The B–Z transition in two synthetic oligonucleotides: d(C-2-amino-ACGTG) and d(m5CGCAm5CGTGCG) studied by IR, NMR and CD spectroscopies

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.