Structural analysis of turkey tendon collagen upon removal of the inorganic phase

Calcified leg flexor tendons in which the inorganic phase content had been lowered by progressive demineralization were studied by small angle X-ray diffraction and thermogravimetry. The X-ray diffraction results agree very well with the data previously obtained on calcified turkey tendon indicating that the method used to decalcify tendons provides good correspondence with the process of calcification. Up to five thermal processes can be detected in the thermogravimetric scans: (1) water release; (2) collagen decomposition; (3 and 4) combustion of the residual organic components; (5) carbonate removal from the apatitic phase. The temperature of collagen decomposition decreases at lower inorganic phase content in agreement with the higher thermal stability of calcified collagen fibrils compared with uncalcified ones. The decrease of collagen thermal stability upon decalification is paralleled by a decrease of the structural order of the collagen fibrils as indicated by small angle X-ray diffraction data. Decalcification down to about 40% wt of inorganic phase does not significantly alter the inorganic blocks that are regularly arranged inside the gap zone of the collagen. Further removal of inorganic phase down to about 15% wt provokes a variation of the intensity distribution of the small angle meridional reflections that can be ascribed to a reduction of the mean height of the inorganic blocks. At inorganic phase contents below 15% wt the gap region is more free to contract upon air drying as a result of the reduction of the mean length of the inorganic blocks.