Helical twist sequence dependence

In addition to large B-form structural variations, [Fe(EDTA)] has been used to probe for subtle changes in the sequence-dependent helical twist of DNA (64). As can be seen in Fig. 10, by depositing the rodlike DNA on an inert surface (in this case calcium phosphate crystals), one can protect an entire face of the helical polymer. Exposed backbone on the other side is then cleaved with a periodicity that reflects the number of base pairs per turn of the helix. Experiments with [Fe(EDTA)] have therefore helped to illustrate that there are sequence-dependent deviations in the number of bases that can pack into a helical turn. 

Bella J (2010). A new method for describing the helical conformation of collagen Dependence of the triple helical twist on amino acid sequence./Struct. Biol, 170,377-391. 

It could also be decisively important to observe the behavior of helices with at least one of the alternative possible amounts of twist per helical coil (helices with integer J 0 [30, Appendix the only helices examined in the literature thus far have J = 0]. Helices pass through combinations of curvature and twist in a different sequence for each J, thus revealing whether filament behavior is indeed some sought-for instantaneous function of filament geometry, or is also history-dependent (perhaps because the shape of the concentration gradients takes time to adjust as the filament bends). 

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