Dual Polarisation Interferometry: An Optical Technique to Measure the Orientation and Structure of Proteins at the Solid-Liquid Interface in Real Time

The study of the orientation, structure, and function of proteins at interfaces in real time is demanding and complex in nature. For the last 40 years the primary means for the study of protein structure has been x-ray crystallography. This is, of course, a solid-state technique and not real time in nature. Over the last 20 years several techniques to address, at least in part, this important area have been adopted. These range from biosensors, which measure the rates of change of processes at interfaces, to analytical techniques such as neutron reflection, which are capable of providing detailed information on (protein) layer structures at interfaces. In this article a new analytical technique is described, dual polarisation interferometry (DPI), which combines the analytical nature of neutron reflection techniques with the real-time, bench-top accessibility associated with biosensors. Several examples of the measurement of protein orientation, structure and function are presented to demonstrate the value of the technique. A description of the fundamental physical principles of the measurement and verification of the technique are also provided.