Polymer movement probed by fluorescent tagging

Biphenyl molecules fluoresce strongly after stimulation with light. If the light used to stimulate fluorescence is polarised, however, only a few molecules will be oriented correctly to be able to absorb the light and become excited.

The excited molecules have life spans of a few nanoseconds, after which time they decay, releasing the light again. While the molecules are excited some will move, changing their orientation. The light emitted from the molecules that changed their orientation while they were excited will be polarised differently to the exciting radiation.

By measuring the intensity of emitted light polarised perpendicular to the stimulating radiation, and comparing it with the intensity of light emitted parallel to the stimulating radiation over time, we obtain a measure of how easily the molecules change their orientation. The measure is an anisotropy decay curve, which tells us what proportion of the emitted light is polarised in the same sense as the stimulating light at various times after stimulation.

Finally, if we attach a biphenyl group to a polymer molecule, then the group will only be able to move if the section of polymer chain to which it is attached also moves. Measuring the anisotropy decay curve now tells us how flexible the polymer chain is.

References

I.C. Barker, J.M.G. Cowie, T.N. Huckerby, D.A. Shaw, I. Soutar and L. Swanson. Studies of the 'Smart' Thermoresponsive Behaviour of Copolymers of N-Isopropylacrylamide and N,N-Dimethylacrylamide in Dilute Aqueous Solution. Macromolecules 36:20, (2003) 7765-7770.

This feature is based on work carried out by Dr Linda Swanson:

Linda Swanson