Solid-state deuterium NMR is used to investigate perturbations of the local, internal dynamics in the EcoRI restriction binding site, -GAATTC- induced by cytidine methylation. Methylation of the cytidine base in this sequence is known to suppress hydrolysis by the EcoRI restriction enzyme. Previous solid-state deuterium NMR studies have detected large amplitude motions of the phosphate-sugar backbone at the AT-CG junction of the unmethylated DNA sequence. This study shows that methylation of the cytidine base in a CpG dinucleotide reduces the amplitudes of motions of the phosphate-sugar backbone. These observations suggest a direct link between suppression of the amplitudes of localized, internal motions of the sugar-phosphate backbone of the DNA and inhibition of restriction enzyme cleavage.

To systematically explore the effects of spin system size and geometry on the precision and accuracy of two-dimensional solid state NMR distance measurements, we have applied two homonuclear dipolar recoupling experiments, 2D DRAWS and 2D RFDR, to five polycrystalline samples of uniformly or selectively 13 Clabeled cytidine. Distance information has been obtained from the intensities and time behavior of crosspeaks observed in the resulting two-dimensional spectra. The experimental cross-peak buildup curves obtained from these crystalline cytidine samples have been analyzed by comparison with simulations. In uniformly 13 C-labeled cytidine, indirect coherence transfer mechanisms lead to low-precision distance measurements not unlike those measured in solution state NOESY experiments. In the selectively labeled cytidines, the distance measurements are considerably more precise, allowing the possibility of very accurate structure determinations from selectively or randomly labeled spin systems. Of the two techniques, the 2D DRAWS method allows identification of indirect coherence transfer mechanisms that hinder accurate distance measurement.

2,2'-Diiodobiphenyl (1) reacts with arenediazonium hexafluorophosphates (ArN(2)(+)PF(6)(-)) in acetonitrile to form biphenyleneiodonium salt (4) and iodoarenes (ArI) as major products. The reaction follows a free-radical-chain mechanism and involves a cyclic diaryliodine, 9-I-2 species (3), which is trapped as the iodonium salt (4) by a single-electron transfer (SET) to the diazonium salt. The results show that diaryliodine intermediates formed by the addition of phenyl radicals to iodoarenes can have sufficient lifetimes to allow trapping by bimolecular processes.

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