Nuclear spectral line broadening

A major limitation in using protein NMR spectroscopy in drug discovery has been the molecular weight limitation imposed by nuclear spin relaxation (line broadening) and increased spectral complexity associated with macromolecules larger than 35 kDa [5]. The most recent developments in NMR spectroscopy aimed at overcoming these problems will be briefly reviewed in Sect. 21.2. 

The analysis of a full tilt series of 2H NMR spectra not only allows the determination of the unique bond angle for a deuteriated methyl group, but also provides an internal check for the consistency of the spectral interpretation. In particular, simulations provide a means for the analysis of line-broadening effects, which arise from the sample mosaic spread as well as the intrinsic line width of the nuclear transition and instrumental factors. When line shapes are fitted to a full tilt series of spectra in a concerted manner and are also compared with the powder spectrum of an unoriented sample, the different contributions can be discerned. In that way an intrinsic line width of around 2 kHz is found for the spectra shown here, together with a mosaic spread between 8° and 10° for the three samples. 

NMR. Quantitative liquid-state carbon-13 nuclear magnetic resonance ( 3c NMR) spectra were recorded for humic and fulvic acid from Como Creek foam and for stream and foam fulvic- and humic- acid samples from the Suwannee River at the U.S. Geological Survey, laboratory in Arvada, CO. C NMR could not be performed on other humic substances due to insufficient sample or instrument availability. The acquisition parameters used were as follows C NMR spectra were recorded on a Varian XL-300 NMR spectrometer at 75 MHz. Each sample (200 mg of freeze-dried material) was dissolved in deuterated water and deuterated sodium hydroxide was added to ensure solution a total solution volume of approximately 6 to 7 mL. Spectra were recorded using a 30,000 Hz spectral window, a 45 pulse width, a 0.199 second acquisition time, and a pulse delay of 10 seconds for quantitative spectra. The number of transients was 10,000, and line broadening was 50 Hz. 

When the line broadening becomes a serious obstacle, one can take advantage of a phenomenon that results in narrower lines. Specifically, substitution of1H atoms with the 2H isotope coupled with 2H NMR spectroscopy results in spectral lines that are up to 40 times narrower than those of corresponding ll NMR, due to slower nuclear relaxation of the 2H nuclei in paramagnetic compounds. 

Fig. 2.6. Solution P nuclear magnetic resonance spectra of a single experiment, showing the effect of line broadening on spectral resolution and the signal-to-noise ratio.

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