Dual polarisation interferometry (DPI) is an analytical technique that can probe molecular scale layers adsorbed to the surface of a waveguide by using the evanescent wave of a laser beam confined to the waveguide. Typically used to measure the conformational change in proteins or other biomolecules as they function (refered to as the Conformation Activity Relationship).

DPI^[1] focuses laser light into two waveguides, one, the "sensing" waveguide, with an exposed surface and one to create a reference beam. A two-dimensional interference pattern is formed in the far field by combining the light passing through the two waveguides. The DPI technique rotates the polarisation of the laser to alternately excite two polarisation modes of the waveguides. Measurement of the interferogram for both polarisations allows both the refractive index and the thickness of the adsorbed layer to be calculated. The polarisation can be switched rapidly, allowing "real time" measurements of chemistry taking place on a chip surface in a flow-through system. These measurements can be used to infer conformational information about the molecular interactions taking place as the molecule size (from the layer thickness) and the fold density (from the RI) change. DPI is typically used to characterise biochemical interactions by quantifying any conformational change at the same time as measuring reaction rates, affinities and thermodynamics.

The technique is quantitative and real time (10Hz) with a dimensional resolution of 0.01nm^[2] .

A novel application for Dual Polarisation Interferometry recently emerged where the intensity of light passing through the waveguide is extinguished in the presence of crystal growth. This has allowed the very earliest stages in protein crystal nucleation to be monitored.^[3] The latest versions of Dual Polarisation Interferometers also have the capability to quantify the order and disruption in birefringent thin films.^[4] This has been used, for example, to study the formation of lipid bilayers and their interaction with membrane proteins^[5].

[edit] See also

• [Farfield Group]

[edit] References

1. ^ GH Cross, A Reeves, S Brand, JF Popplewell, LL Peel, MJ Swann and NJ Freeman A New Quantitative Optical Biosensor for Protein Characterisation. Biosensors and Bioelectronics (2003) 19 383-390.
2. ^ M.J. Swann, N.J. Freeman and G.H. Cross “Dual Polarization Interferometry: A Real-Time Optical Technique for Measuring (Bio)Molecular Orientation, Structure and Function at the Solid/Liquid Interface” In: Handbook of Biosensors and Biochips, 2 Volume Set (2007). Eds: R. S. Marks, C. R. Lowe, D. C. Cullen, H. H. Weetall, I. Karube. Wiley. ISBN: 978-0-470-01905-4, Vol1, part 4, ch33, pp549-568.
3. ^ Attia Boudjemline et al. Early stages of protein crystallization as revealed by emerging optical waveguide technology J. Appl. Cryst. 41, 523–530 (2008)
4. ^ Alireza Mashaghi et al. Optical anisotropy of supported lipid structures probed by waveguide spectroscopy and its application to study of supported lipid bilayer formation kinetics Anal. Chem., 80 (10), 3666–3676 (2008) [1]
5. ^ Narinder Sanghera, Marcus J. Swann, Gerry Ronan, Teresa J.T. Pinheiro, Insight into early events in the aggregation of the prion protein on lipid membranes, Biochimica et Biophysica Acta (BBA) - Biomembranes, Volume 1788, Issue 10, October 2009, Pages 2245-2251.

Further Reading

• Young's fringes from vertically integrated slab waveguides: Applications to humidity sensing[2]
• A new quantitative optical biosensor for protein characterisation [3]
• Neville J. Feeman et al. Real time, high resolution studies of protein adsorption and structure at the solid–liquid interface using dual polarisation interferometry. Journal of physics - Condensed Matter, 16, S2493–S2496 (2004) [4]
• Tabrisur Rhaman Khan et al. Lipid redistribution in phosphatidylserine-containing vesicles adsorbing on titania. Biointerfaces 3(2), FA90 - FA95 (2008)

[edit] External links