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.

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.

An evaluation of the relationship between signal‐to‐noise ratio (S/N) and integral accuracy, as a function of tip angle and relaxation delay, shows that the SIN decreases with increasing integral accuracy. At the expense of a slight decrease in integral accuracy, a substantial increase in SIN can be realized, and a procedure is presented for selecting the appropriate tip angle and relaxation delay.

C13 chemical shifts of the carbonyl group of acetic acid were determined in the solvents water, acetone, chloroform, and cyclohexane over the range of volume fractions of acetic acid from 1.0 to about Relatively large dilution effects on the C13 shifts were observed with acetone as solvent, a smaller effect with chloroform, and only small effects in water or cyclohexane as solvents. A curious hump was found in the water dilution curve. These results are understandable in terms of the influences of making and/or breaking of hydrogen bonds between the carboxyl group and various species.he structural identity of the species present when

No abstract

All probes in NMR spectrometers must be “tuned” and “matched” to obtain optimum sensitivity. In the simplest electronic circuits, tuning and matching are accomplished by adjusting variable capacitors. These adjustments determine the impedance of the circuit; i.e., the extent to which the circuit opposes, or impedes alternating‐current flow. This impedance is the vector sum of the individual oppositions owing to the presence of resistors, inductors and capacitors in the circuit. When the impedance of the probe circuitry is equal to (matches) that of the high‐power, pulsed, radio‐frequency transmitter, power is efficiently transferred to the probe and pulse‐widths are minimized. Similarly, when the impedance of the probe circuitry matches that of the small‐signal preamplifier, the detected signal power is efficiently transferred to the amplifier and the signal‐to‐noise ratio is maximized.

No abstract

A referee has suggested an alternative interpretation for the MCE results on CuOEP. This is that the peak at 6912 Á is from the trip-doublet (2T,) designated 2EU In our text while the 6820 A signal arises from the second trip-doublet (2T2) arising from coupling with the B state triplet and thought also to be in this energy region.12 Field-Induced mixing of these states might show similar effects to those observed. We do not pursue this argument here but hope to clarify this question in a future publication describing MCE results on CuOEP at temperatures below 4.2 K