The application of polyelectrolyte limiting laws to the helix–coil transition of DNA. VI. The numerical value of the axial phosphate spacing for the coil form

It is concluded on the basis of comparison of polyelectrolyte theory with published data that the mean phosphate spacing b along the contour axis of an unfolded polynucleotide single strand is in the range 3–4 Å (polyelectrolyte parameter ξ ≈ 2), regardless of temperature, base composition, or extent of stacking. This result is consistent with the low‐angle X‐ray scattering measurements of Gulik, Inoue, and Luzzati on poly(C). No conclusion may be drawn from this value of b concerning the structure of the chain skeleton or the spatial arrangement of the bases other than that the chain is far from an all‐ trans local conformation (for which b would be about 6–7 Å, the length of a nucleotide unit). The structural implications, or lack thereof, are discussed in detail in the following paper.