One-dimensional NMR spectra

It is usual to plot a normal (one-dimensional) NMR spectrum along each of the axes to give referenee speetra for the peaks that appear in the two-dimensional spectrum. 

The TOCSY spectrum is symmetrical about the diagonal and has the H NMR spectrum of the substance as both of the ehemical shift axes (Fi and F2). A sehematic representation of TOCSY spectrum is given below. Again, it is usual to plot a normal (one-dimensional) NMR spectrum along each of the axes to give reference spectra for the peaks that appear in the two-dimensional spectrum. 

H is particularly important in NMR experiments because of its high sensitivity and natural abundance. For macromolecules, 1H NMR spectra can become quite complicated. Even a small protein has hundreds of 1H atoms, typically resulting in a one-dimensional NMR spectrum too complex for analysis. Structural analysis of proteins became possible with the advent of two-dimensional NMR techniques (Fig. 3). These methods allow measurement of distance-dependent coupling of nuclear spins in nearby atoms through space (the nuclear Overhauser effect (NOE), in a method dubbed NOESY) or the coupling of nuclear spins in atoms connected by covalent bonds (total correlation spectroscopy, or TOCSY). 

FIGURE 2 A one-dimensional NMR spectrum of a globin from a marine blood worm. This protein and sperm whale myoglobin are very close structural analogs, belonging to the same protein structural family and sharing an oxygen-transport function. 

Figure 9.1 represents a one-dimensional NMR spectrum (the intensities of the peaks are not considered to be a second dimension). More sophisticated NMR studies, in two, three or four dimensions can be used to determine the position of ail the atoms present in a molecule. This chapter only deals with one-dimensional (1 -D) NMR. 

NMR spectrum The one-dimensional NMR spectrum of buclizine dissolved in CDCI3, which was recorded at 24 °C and internally referenced fo TMS. The NMR assignmenfs are presenfed in... 

Figure 15.15 COSY spectrum for ethanol. Frequencies are expressed in terms of chemical shilis 5. The corresponding one-dimensional-NMR spectrum is shown at the top.

A one-dimensional NMR spectrum provides a wealth of information, which for simple molecules may yield enough detail for complete structural characterization. The spectrum of a 5 mg sample of ibuprofen shown in Fig. 2 was taken in less than 1 min. Given the relative simplicity of this molecule, all of the resonances... 

Figure 5-51 (A) The low-field region of the one-dimensional NMR spectrum of E. coli tRNAj at 27°C in H2O. Resonances are identified by letters A - X. (B) NOESY spectrum of the same tRNA under similar conditions showing the imino-imino NOEs. in the lower right sector the connectivity traces of the acceptor helix and dihydrouridine helix are shown as solid and dotted lines, respectively. In the NOESY sample the two protons in peak EF are partially resolved whereas the two protons in peak T have coalesced. (C) NOESY spectrum of E. coli tRNAj at 32°C showing the imino and aromatic proton regions. AU-type imino protons have been connected horizontally by a dotted line to the cross-peak of their proximal C2-H or C8-H in the 7 to 9 ppm region, which has been labeled with the corresponding lower-case letter. From Hare et Courtesy of Brian Reid.

FIGURE 3.4 Illustration of the process of acquiring a one-dimensional NMR spectrum. The steps involved in obtaining an NMR spectrum are shown. The sample is a tetra-peptide (Val-Ala-Ser-Ala). A short (10 asec) intense RF pulse is applied to the sample. This pulse excites all of the nuclei and they emit energy at their characteristic absorption frequencies. This signal is called the free induction decay (FID) and is collected as a function of time. This time domain signal is converted to spectrum by Fourier transformation. Note the characteristic chemical shifts for amide protons (H v), a-protons, (i-protons, and methyl protons. Also note that the two alanine residues, although chemically equivalent, have different chemical shifts because they experience different local environments. 

The one-dimensional NMR spectrum shows amplitude as a function of frequency. To generate this spectrum, an ensemble of a particular NMR-active nuclide is excited. The excited nuclei generate a signal that is detected in the time domain and then converted mathematically to the frequency domain by using a Fourier transform. 

Figure 2 One-dimensional NMR spectrum of lysozyme in aqueous solution obtained with presaturation of the water signal, illustrating the range of proton chemical shifts expected for a random coil, or denatured, protein. The positions of upfield shifted methyl signals and downfield shifted alpha and amide proton signals that are indicative of a non-random, ordered protein structure are also shown.

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