A study of the thermal stability of the Z form of (m5dC–dG)3by resonance Raman spectroscopy

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Abstract
The thermal stability of the hexanucleoside pentaphosphate m5dCpdGpm5dCpdG has been studied by resonance Raman spectroscopy with 257 nm excitation wavelength. At low temperature and in 3M NaC104, the Raman spectrum resembles that of poly (dG–dC).poly(dG-dC) in the Z conformation. As the temperature is increased, the position and the intensity of several bands (1312 cm−1, 1482 cm−1, 1584 cm−1 and 1632 cm−1) are modified. The variation of intensity versus temperature is biphasic. Analysis of the results suggests that the increase of temperature induces first a transition from the Z form to an intermediate stable form which then melts. These results and those previously obtained by circular dichroism and 31p nuclear magnetic resonance suggest that the intermediate form belongs to the left family but with changes in the stacking of the bases and the geometry of the phosphate groups as compared to the canonical Z form.