Double-helical structures for polyxanthylic acid

Polyxanthylic acid has been found to exist in two different duplex forms, A I and A II. a I, formed at pH5·7, occurs in a compact lattice with nearest neighbor molecules spaced at 2.11 nm. It has an axial translation per residue, h = 0·301 nm, and a rotation per residue, t = 36·0 °. The intensity distribution in its X-ray fiber diffraction pattern is analogous to that of A-RNA ( h = 0·281 nm, t = 32·7 °). On the other hand A II, formed at pH 8·0, has a less compact, statistically disordered crystal packing with nearest neighbors 2·35 nm apart. It has h = 0·252 nm and t = 32·7 ° and gives an X-ray intensity distribution essentially identical to A-DNA ( h = 0·256 nm, t = 32·7 °). Similar right-handed helical duplex models, with flexible C-3′- endo sugar rings, have been developed for each molecular structure. Both have purine purine base-pairs, possibly triply hydrogen-bonded, and certainly with the same symmetry as Watson-Crick pairs but with a 0·2 nm greater C-1′ … C-1′ separation.