B-Z transition of poly(dG-m5dC).poly(dG-m5dC) in the presence of basic oligopeptides

The effect of basic oligopeptides (Lys-Ala-Ala)n and (Lys-Leu-Ala)n (n = 1-4) on the B-Z transition of poly(dG-m5dC).poly(dG-m5dC) in aqueous solution and in methanol-water mixtures was investigated by c.d. spectroscopy. In aqueous solution dimers and higher oligomers induce a transition to the Z conformation. These temperature dependent transitions are consistent with positive delta HB-Z values which depend on peptide composition. In the absence of peptides no transition can be observed. In the presence of peptides B-Z transition can be induced by small amount of methanol. The temperature dependence of this transition is consistent with a small, but definitely negative delta HB-Z. At high methanol concentration a transition to Z' type conformation was observed. In subcritical methanol concentrations B-Z transition can be induced by the addition of peptides. In this case the delta HB-Z values are again very small, but definitely positive. The effects of bulky hydrophobic peptide side chains, of the presence of methanol and the temperature dependences are consistent with an important contribution of hydrophobic interactions in maintaining the stability of the Z DNA-peptide complex.