A macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change. A macromolecular conformational change may be induced by many factors such as a change in temperature, pH, voltage, ion concentration, or the binding of a ligand.
A specific nonlinear optical technique called second-harmonic generation (SHG) has been recently applied to the study of conformational change in proteins. In this method, a second-harmonic-active probe is placed at a site that undergoes motion in the protein by mutagenesis or non-site-specific attachment, and the protein is adsorbed or specifically immobilized to a surface. A change in protein conformation produces a change in the net orientation of the dye relative to the surface plane and therefore the intensity of the second harmonic beam. In a protein sample with a well defined orientation, the tilt angle of the probe can be quantitatively determined, in real space and real time. Second-harmonic-active unnatural amino acids can also be used as probes.
- Book Chapter on Protein Motions from website of RH Austin  at Princeton University
- Frauenfelder, H. New looks at protein motions Nature 338, 623 - 624 (20 April 1989).
- Detection of Protein Conformational Change by Optical Second-Harmonic Generation
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