Detection of protein conformational change by optical second-harmonic generation

An ability to detect and track conformational change in real time is essential to understanding the dynamic relationship between structure and function in biological molecules. Here I show that second-harmonic generation (SHG), a surface-selective technique, offers a new means to probe structural dynamics. A protein, calmodulin, was labeled with a second-harmonic-active dye and immobilized to a surface. Although neither the labeling nor the immobilization was done in a site-specific manner, an overall net orientation of the labels was produced relative to the surface plane. Conformational change of the protein induced by calcium alters the average tilt angle of the labels and causes a change in the intensity of second-harmonic radiation generated by the surface. As SHG is spatially and temporally coherent and depends quantitatively on the structural details of a surface, the method described here should serve as the starting point for more detailed studies of the mechanism of conformational change in molecules, as well as related topics such as protein folding. SHG's surface selectivity also suggests its use in a range of biological applications which require detection of surface-bound molecules.