A growing area of ISIS research is the study of 'soft matter' in which weak electrostatic interactions play a significant role in creating complex structures and behaviour. Soft matter includes polymers, surfactants and biochemical materials, and so its study is important in health and biological research.
Peter Griffiths and his research group in the Chemistry Department at Cardiff University investigate soft matter and are frequent ISIS users.
Peter heard a colleague Ruth Duncan give a talk on polymer therapeutics - using long-chain molecules to ferry small drug molecules into cells. Targeted delivery of pharmaceuticals and genes to particular locations in the body is a key area of medical research but is difficult to achieve.
Professor Duncan commented that there was a lack of fundamental information on the three-dimensional arrangements of the polymer chains (their conformation) and how quickly they diffused in biological tissues. Peter realised he had the experience and experimental tools to uncover the information needed.
In collaboration with Cardiff’s School of Pharmacy, Peter started looking at factors affecting how a polymer-bound therapeutic negotiates the complex delivery-pathway from initial injection, or absorption, to the targeted cellular structure. The central issue was how interactions between polymers and the local tissue environment affected their rates of diffusion. A combination of experiments which include small angle neutron scattering (SANS) could provide some answers.
The power of neutrons
SANS can probe structures at scales from around 1 nanometre to more than 100 nanometres, which makes it ideal for studying the conformation and behaviour of synthetic polymers, as well as biomaterials.
Neutrons are particularly effective at homing in on the positions of the hydrogen atoms. The locations of surrounding water molecules can also be established,as they have a significant role in modifying the conformation and activity of biomolecules.
Now you see it...
One technique that neutron studies have to offer is that of isotopic substitution combined with ‘contrast matching’, in which specific parts of a complex molecular assembly can be highlighted. This is done by substituting selected hydrogen atoms with deuterium (D), which scatters differently.
For example, in a two-component system such as a protein mixed with a polymer in water, a proportion of the aqueous solvent may be substituted with heavy water (D2O) such that its scattering strength matches that of one of the components. This component is then rendered invisible so that only the other component is seen. “We make as much use of this approach as we can,” says Peter.
The mucus barrier
Peter has been working with one of his PhD students Paola Occhipinti on a particular drug-delivery issue.
Organs such as the respiratory, gastrointestinal and reproductive tracts are coated with a layer of mucus, tens of micrometres thick, which may pose a significant barrier to drug delivery agents. In the case of diseases like cystic fibrosis, the mucus layer may be many times thicker.
Mucus is composed of mucin, a gel-like network of glycoproteins, and Peter and Paola were interested in what factors affected how various polymers diffuse through it. Does the structure of mucin change on addition of the polymer? Does it bind to the gel?
SANS carried out on appropriately deuterated samples, together with measurements of viscosity, rates of diffusion, and nuclear magnetic resonance studies, can build up a picture of polymer-mucin interaction.
They carried out experiments at ISIS investigating the interaction of mucin with a typical model, polymer polyethylene glycol (PEG), and also with a dendrimer - a small but highly branched polymer.
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