Simplified Probe Synthesis

Most methods for generating imaging probes and other molecular constructions with multiple functions, such as target recognition and fluorescence, require the different portions of the system to be added sequentially. This sequential process makes it difficult to create reproducible materials with well-characterized stoichiometry—a big plus when it comes to medical applications. To address this limitation, researchers at Massachusetts General Hospital have developed a more readily controllable synthesis of multifunctional imaging probes and nanomaterials.

The investigators’ approach is to attach multiple functional moieties to a peptide scaffold that has a single reactive site (Chem. Commun., DOI: 10.1039/b809537j). They then use that site to attach the entire construct to a substrate, such as a gold nanoparticle, in one step. This approach yields a final product with a well-defined stoichiometry, according to Elisabeth Garanger, who presented the work at the American Chemical Society national meeting in Philadelphia at a symposium sponsored by the Division of Analytical Chemistry. She is a postdoctoral associate working with Lee Josephson at the hospital’s Center for Molecular Imaging Research.

Garanger described two types of “multifunctional single-attachment point” (MSAP) reagents. Bifunctional scaffolds consist of a dipeptide of lysine and cysteine; trifunctional scaffolds are made of tetrapeptides with two lysines, β-alanine, and cysteine. The functional moieties are attached to the primary amines of the lysines. The cysteine’s thiol is modified by a maleimide-containing cross-linking agent to display the desired reactive group.

Garanger reported two probes made by the simplified process. In one, an MSAP reagent bearing polyethylene glycol for stabilization and fluorescein and diethylenetriaminepentaacetic acid (a metal-chelating agent) to add a detection function were attached to gold nanoparticles. Usually, multifunctional gold nanoparticles are difficult to make reproducibly by sequential synthesis, Garanger said. In the other type of probe, she and her colleagues attached a scaffold containing a metal-chelating agent and a fluorophore to a tumor-targeting cyclic peptide.

“The ability to control the stoichiometry of different groups using the peptide scaffold is very elegant,” says Mansoor M. Amiji, professor of pharmaceutical sciences at Northeastern University.