The slope of the line relating the distances at initial contact D C and the number of neck repeats gives a value of 3.87 ( 0.23 nm/ repeat (N = 22) ( Figure 3B ), which matches almost exactly the length of 3.7 nm for a single repeat determined from the crystallographic analysis of the isolated neck region. 17 A similar Biochemistry ARTICLE plot of D eq versus the number of repeats is shifted slightly, but the calculated slope is 3.6 ( 1.4 (N = 22). The larger error in the latter reflects the greater uncertainty in the estimated value of D eq , as discussed above. However, the comparative length measurements presented here provide exceptionally important direct experimental evidence that the neck repeat structure, which is observed essentially as a fragment out of context in the crystal structure, is an accurate reflection of the conformation of the neck in the intact molecule.