Hydrogen interhelix

DNA, functional groups of nucleic acid bases are completely dehydrated and located inside the hydrophobic domain of duplex, forming efficient interhelix hydrogen bonds as shown in Figure 6. 

All of the unit-cell dimensions increase when converting from the "dry" form to the "hydrate". Thea plane expands equally in both directions and it is believed that the water of hydration is disposed between the triple helices in a columnar fashion (Figure 8). Interhelix hydrogen bonds are probably broken and/or replaced by new bonds involving water molecules. This mechanism could lead to the loss of three-fold symmetry. The three strands are no longer equivalent, possibly due to different hydroxymethyl group rotameric positions. 

When the a helices in wool are stretched, intrahelix hydrogen bonds are broken as are some of the interhelix disulhde bridges maximum stretching yields an extended P sheet structure. The Cys cross-links provide some resistance to stretch and help pull the a helices back to their original positions. In silk, the P sheets are already maximally stretched to form hydrogen bonds. Each P pleated sheet resists stretching, but since the contacts between the sheets primarily involve van der Waals forces, the sheets are somewhat flexible. 

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