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Secondary structures of poly(dG).poly(dC) and poly(dG).poly(dm5C) in solution are determined by nuclear Overhauser effect (NOE) measurements on GH8-deuterated and -nondeuterated DNAs with low presaturation pulse lengths (10-25 ms) and low-power and prolonged accumulations in the range of 50,000-72,000 scans. Under these conditions, the NOE difference spectra were free from diffusion. Primary NOEs between base protons GH8/CH6 and sugar protons H1', H2'/H2'', and H3' suggest that in poly(dG).poly(dC) both guanine and cytosine nucleotides adopt a C3'-endo, low anti X = 200-220 degrees conformation. Computer modeling of the NOE data enable identification for the first time, in terms of the geometry of the nucleotide repeat, handedness, and helix geometry, of the structure of poly(dG).poly(dC) to be the A form, and the derived structure for the polymer duplex is very close to the single crystal structure of the double-helical d-GGGGCCCC [McCall, M., Brown, T.,Kennard, O. (1985) J. Mol. Biol. 183, 385-396]. Similar nuclear Overhauser effect data on poly(dG).poly(dm5C) revealed that G and m5C adopt a C2'endo, anti X = 240-260 degrees conformation, which indicates that this DNA exhibits the B form in solution. In summary, the results presented in this paper demonstrate that methylation of cytosines in poly(dG).poly(dC) causes A----B transition in the molecule. |
