Figure 2.

(a) Zoomed image of Fig. 1c (white frame). Individual mineralized fibrils are observed by Fourier filtering using the HAP (002) reflections only. Three individual alpha-chains entangling each other and forming a triple–helix are seen close to the white arrow. (b) The triple–helices form bundles, which are mineralized by apatite. The model beneath the experimental observed triple–helix demonstrates the idealized arrangement of the individual chains. The fine striation of 3.44 Å along the long axis of the triple–helices corresponds to the (002) reflection and thus the c–axis of HAP as indicated at the left bottom. (c) Bundle of mineralized triple–helices with the typical width of 1.5 nm of an individual fibril. (d) At thinner areas and at higher magnification (digitally zoomed) the winding of the alpha–chains becomes even more evident. Within the triple–helix (borders marked with white lines) the steep angle of the alpha–chain with respect to the long–axis and the spiral nature is revealed. (e) Comparison of predicted superstructure along the triple–helix of 2.86 nm (left) and supercoiling as read out from the TEM high–resolution (right). (f) Indication of winding period of alpha–chains. The periodicity along the triple–helix amounts to 0.86 nm and is evoked by the alpha–chain helix indicated on the model at the right.