Raman spectroscopic elucidation of DNA backbone conformations for poly(dG‐dT) · poly(dA‐dC) and poly(dA‐dT) · poly(dA‐dT) in CsF solution

Raman spectra have been recorded for poly(dG‐dT) · poly(dA‐dC) and poly(dA‐dT) · poly(dA‐dT) in low salt and at high concentrations of CsF. Poly(dG‐dT) · poly(dA‐dC) shows no change in the 682‐cm−1 guanine mode, demonstrating the absence of the Z‐structure at high salt. The 790‐cm−1 phosphodiester symmetric stretch, however, shifts up 5 cm−1 in 4.3M CsF, suggesting a slight conformational change, associated with ion binding or hydration changes. Poly(dA‐dT) · poly(dA‐dT) shows an additional broad band at 816 cm−1, attributed to the phosphodiester modes associated with the C3′‐endo deoxyribose units in the alternating B‐structure. In this case, both the 841‐ and the 816‐cm−1 asymmetric phosphodiester stretches, associated with the C2′‐ and C3′‐endo units, shift down on addition of CsF in a sequential manner. Correlation of this sequence with that previously observed for the two 31P‐nmr resonances, establishes that the phosphodiester stretching frequencies depend on the conformation of the 5′‐sugar, and not on the 3′‐sugar.