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In biochemistry, a conformational change is a change in the shape of a macromolecule, often induced by environmental factors.
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. Factors that may induce such changes include temperature, pH, voltage, light in chromophores, ion concentration, phosphorylation, or the binding of a ligand.Transitions between these states occur on a variety of length scales (tenths of Å to nm) and time scales (ns to s),
and have been linked to functionally relevant phenomena such as allosteric signaling and enzyme catalysis.
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.
Another method applies electro-switchable biosurfaces where proteins are placed on top of short DNA molecules which are then dragged through a buffer solution by application of alternating electrical potentials. By measuring their speed which ultimately depends on their hydrodynamic friction, conformational changes can be visualized.
^Salafsky, Joshua S.; Cohen, Bruce (2008). "A Second-Harmonic-Active Unnatural Amino Acid as a Structural Probe of Biomolecules on Surfaces". Journal of Physical Chemistry. 112 (47): 15103-15107. doi:10.1021/jp803703m. PMID18928314.