Quarter-stagger

Assembly of collagen fibers in quarter-staggered alignment... 

Polyalanine PABPN1 Quarter-staggered /1-sheet Shinchuk et al. (2005)... 

Refinement of the four quarter staggered models led to the following residuals Rp1=0.255, Rp2 = 0.247, Rai=0.254 and Ra2=0.294. All of these values are relatively close. It is significant however that model pj is very close to that refined by x-ray work, whereas the other three differ significantly from their x-ray counterparts. The agreement for the meridional reflections is unacceptable for both the a and a2 models in particular a strong 002 intensity is predicted, compared to the very weak observed reflection. In addition, the two chains in the unit cell have different orientations about their axes such... 

Figure 2.22. Cross-linking of collagen molecules in quarter-stagger packing pattern of collagen in fibrils. Each molecule is 4.4-d long (where D is 67 nm) and is staggered by D with respect to its nearest neighbors. A hole region of 0.6 D occurs between the head (circles) of one molecule and the tail of the preceding molecule (arrowheads).

Figure 3.25. Diagram illustrating the relationship between quarter-staggered collagen molecules in the microfibril (top) and the flexible and rigid regions of the amine acid sequence (bottom).The D period is one repeat containing 4 molecules (hole) followed by 5 molecules in cross-section (overlap). Mineral is deposited in the hole region in bone.

We think that the structural principles that imderlie quasicrystals (discussed in Chapter 2) are relevant in order to describe the superstructure of tropocollagen molecules, which must accommodate the wide variety of tissue structures that can be formed. There are no possibilities of packing triple helices in a space-filling arrangement. A structure with five-fold symmetry in the plane and with perfect periodicity in the perpendicular direction, however, which is consistent with the quarter stagger pentafibril model, is in fact a quasicrystal of so-called T-type (also known as the decagonal phase). 

Ja,b Skin, bone, tendon, dentin Most abundant, banded quarter-staggered fibrils 7,17... 

It should be added that the two films analysed are especially cross-linked and do not contain the "quarter stagger" shift and that the film formed at 50% no longer retain the triple helix structure. 

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