Dual-polarization interferometry (DPI) is an analytical technique that probes molecular layers adsorbed to the surface of a waveguide using the evanescent wave of a laser beam. It is used to measure the conformational change in proteins, or other biomolecules, as they function (referred to as the conformation activity relationship).
DPI focuses laser light into two waveguides. One of these functions as the "sensing" waveguide having an exposed surface while the second one functions to maintain 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 polarization of the laser, to alternately excite two polarization modes of the waveguides. Measurement of the interferogram for both polarizations allows both the refractive index and the thickness of the adsorbed layer to be calculated. The polarization can be switched rapidly, allowing real-time measurements of chemical reactions 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 characterize 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 (10 Hz) with a dimensional resolution of 0.01 nm.
A novel application for dual-polarization interferometry emerged in 2008, 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. Later versions of dual-polarization interferometers also have the capability to quantify the order and disruption in birefringent thin films. This has been used, for example, to study the formation of lipid bilayers and their interaction with membrane proteins.
- Cross, G; Reeves, AA; Brand, S; Popplewell, JF; Peel, LL; Swann, MJ; Freeman, NJ (2003). "A new quantitative optical biosensor for protein characterisation". Biosensors and Bioelectronics. 19 (4): 383–90. doi:10.1016/S0956-5663(03)00203-3. PMID 14615097.
- Swann, MJ; Freeman, NJ; Cross, GH (2007). "Dual Polarization Interferometry: A Real-Time Optical Technique for Measuring (Bio)Molecular Orientation, Structure and Function at the Solid/Liquid Interface". In Marks, R.S.; Lowe, C.R.; Cullen, D.C.; Weetall, H.H.; Karube, I. Handbook of Biosensors and Biochips. Vol. 1. Wiley. Pt. 4, Ch. 33, pp. 549–568. ISBN 978-0-470-01905-4.
- Boudjemline, A; Clarke, DT; Freeman, NJ; Nicholson, JM; Jones, GR (2008). "Early stages of protein crystallization as revealed by emerging optical waveguide technology". Journal of Applied Crystallography. 41: 523. doi:10.1107/S0021889808005098.
- Mashaghi, A; Swann, M; Popplewell, J; Textor, M; Reimhult, E (2008). "Optical Anisotropy of Supported Lipid Structures Probed by Waveguide Spectroscopy and Its Application to Study of Supported Lipid Bilayer Formation Kinetics". Analytical Chemistry. 80 (10): 3666–76. doi:10.1021/ac800027s. PMID 18422336.
- Sanghera, N; Swann, MJ; Ronan, G; Pinheiro, TJ (2009). "Insight into early events in the aggregation of the prion protein on lipid membranes". Biochimica et Biophysica Acta. 1788 (10): 2245–51. doi:10.1016/j.bbamem.2009.08.005. PMID 19703409.
- Lee, TH; Heng, C; Swann, MJ; Gehman, JD; Separovic, F; Aguilar, MI (2010). "Real-time quantitative analysis of lipid disordering by aurein 1.2 during membrane adsorption, destabilisation and lysis". Biochimica et Biophysica Acta. 1798 (10): 1977–86. doi:10.1016/j.bbamem.2010.06.023. PMID 20599687.
- Cross, GH; Ren, Y; Freeman, NJ (1999). "Young's fringes from vertically integrated slab waveguides: Applications to humidity sensing" (PDF). Journal of Applied Physics. 86: 6483. Bibcode:1999JAP....86.6483C. doi:10.1063/1.371712.
- Cross, G (2003). "A new quantitative optical biosensor for protein characterisation". Biosensors and Bioelectronics. 19 (4): 383–90. doi:10.1016/S0956-5663(03)00203-3. PMID 14615097.
- Freeman, NJ; Peel, LL; Swann, MJ; Cross, GH; Reeves, A; Brand, S; Lu, JR (2004). "Real time, high resolution studies of protein adsorption and structure at the solid–liquid interface using dual polarization interferometry". Journal of Physics: Condensed Matter. 16: S2493. Bibcode:2004JPCM...16S2493F. doi:10.1088/0953-8984/16/26/023.
- Khan, TR; Grandin, HM; Mashaghi, A; Textor, M; Reimhult, E; Reviakine, I (2008). "Lipid redistribution in phosphatidylserine-containing vesicles adsorbing on titania". Biointerphases. 3 (2): FA90. doi:10.1116/1.2912098. PMID 20408675.