Triacylglycerols affect the water content and cohesive strength of collagen fibrils

The function of lipids in collagen fibrils is not well understood. The types and amounts of lipids present in collagen fibrils are particularly unknown. Triacylglycerols (TAGs) are a major class of lipids found in tendons and connective tissue. In this study, we measured the proportion of TAGs in single collagen fibrils obtained from chicken calcaneal tendons and investigated how TAGs affect the mechanical properties of collagen fibrils. An analytical protocol using 3D depth profiling with atomic force microscopy (AFM) was developed to measure the proportion of TAGs in single collagen fibrils. We found that collagen fibrils from chicken tendons contain an unexpectedly large amount of TAGs, on average 9% of the dry fibril volume, and that TAGs act as plasticizers, softening the fibrils and reducing their water content. Spectroscopic data from Raman and ¹H NMR revealed the chemical identity of TAGs in collagen fibrils. We propose that TAGs are irregularly intercalated within the collagen crystal lattice and display liquid-like disorder. This challenges the current understanding of the chemical composition of native collagen fibrils. Furthermore, our findings demonstrate that structurally embedded lipids, such as TAGs, can modulate cohesive forces between proteins at the molecular level.