A method of conjugating a carboxylate-containing molecule to an amine-containing macromolecule to form a substituted macromolecule is provided. The method includes (a) forming a first solution by dissolving a substance comprising at least one macromolecule having free amine groups in an aqueous basic solution; (b) forming a second solution by combining at least one carboxylate-containing molecule with at least one acid acceptor in an aprotic solvent; (c) adding at least one carboxylate activating agent to the second solution to form an activated carboxylate-containing molecule; (d) combining the first solution and second solutions to form a multiphasic solution; and (e) isolating the resultant substituted macromolecule from the mixture. A substituted macromolecule produced by the aforementioned method is also provided. Further provided is a method of enhancing a magnetic resonance image of a subject including administering the carboxylate-containing molecule-substituted macromolecule produced by the aforementioned method.