Naturally Occurring Polymer Biomaterials:
Naturally occurring polymers are used as biomaterials largely because their structures are similar to the human tissue they intend to replace. They are also available cheaply and easily in large quantities. Usually, the naturally occurring biomaterials can be degraded by naturally occurring enzymes and therefore they are biodegradable, which offers an additional advantage over the use of synthetic non-biodegradable polymers. However, the use of naturally occurring polymers often has the problem to provoke immune reaction of the host tissue. Therefore, many of the naturally occurring polymers have to be chemically modified before they are used as biomaterials.
1- General Introduction to Proteins:
Proteins are monodisperse polymers of amino acids. They are essential components of plants and animals. There are twenty different ?-amino acids, which can join
together by peptide linkages to form polyamides or polypeptides. Polypeptides are often used by biologists to denote oligomers or relatively low molecular weight proteins. All ?-amino acids found in proteins, except glycine (Gly), contain a chiral carbon atom and are L-amino acids.
Because these amino acids can be ionized, they are water-soluble polar compounds, which migrate toward an electrode at pH values other than that of the isoelectric point in a process called electrophoresis.
2- Collagen:
Collagen, the most abundant protein in mammalian tissues, accounts for up to one-third of all protein mass in a mammal. Collagen fibers form the matrix or cement material in human bones where bone mineral precipitate. Collagen fibers constitute a major part of tendons and act as a major part of skin. The main function of collagen is the mechanical reinforcement of the connective tissues of vertebrates .
The individual polypeptide chains of collagen contain 20 different amino acids and the precise composition varies between different tissues. The variation in specific amino acid sequence gives rise to the different types of collagen labeled as Type I , Type II up to Type XIX. The most commonly occurring collagens are Types I , and M, which form the long-recognized characteristic fiber bundles seen in many tissues. Type I collagen is mostly found in skin, tendon, and bone, and Type HI in blood vessels. The various collagen types show differences in degrees of glycosylation, which means that glucose and galactose are covalently coupled to the collagen molecules .
The lysine (Lys) and proline (Pro) residues present in the collagen are partly hydroxylated yielding the rare amino acids hydroxyproline (Hyp) and hydroxy lysine (Hyl), respectively. The name collagen is used as a generic term to cover a wide range
of protein molecules, which form supramolecular matrix structures. The basic building block of collagen is a triple helix of three polypeptide chains called the tropocollagen unit. Each chain is about 1,000 amino acid residues long. These three individual a-chains are cross-linked biosynthetically and fold to form a triple helix (tertiary structure) with a molecular weight of approximately 300.000 g/mol, a length of approximately 300 nm and a diameter of 1. 5 nm . This triple-helix generates a symmetrical pattern of three left-handed helical a-chains (secondary structure), forming an additional "supercoil" with a pitch of 86 ?. The amino acids within each chain are displaced by a distance of 2.91 ?, with a relative twist of -110°, making the number of residues per turn 3.27 and the distance between each third glycine 8.7 ? (See Fig. 1).
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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