Hydroxyapatite Adsorbed at the Surface of the Byssus Fibers of Pinna Nobilis

Mussel-forming fibers manifest fascinating properties due to the numerous reactive functional groups present in their primary structure besides their secondary structures. Humans adopted these protein-based fibers to produce an extremely fine and valuable fabric, named byssus. Byssus is the bundle of filaments secreted by Pinna nobilis species employed to colonize the marine substrates. The byssus fibers coming from other species, such as Mytilus edulis , have been widely investigated in particular for their self-healing properties attributed to the collagen presence and their core-shell structure. Surprisingly, the Pinna nobilis byssus is non-collagenous based and the fibers nanostructure is still poorly understood. Here, we are reporting an investigation on the Pinna nobilis byssus samples retrieved intact in each stage of treatment. The scanning electron microscope (SEM) provides a peculiar feature of Pinna nobilis byssus, i.e. the elliptical cross-section of the fibers. Wide-angle X-ray scattering (WAXS), as well as infrared spectroscopy (IR), reveal the presence of hydroxyapatite adsorbed at the surface of the initial fibers and the presence of crystalline beta-sheet domains within the fibers. Finally, a WAXS-based method reveals a decrease in the crystalline/amorphous ratio during byssus treatment.