The secondary structures of double-stranded poly[d(A-T)].poly[d(A-T)] in films have been studied by IR spectroscopy with three different counterions (Na+, Cs+, and Ni2+) and a wide variety of water content conditions (relative humidity between 100 and 47%). In addition to the A-, B-, C-, and D-form spectra, a new IR spectrum has been obtained in the presence of nickel ions. The IR spectra of Ni2+-poly[d(A-T)].poly[d(A-T)] films are analyzed by comparison with previously assigned IR spectra of left-handed poly[d(G-C)].poly[d(G-C)] and poly[d(A-C)].poly[d(G-T)], and it is possible to conclude that they reflect a Z-type structure for poly[d(A-T)].poly[d(A-T)]. The Z conformation has been favored by the high polynucleotide concentration, by the low water content of the films, and by specific interactions of the transition metal ions with the purine bases stabilized in a syn conformation. A structuration of the water hydration molecules around the double-stranded Ni2+-poly[d(A-T)].poly[d(A-T)] is shown by the presence of a strong sharp water band at 1615 cm-1.

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Infrared spectroscopic studies demonstrate the ability of poly[d(A-C)].poly[d(G-T)] to adopt a Z-type conformation. The Z form of the unmodified polynucleotide is induced by Ni2+ counterions and not by Na+. The B----Z equilibrium is shifted at room temperature, in the presence of 1 Ni2+/nucleotide, by an increase in the concentration of poly[d(A-C)].poly[d(G-T)]. The importance of specific binding of Ni2+ ions on the N7 site of purines in the stabilization of the Z form is also discussed.

The sequences CA'CGTG (where A' = 2-aminodeoxyadenosine) and m5CGCAm5CGTGCG are prepared and studied by IR, CD and 1H-NMR. Infrared spectra demonstrate the capacity of the modified hexamer and decamer to adopt a Z conformation. The influence of the NH2 substitution on the adenine or of the methylated terminal part of the decamer acting with the increase of the DNA concentration stabilizes the Z conformation at room temperature in low humidity films. Very weak proportion of Z conformation is detected in UV dilute solutions. In more concentrated NMR solutions, the Z proportion induced by high salt content is only 20-25%. The effects of the concentration and of the covalent modification of the bases are discussed.