In 4H NaCl solutions (dC-dG)n (n=3,4; ̃9mM) exist as a mixture ot B and Z forms. The low and high field components of two 31P NMR resonances originating from internal phosphodiester groups are assigned to the GpC and CpG linkages, respectively. Low temperatures stabilize the Z-forms, which completely disappear above 50°C (n=3) and 65°C (n=4). ΔH=-44 and -17 kJ/mol for B to Z transition in the hexamer and octamer duplexes, respectively. Temperature dependent changes (0-57°C range) in the spin-lattice relaxation times at 145.7 MHz are distinctly different for the 31p nuclei o± GpC and CpG groups. The relaxation data can be explained by assuming that the GpC phosphodiester groups undergo more local internal motion than do the CpG groups.
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