By Carolyn A. Wilson, Ph.D.
The word spin might make you think of someone trying to influence your opinion. But in physics, spin refers to an intrinsic property of certain subatomic particles that make some nuclei act like small magnets. That kind of spin is key to a powerful laboratory technique scientists use to study complex, carbon-based biological products at the atomic and molecular level.
This technique, called nuclear magnetic resonance (NMR), uses a strong, external magnetic field and radio waves to trigger the release of electromagnetic energy from atoms with nuclei that have spin. Computers convert these data into contour plots that resemble topographic land maps. Scientists use these data to determine the locations of atoms in relation to each other in molecules. This enables them to create three-dimensional models they can hold in their hands and study, or 3D images they can rotate on a computer screen.
Scientists in FDA’s Center for Biologics Evaluation and Research (CBER) are using NMR to study two types of molecules relevant to the vaccines against bacterial and viral disease that we regulate: polysaccharides (long chains of sugar molecules), which occur in either the cell wall or the capsule surrounding some disease-causing bacteria, and shorter chains of sugars called oligosaccharides. Oligosaccharides are found in viruses and are also part of bacterial vaccines.
The microbes need these molecules to cause disease, so CBER scientists are using NMR to study how the structure of such polysaccharides and oligosaccharides triggers production of antibodies against the microbes that carry these molecules. The methods developed by CBER scientists will allow evaluation of licensed and investigational polysaccharide vaccines by using NMR to determine if those vaccines were developed in a manner consistent with these insights into how the structure of these molecules triggers antibody production. In addition, the outcomes of these studies might provide information that manufacturers could use to design novel polysaccharide vaccines that are safe and effective.
Scientists at FDA discuss construction of the facility that will house the new NMR. From left to right: Hugo F. Azurmendi (CBER), Kang Chen (CDER), Darón I. Freedberg (CBER) & Marcos D. Battistel (CBER). Get this and other FDA photos on Flickr.
Insights into the structures of polysaccharides that play critical roles in generating protective immune responses would be especially useful in confronting dangerous pathogens for which there are no vaccines. Two such pathogens, the bacteria Neisseria meningitidis B and Escherichia coli K1, cause meningitis (a potentially fatal inflammation of the brain and spinal cord). The capsules surrounding these bacteria contain a polysaccharide called polysialic acid. This molecule is unusual because it doesn’t trigger antibody production when injected by itself into adult humans, but people infected with bacteria that have polysialic acid in their cell walls or capsules do produce antibodies against it. One logical explanation for this difference is that “free” polysialic acid has a somewhat different structure than polysialic acid on bacterial cell walls. But, using NMR, CBER scientists found that polysialic had the same structure whether free or as part of the pathogen. Figuring out why only bound polysialic acid triggers antibody production might help researchers develop much needed vaccines for these bacteria. Soon they will have a new NMR facility at the White Oak campus that could help them solve that puzzle.
NMR “Stick” and “Solid Spheres” Models: NMR studies at CBER are providing insights into the atomic and molecular ins and outs of polysialic acid, a molecule found on the surface of bacteria, including some that cause meningitis. This work is aimed at helping researchers develop safe and effective vaccines against such bacteria that are based polysialic acid. Using the NMR data from their studies, the CBER scientists created two models of polysialic acid, a “stick” model (above) and a “solid spheres” model (below), shown in these two short animations.
The NMR spectrometer in the new facility will have a magnet that is much stronger than those previously used at FDA. The stronger the magnet, the more precise the data generated by NMR and the more precise the models that can be developed from this data.
To put this into perspective, the clinical application of NMR, called magnetic resonance imaging (MRI), uses magnets with strengths of 1.5 to 3 units of magnetic power called Tesla. The strength of NMR magnets at CBER is now about 16.4 Tesla (700 megahertz). The new NMR facility at White Oak will have a strength of 19.9 Tesla (850 megahertz) – about 6 times that of hospital MRI machines. In fact, the magnet is so powerful that the machine is isolated in a special room with walls thick enough to block its magnetic field from pulling unsecured metallic objects toward it. In the photograph you can see the NMR team visiting the facility as it is being prepared for the arrival of the machine.
CBER will share the new NMR spectrometer with the Center for Drug Evaluation and Research (CDER), which will use it to do extremely sensitive assessments of the purity of heparin and of the structures and properties of protein therapeutics.
This powerful magnetic molecular “microscope” is one way that FDA incorporates new technology into its regulatory science work to protect and promote the nation’s health.
Carolyn A. Wilson, Ph.D., is Associate Director for Research at FDA’s Center for Biologics Evaluation and Research.