br Multi hierarchical structure of collagen Protein structur
Multi-hierarchical structure of collagen Protein structure is the foundation of protein functions and properties. It is generally accepted that the protein has biological function only when it has tertiary structure or above. Among the identified 28 types of collagen, most is known about type I collagen. Evidence to date indicates that from the perspective of collagen's biosynthesis in vivo, type I collagen has been demonstrated as a natural polymer with a multi-layered hierarchical structure. With the interdisciplinary application of cell biology in collagen chemistry, researchers have conducted in-depth studies of the biosynthetic process of collagen in the body, which contains a large number of intracellular and extracellular physiological steps. The specific process is shown in Fig. 1 . Briefly, the procollagen alpha chain is first synthesized from the mRNA carrying the genetic information, and then transcribes the information required for encoding the collagen gene into the ribosome, and finally translates the collagen polypeptide chain corresponding amino retinoic acid receptor sequence on the ribosome. When the signal peptide at the collagen polypeptide enters into the lumen of the endoplasmic reticulum, it would be specifically located and removed by biological enzyme. At the moment, the procollagen alpha chain is mainly composed of five regions including an N-terminal propeptide, an N-terminal peptide, a triple helix region, a C-terminal peptide, and a C-terminal front. Afterwards, the procollagen alpha chain would be hydroxylated and glycosylated by the action of hydroxylase, galactose invertase and glucose invertase, resulting in a large amount of hydroxyproline of the alpha chain. Then the α chains of procollagen close to each other at the sites of spherical C-terminus polypeptides and further generate disulfide bonds to form a three-helix core. With the triple helix extending from the C-terminus to the N-terminus, the heterophasic trimeric procollagen molecules would be formed. Finally, the procollagen molecules are secreted to the outside of cells and transformed into tropocollagen molecules after the site-directed and restrictive cleavage of biological enzymes . However, tropocollagen molecule is not the final form of collagen in the body. Under physiological conditions, procollagen molecules continue to assemble and aggregate into supramolecular collagen aggregates recognized by a 65–67 nm axial periodicity on the basis of the so-called “1/4 staggered permutation rules”. Therefore, the basic molecular conformation of nature collagen include five hierarchical structures: primary structure (amino acid triplet), secondary structure (the α-helix), tertiary structure (triple helix) and quaternary structure (fibrils) and supermolecular aggregation structure .
Modification of collagen Collagen-based materials have been widely used in biomedical applications. But it also suffers from some poor physicochemical properties, such as thermostability, mechanical strength, resistance to enzymatic degradation . Therefore, the modification of collagen is necessary before use. There are many crosslinking methods of collagen including physical, chemical and biochemical modification. Besides blending other biomaterials with collagen to prepare collagen based composites could be also an effective modification method for collagen. Each crosslinking method has its advantages with its disadvantages at the same time. The methods of physical crosslinking for collagen are usually used in biomedical applications, such as ultraviolet (UV) radiation , dehydrathermal (DHT) , gamma (γ) irradiation  and so on. The physical crosslinking methods could improve the physical and mechanical properties without bringing in toxic or cross-linker residue compared to chemical crosslinking. However, collagen could be degraded by a long exposure to physical treatments. Meanwhile, physical treatments could not attain collagen materials with high strength and uniformity of crosslinking degree . Therefore, the properties of collagen based materials might be benefited from the synergistic effect between physical modification and other crosslinking methods .