Chemical shifts introduction

After the introduction of C-labels into the protein or glycoprotein molecule, the ability to assign the resonances to specific carbon atoms is essential. In the case of glycophorin (see Fig. 1), it may readily be seen that 5 lysine residues and 1 N-terminal amino acid (per species) can be reduc-tively di[ C]methylated. This could theoretically lead to 6 resonances (or possibly more, if chemical-shift nonequivalence is observed for the dimethyl species) in the C spectrum of methylated glycophorin A. However, in most cases, the N, N -di[ C]methyllysine resonances all occur near, or at, the same frequency. It is then necessary to be able at least to assign, or... 

If one wishes to obtain a fluorine NMR spectrum, one must of course first have access to a spectrometer with a probe that will allow observation of fluorine nuclei. Fortunately, most modern high field NMR spectrometers that are available in industrial and academic research laboratories today have this capability. Probably the most common NMR spectrometers in use today for taking routine NMR spectra are 300 MHz instruments, which measure proton spectra at 300 MHz, carbon spectra at 75.5 MHz and fluorine spectra at 282 MHz. Before obtaining and attempting to interpret fluorine NMR spectra, it would be advisable to become familiar with some of the fundamental concepts related to fluorine chemical shifts and spin-spin coupling constants that are presented in this book. There is also a very nice introduction to fluorine NMR by W. S. and M. L. Brey in the Encyclopedia of Nuclear Magnetic Resonance.1... 

Analysis of nitronates by 14N and 15N NMR spectroscopy has an auxiliary character (see Table 3.12). The 14N NMR signals are often broadened and, hence, are difficult to observe and are poorly informative, although magnitudes of their chemical shifts could in principle help in distinguishing between covalent nitronates and salts. It is difficult to observe 15N NMR signals in natural-abundance NMR spectra of nitronates, while an introduction of a label is an expensive procedure. 

Intraactions between elections in three-membered rings and unsaturated groups in the same molecule have been detected via 13C chemical-shift variations in a number of instances. Thus, introduction of the carbonyl function in tricy-clo[3.2.1,02,7]decane (e.g., 274) leads to significant downfield shifts of the signals of C(l) (+8.0), C(2) (+15.5), and C(7) (+7.7) (385), whereas corresponding effects in bicyclo[3.1.0]hexan-2-one (275) are smaller (385,386). A corresponding dependence was reported for 276 and 277 and related to more effective electron withdrawal in 276 (387). An even more pronounced deshielding effect was observed by Murata and co-workers (388,389) in the ketone 278 when they compared it with 279. 

As can be seen from Table 19, the saturation between Cl and Cl2 in 67 causes an upheld shift of 1114 (ca 0.2 ppm) and of H15 and 1115 (ca 0.1 ppm) compared with 66. The remaining polyene protons in 67 are only slightly affected. Similar features are observed upon comparison of the chemical shift values of the protons of the polyene chains of 68, 69, 70 and 71 with those of 66. Thus, saturation of a double bond in a polyene chain generally leads to an upheld shift of ca 0.2 ppm for the protons connected to the y- and the -carbons the upheld shift of the remaining polyene protons is generally less than 0.05 ppm. Comparison of the chemical shift values of 72 with those of 66 shows that the introduction of the 15,15 -triple bond in 72 leads to an upheld shift of the signals of H14 and H14 (ca 0.5 ppm), whereas the chemical shift values of the other polyene protons are only slightly affected. 

The particular array of chemical shifts found for the nuclei of a given polymer depends, of course, on such factors as bond orientation, substituent effects, the nature of nearby functional groups, solvation influences, etc. As a specific example, derivatives of the carbohydrate hydroxyl moieties may give rise to chemical shifts widely different from those of the unmodified compound, a fact that has been utilized, e.g., in studies (l ) on commercially-important ethers of cellulose. Hence, as illustrated in Fig, 2, the introduction of an 0-methyl function causes (lU,15) a large downfield displacement for the substituted carbon. This change allows for a convenient, direct, analysis of the distribution of ether groups in the polymer. Analogously, carboxymethyl, hydroxyethyl and other derivatives may be characterized as well... 

Figure 2. Stick diagram representing the C-13 spectra (18) of methyl R-cello-bioside, as a model for cellulose (upper), and a partially substituted O-methyl-cellulose (2- and 6-0-methyl) (lower). The light lines emphasize the changes in chemical shift associated with the introduction of ether substituents.

The introduction of isotopes into a compound alters the coupling pattern and the chemical shifts of the observed spectrum. As shovm in Eigure 1.18, deuterium-induced chemical shift variations have allo ved the estimation of the ratio of isomers 90a-d formed in Eq. (2) vhen R = Ph, R = CHjCOOH, and DCOOD/ Et3N is used for the hydrogen transfer [136]. The three sp -carbons Cl, C2 and C3 each afford a distinct singlet for the four possible isotopomers 90a-d (replacement of by shifts the resonances of the adjacent carbon nuclei to lo ver frequency) [137]. 

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