"from single atom to bulk structure with second harmonic generation Microscopy"

Understanding of complex biological processes requires knowledge of molecular structures and measurement of their dynamics in vivo. Several techniques must be used to encompass the spatial and temporal scales involved in such processes. In fact, protein crystallography provides static structures with atomic resolution, while complementary techniques are needed to follow structural dynamics in the cellular context. The cooperative chemo-mechanical action of myosin molecules in complex sarcomeric muscle structures represents a paradigmatic example in this respect. Here we describe a label-free imaging method for measuring protein structural dynamics in vivo. We employed the order-based contrast enhancement by second-harmonic generation (SHG) for the functional imaging of muscle cells in different physiological states. Identification of the hyper-Rayleigh scatterer (HRS) in the peptide bond and modeling of the full acto-myosin array allow the characterization of polarization-sensitive SHG in terms of myosin conformation.