| One- and two-dimensional 19F NMR spectroscopy have been 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 19F 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 1H NMR results. Slow interconversion between the two conformations of II was established by temperature-dependent two-dimensional 19F NMR chemical exchange spectra. On the basis of magnetic anisotropy effects and isotopic solvent-induced shifts, the 19F signals at -117.31 and -118.09 ppm in the 19F 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 (DeltaG) of 14.0 kcal/mol with a chemical exchange time of 2 ms at 30°C. This new 19F approach will be very useful in investigating the sequence-dependent conformational heterogeneity of arylamine-modified DNA.;It has previously been believed that whenever a window function is employed to treat a free induction decay (FID) an improvement in the signal-to-noise ratio (S/N) is always in conflict with an improvement in resolution. According to this view, a spectroscopist must compromise resolution to achieve better S/N. Furthermore, it has been assumed that it is impossible to treat noise differently than signal when using window functions. In this study it will be shown that a process that uses window functions can not only treat noise differently than signal, but it can also produce an improvement in S/ N with no change in resolution. It will also be mathematically shown that the TRAF window function is indeed the optimum window function for sensitivity enhancement of NMR signals. |
