HELICAL CONFIGURATION OF SINGLE-STRANDED DNA

Luzzati, Mathis, Masson & Witz (1964) have recently discovered an intermediate form of DNA which may be a single-stranded DNA in helical form. Its linear mass density is one nucleotide per 3;-4 Å and its radius of gyration, Rs, is 68% of that of the double helix form, Rd. Based on the current notions of the intermolecular forces and the solvent effect, we deduce the most stable configuration of a single-stranded DNA. The London forces between the adjacent bases and the hydrophobic bondings are assumed to be the dominant factors. Calculations are carried out with the help of a model of simple geometry, the bases being treated as identical square plates. The result is a single helix of 5.5 bases per turn, having a radius of gyration 72% of Rd, which is in good agreement with the experimental values. An alternative model using identical circular plates is also considered; it leads to similar results. Thus the choice of model is not crucial to our conclusions. © 1969.