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Ring current shielding effects in nucleic acid double helices.

Values of ring current shielding in parts-per-million have been calculated for double helical nucleic acids in the A-RNA (RNA-11), A' -RNA (RNA-12) and B-DNA geometries. Atomic coordinates determined previously from x-ray diffraction of fibers were used to calculate the positions of protons relative to both nearest and second nearest neighboring bases, including those on the complementary strand. The magnitude of the diamagnetic shielding was then calculated for each aromatic ring. From these calculations tables were constructed for use in determining the shielding expected for carbon-bound and ring nitrogen-bound protons of any double helical nucleic acid sequence. The results are compared with available experimental data for several oligonucleotides and with previous ring current shielding calculations where differences of up to 0.4 ppm are found.