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  • FTIR microspectroscopic analysis: future perspectives. Bone mineral

Bone mineral

Bone mineral is a poorly crystalline hydroxyapatite [Ca5(PO4)3OH] phase. Ion substitutions are abundant. For example, Na+1, and Mg+2 are substituting Ca+2 ions, HPO4-2 ions substituting the phosphate ions, Cl-1 and F-1 substituting OH-1, and CO3-2 sub­stituting for either phosphate or hydroxyl groups. Once mineral is deposited in bone by osteoblasts, it is not a static moiety, but rather a dynamic one. Since it is bathed in aqueous biological fluids, the type and extent of these substitutions changes with time resulting in alterations of the mineral maturity, which is ac­companied by changes in mineral crystallite size and /or shape.

The contribution of mineral maturity, and crystallite size and shape to bone strength is very apparent in the case of fluoride treated bone.

In both animal models and in humans it has been reported that osteoporotic bone mineral characteristically consists of crystals which are larger and more perfect than in normal bone, smaller and less perfect, or that there are no differ­ences. Typically in these studies, tissues were homoge­nized prior to analysis, concealing the effect of spatial varia­tions in mineral properties. Recently, utilizing techniques such as Small Angle X-ray Scattering (SAXS), and quantitative backscattered electron imaging (qBEI), the analysis of bone mineral (poorly crystalline hydroxyapatite) at the microscopic level and the contribution of mineral crystallinity (crystallite size) and maturity (chemical composition) to bone strength is being actively pursued. Based on such stud­ies, models for the importance of mineral crystallite shape and size in determining bone strength have been put forth.

Bone collagen & collagen cross-links

The organic matrix of bone consists of collagen and a series of non-collagenous proteins and lipids. Some 85%-90% of the to­tal bone protein consists of collagen fibers. Type I colla­gen, the principal component of the organic matrix of bone, as well as other connective tissues, is a large fibrous protein with a highly repetitive amino acid sequence [Gly (glycine) – X – Y]n (often X is proline and Y is hydroxyproline). This repeti­tive sequence allows three polypeptide chains (called a chains; type I collagen is composed of two a1 and one a2 chains) to fold into a unique triple-helical structure. It consists of three do­mains: the —NH2 terminal nontriple helical, the triple helical, and the —COOH terminal nontriple helical domains. The single uninterrupted triple helical domain represents more than 95% of the molecule.

The most distinct feature of type I collagen in mineralized tis­sues can be seen in its cross-linking chemistry and molecular packing structure. The intermolecular cross-linking pro­vides the fibrillar matrices with various mechanical properties such as tensile strength and viscoelasticity. All the known cross-links of type I collagen are condensation products be­tween the prosthetic groups of juxtaposed specific peptidyl residues of lysine (Lys), hydroxylysine (Hyl), and histidine (His). At present, seven major collagen cross-links have been established as naturally occurring intermolecular cross-links. They are dehydrodihydroxylysinonorleucine (deH-DHLNL), dehydrohydroxylysinonorleucine (deH-HLNL), dehydro- histidinohydroxymerodesmosine (deH-HHMD), pyridinoline (Pyr), deoxypyridinoline (d-Pyr; lysyl analog of Pyr), pyr­role, and histidinohydroxylysinonorleucine (HHL). The first three are NaBH4-reducible (their reduced forms are referred to as DHLNL, HLNL, and HHMD, respectively) and the rest are non-reducible compounds.

Altered collagen structure and inferior bone mechanical proper­ties are encountered in the case of osteogenesis imperfecta, in both humans and animal models. The importance of collagen intermolecular cross-links to the mechanical perfor­mance of bone is also very apparent in the pyridoxine deficient chick animal model, as well as in lathyrism.
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