Second order hyperpolarizability of the collagen triple helix: Measurement and determination of its physical origin

Collagen is the major protein of the extracellular matrix and plays a central role in the formation of fibrillar and microfibrillar networks, basement membranes, as well as other structures of the connective tissue. As a fundamental brick of the architecture of tissues, it guarantees organs functioning and is crucial in the adaptative response to various tissue injuries. This protein is characterized by triple helical domains and possesses remarkable non linear optical properties. Indeed, collagen fibers exhibit efficient Second Harmonic Generation (SHG) in tissues and SHG microscopy has proved to be a valuable technique to probe the three-dimensional architecture of fibrillar collagen in native and biomimetic tissues and to assess the progression of fibrotic pathologies. However, the nonlinear optical response of fibrillar collagen is not fully characterized yet and quantitative data are required to further process SHG images. We therefore performed Hyper-Rayleigh Scattering (HRS) experiments in order to measure quantitatively the nonlinear optical response of the collagen molecule, and to get insight into the physical origin of high SHG signals observed for fibrillar collagen in tissues.