The high salt form of poly(dG-dC)·poly(dG-dC) is left-handed Z-DNA: Raman spectra of crystals and solutions

The laser-Raman spectra of crystalline d(CpGpCpGpCpG) and of aqueous poly(dG-dC)·poly(dG-dC) in high salt (4M NaCl) and low salt (0.lM NaCl) solutions have been measured and compared. The spectra of the crystal and the high-salt solution show a striking congruence, which indicates clearly that the high-salt form of the aqueous polymer has the left-handed Z-DNA structure of the crystalline oligomer. These two spectra differ substantially from that of the low-salt form of the polymer, which has been found previously to have spectral characteristics of the B-form of DNA. The high salt spectrum shows a unique line due to guanine residues at 625 cm^-1 which should be useful for qualitative and possibly quantitative assessment of the amount of Z-structure present in a sample of DNA.