Oriented linear dichroism to probe the organization of highly-ordered collagen biomaterials

Controlling the assembly of high-order structures is a central aim of soft-matter research. This is also crucial when working with biopolymers in the context of engineered biomaterials in view of medical applications. Here, we show how oriented linear dichroism (OLD) spectroscopy, coupled with other techniques, is used to probe the ordering of chiral collagen molecules in the dense state. Collagen triple helices were aligned by solvent evaporation, relying on the high aspect ratio of the triple helices (ca. 300 nm in length, 1.5 nm in diameter). The high-order packing of the helical structure in the dense state gives rise to a strong LD signal that changes sign and intensity with varying sample orientations with respect to the linear polarized beam. LD, which allows to identify orientation of samples, is complementary to circular dichroism (CD), which is routinely used to probe the structure of chiral (bio)molecules. LD allows to shift from the molecular to the supramolecular scale, and from the investigation of the conformation to interactions. In this work, supported by two-photon microscopy and X-ray scattering, we show that LD spectroscopy provides a straightforward route to probe the alignment of collagen triple-helices packed in the dense state, determine the packing density, and monitor the denaturation process. This approach could be readily adapted to any biological chiral molecule and assembly, as well as synthetic polymers, with a key advantage being its ability to detect large-scale assemblies with high specificity to both aligned and chiral molecules, and improved sensitivity compared to more conventional techniques.