The synthetic DNA polymerpoly d(GC) . poly d(GC) has been studied by X-ray diffraction of orientated fibres in which it has either the well known (A or B) randed conformations have been observed with poly d(AC) . poly d(GT) and with poly d(As4T) . poly d(As4T), demonstrating that the novel structure is accessible to any DNA segment with an alternating purine-pyrimidine base sequence.

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A fibrous form of poly d(A):poly d(T) has a heteronomous secondary structure which is the first to be confirmed for a polynucleotide duplex: although both chains are 10(1) helices, mutually hydrogen-bonded in the standard (Watson-Crick) fashion, each has a quite different conformation. One chain -- probably poly d(A) -- has C3'-endo-puckered furanose rings characteristic of the A family of polynucleotide secondary structures while the other -- probably poly d(T) -- has the C2'-endo-puckered rings of the B family. Since analogous heteronomous structures could be assumed by DNA-DNA or DNA-RNA duplexes containing more general base sequences the polymorphic range of polynucleotide double-helices may be even greater than we have come to suppose.

X-ray-diffraction analysis of oriented, partially crystalline fibres of polyinosinic acid has resulted in a new molecular model. This model consists of four identical polynucleotide chains related to one another by a fourfold rotation axis. The coaxial helices are righthanded (screw symmetry 23(2)) and have an axial translation per residue h=0.341nm and a rotation per residue t=31.3 degrees . Incorporated in the model are standard bond lengths, bond angles and C-2-endo furanose rings. The nucleotide conformation angles, determined by linked-atom least-squares methods, are orthodox and the fit with the X-ray intensities is good. Each hypoxanthine base is linked to two others by hydrogen bonds involving O-6 and N-1. Further stability may arise from intrachain hydrogen bonds between each ribose hydroxyl group and the phosphate oxygen O-3. If guanine were to be substituted for hypoxanthine in an isogeometrical molecular structure, additional hydrogen bonds could be made between every N-2 and N-7.

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