamido, and epoxy groups have been incorporated into amino-terminal dendrimers by their reactions with appropriate heterobifunctional reagents. Two independent sets of reactions have been used to prepare the reactive N-hydroxysuccinimidyl esters from dendrimers containing the terminal carboxyl groups. Quantitation of the reactive groups has been carried out by direct titration of these activated dendrimers and the products obtained by reactions of these dendrimers with small molecules and proteins.