F NMR spectroscopy was used to investigate the conformational heterogeneity of two arylamine-modified DNA duplexes, d[CTTCTTG*ACCTC]‚d [GAGGTCAAGAAG], in which G* is either N-(deoxyguanosin-8-yl)-4′-fluoro-4-aminobiphenyl (dG-C8-FABP) (I) or N-(deoxyguanosin-8-yl)-7-fluoro-2-aminofluorene (dG-C8-FAF) (II). The 19 F NMR spectrum of I showed a single peak, while that of II revealed two prominent signals with a 55:45 ratio, in good agreement with previous 1 H NMR results (Cho et al. Biochemistry 1992, 31, 9587-9602;1994, 33, 1373-1384. Slow interconversion between the two conformations of II was established by temperature-dependent two-dimensional 19 F NMR chemical exchange spectra. On the basis of magnetic anisotropy effects and isotopic solvent-induced shifts, the 19 F signals at -117.31 and -118.09 ppm in the 19 F NMR spectrum of II were assigned to a relatively undisturbed "B-type" conformer and a highly perturbed "stacked" conformer, respectively. Analysis of the temperature dependent (5-40 °C) line shapes by computer simulation yielded an interconversion barrier (∆G q ) of 14.0 kcal/mol with a chemical exchange time of 2 ms at 30 °C. This new 19 F approach should be very useful in investigating the sequence-dependent conformational heterogeneity of arylamine-modified DNA.

No abstract

13C NMR Substituent chemical shift (SCS) increments have been determined for the carbonyl carbon of a variety of substituted benzaldehydes and acetophenones. The 13C NMR chemical shift of the carbonyl carbon can be predicted for many di- and trisubstituted benzaldehydes and acetophenones through simple additivity of the SCS increments. The magnitude and sign of the SCS increments have been explored using Hartree-Fock 6-31G* calculations to determine the natural atomic charges of the carbonyl carbon. When a substituent capable of intermolecular hydrogen bonding is present, deviations from additivity on the order of 2 ppm are observed in dilution experiments; deviations of up to 6 ppm can result from intramolecular hydrogen bonding.

For the quantitative analysis of N M R spectra, the relationship between the tip angle and the relaxation delay is examined. It is shown that the common procedure of using a small tip angle combined with a short delay is not the optimum. Instead, an angle of approximately 83" with a pulse-repetition period of 4.5 multiplied by the longest T, is the optimum procedure. However, the results obtained from this procedure are only slightly better than those obtained from the standard "90" -wait 5T," method. It is further shown that the 83" angle is independent of TI, but that the integral accuracy is relatively dependent on the estimate of TI.