Copper ions are required for human health, but are toxic in excessive quantities. Numerous conditions, including Alzheimer's (fatal dementia) and Wilson's (liver and psychiatric problems) disease, are caused, or are thought to be caused, in whole or part, by copper toxicity.
Consequently, it is of medical interest to develop molecules that chemically bind copper ions, thereby removing them from circulation in the blood. A primary challenge here is to discover a strongly binding molecule that is selective for biologically relevant forms of copper ions, in preference to other metal ions.
Pascale Delangle (Institute for Nanoscience and Cryogenics, France) and coworkers have synthesized such a molecule. Their molecule, based on the amino acid cysteine, has a much higher affinity for copper ions than copper ion transporters found in cells, and is similarly selective for copper ions over other metal ions.
The workhorse of the scientists' molecule is its three cysteine-based units. Each one of these units is appended to the central nitrilotriacetic acid unit of the molecule.
Nitrilotriacetic acid is a biodegradable molecule used for water treatment. Cysteine is a nonessential amino acid (a subunit of proteins).
The copper-binding components of the molecule are the sulfur atoms of the cysteine units. The scientists found that their molecule forms chemical bonds with up to two copper ions, of the +1 oxidation state commonly found in cells.
Probing copper ion binding.
The scientists first investigated what happens when up to one copper ion is available for binding per molecule. They used nuclear magnetic resonance spectroscopy, electrospray ionization mass spectroscopy, and circular dichroism spectroscopy for these investigations.
Nuclear magnetic mass spectroscopy uses a magnetic field and radio waves, most commonly to determine the type and number of chemical bonds that are formed by the hydrogen and carbon atoms in a molecule. This technique indicated that the scientists' molecule wraps around the copper ion, forming strong chemical bonds, with at most one copper ion per molecule.
Electrospray ionization mass spectroscopy ionizes a molecule, fragments it, and computes the mass to charge ratio of the fragments based on its transit through an electromagnetic field, providing chemically identifying information. This technique confirmed the three cysteine units (one molecule) per one copper ion relationship.
Circular dichroism spectroscopy uses directionally controlled light for numerous chemical identification purposes, in this case to probe the metal binding properties of the scientists' molecule. This technique indicated that there was considerable electron transfer from the scientists' molecule to the copper ion, a confirmation of chemical binding between the cysteine units and the copper ion.
When more than one copper ion is available for binding per molecule, the situation is quite different. All of the spectroscopy techniques inidcated that a copper ion cluster had formed, up to two copper ions per cysteine unit.
Stability and selectivity of copper ion binding.
Having demonstrated that their molecule chemically binds to biologically relevant variants of copper ions, the scientists needed to demonstrate that the bonds are stable. They further needed to demonstrate that their molecule is selective for copper ions in preference to other relevant metal ions, especially those it will be likely to encounter in a biological medium such as blood.
Titrations with a molecule of known stability (bathocuprione disulfonate) indicated that cysteine-copper chemical bonds are 18 times stronger than those of a copper ion transporter found in cells. Furthermore, in comparison to the ion transporter, the scientists' molecule was similarly selective for copper ions relative to calcium, lead, zinc, cadmium, and mercury ions.
These scientists clearly have a molecule that shows promise for biologically relevant copper ion binding. However, further work may be necessary for this molecule to be of clinical benefit (the scientists' stated aim of the research).
These are all questions that need to be addressed, and if a limitation is found, many protocols have been developed that should be applicable to solving the problem within this context. Although the research described here may be viewed as the beginning of the road for clinically relevant applications, a promising start has been made that is worth pursuing.
for more information:
Pujol, A. M., Gateau, C., Lebrun, C., & Delangle, P. (2009). A Cysteine-Based Tripodal Chelator with a High Affinity and Selectivity for Copper(I) Journal of the American Chemical Society, 131 (20), 6928-6929 DOI: 10.1021/ja901700a