Molecular-dynamics simulations reveal internal tension in native-state collagen fibrils

Collagen fibrils are the building block of many biological tissues, the viability of which depends on the fibrils' properties. Altered properties of collagen fibrils are central to the appearance of many diseases, while physiological or native properties must be reproduced for tissue engineering. However, the self-assembly, the structure, and therefore the properties of collagen fibrils remain elusive. One main reason is the extreme sensitivity of the fibrils to their environmental conditions, and, in particular, hydration, which is only loosely bound by experimental measurements. Furthermore, mechanics are an integral part of the self-assembly process; forces exerted by cells or osmotic pressure may result in internal stresses in collagen fibrils in native conditions. Here, we propose to investigate internal stresses in collagen fibrils by means of molecular-dynamics simulations of the collagen microfibril model. Our simulations reveal the quantitative evolution of internal stresses in collagen fibrils with hydration. We establish a value of native hydration of collagen fibrils at 0.78 g/g based on an absence of cross-sectional stresses. In turn, we determine a quantitative estimate of internal longitudinal stresses in collagen fibrils in native conditions of 210 MPa. We find that internal longitudinal stresses are caused by an over-extended protein backbone rather than partial hydration, which appears to be a remnant of the local forces driving collagen self-assembly. We also demonstrate the consequences of internal longitudinal stresses on the mechanical properties of collagen fibrils, the absence of which induces more than a 20% decrease in the Young's modulus. Overall, our findings provide insights into the native structure and properties of collagen fibrils. More than ever, collagen fibrils appear to be assembled via an out-of-equilibrium process key to the synthesis of viable tissues.