Chemical shift carbonyl carbon

Fig. 22. Carbon-13 nuclear magnetic resonance of carbonyl carbon chemical shifts on complexation of K.+.

Carbon chemical shifts were employed in studies of conformational effects on aldoximes and ketoximes. They appear in the region 145 to 163 ppm and several values are illustrated in Table 2. Such resonances are some 50 ppm to higher field relative to the corresponding carbonyl resonance. Where substimtion is asymmetrical, differences in oxime carbon chemical shifts are observed, depending upon the conformation of the oxime N—OH. Such conformational isomerism also has a profound effect upon the chemical shift of the a-carbon. 

The l3C isonitrile carbon chemical shift exhibits considerably greater sensitivity to changes in bonding and molecular structure (254, 256) than carbonyls, but is generally observed in the range 105-175 ppm (254, 258) [cf. free isonitriles resonate between 154 and 165 ppm (254, 259)]. 

The dependence of the principal components of the nuclear magnetic resonance (NMR) chemical shift tensor of non-hydrogen nuclei in model dipeptides is investigated. It is observed that the principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. On the other hand, there is no clear relationship between the alpha carbon chemical shift tensor and the molecular framework. However, the projection of this tensor on the C-H vector reveals interesting trends that one may use in peptide secondary structure determination. Effects of hydrogen bonding on the chemical shift tensor will also be discussed. The dependence of the chemical shift on ionic distance has also been studied in Rb halides and mixed halides. Lastly, the presence of motion can have dramatic effects on the observed NMR chemical shift tensor as illustrated by a nitrosyl meso-tetraphenyl porphinato cobalt (III) complex. 

H and 13C NMR Data. A ketone or aldehyde carbonyl group bound to a CF2H group shields its proton slightly (0.1 ppm), and even more surprisingly it also has a shielding effect upon its carbon chemical shift of about 8 ppm (Scheme 4.42). By comparison, a hydrocarbon... 

The conformation of M73R SSI in the free state was nearly identical to that of the WT SSI since there was almost no chemical shift difference between these two inhibitors. The carbonyl carbon chemical shift profile of the M73R SSI-subtilisin BPN complex was also very similar to that of the WT SSI-subtilisin BPN complex, showing that the conformations of these complexes are also nearly the same. The shift profile of the M73R SSI-bovine trypsin, however, was quite different from those of the subtilisin complexes. This can be taken as a good indication that the conformational flexible segments are responsible for the wide inhibitory activity of SSI. 

Table 22.4 shows the values of Rn for some solid polypeptides, determined by using Equations (22.1a-e) through the observation of the amide carbonyl-carbon chemical shift as listed are solid polyglycine[(Gly) ], poly(L-alanine)[(Ala) ], poly(L-valine)[(Val) ], and poly(L-leucine)[(Leu) ] with several conformations such as righthanded a-helix (aR-helix), j8-sheet, 3i- and (UL-helix. In these homopolypeptides, the Rn o values determined for the /3-sheet form are constant in the range of 3.0-3.1 A, regardless of amino acid residue species. The Rn-.-o value for the 3i-helix in (Gly) is... 

Carbonyl carbon chemical shifts, p.p.m. from CS2 and the n— n absorption of some bicyclic compounds... 

Urry, D. W., Mitchell, L. W., and Ohnishi, T. (1974). Biochem. Biophys. Res. Comm. 59, 62. Solvent Dependence of Peptide Carbonyl Carbon Chemical Shifts and Polypeptide Secondary Structure The Repeat Tetrapeptide of Elastin. 

The chemical shift anisotropies for the carbonyl and aromatic carbons of Hytrel were reconstructed from a Herzfeld-Beiger analysis (24) of the intensities of the sidebands from NMR magic angle spinning experiments. The results in Table III indicate that the carbonyl carbon chemical shift anisotropy is axially symmetric for each terephthalate ester. We attribute this axial symmetry to a general property of terephthalate esters, rather than as a consequence of molecular motion, as the highly crystalline dimethyl terephthalate also has an axially symmetric carbonyl carbon chemical shift tensor. 

Carbon-13 NMR spectra of tungsten derivatives (CO)sWC(CH3)(SC6H4Y) were obtained by Ward et al. (127) (Table XXXVII). Only small variations of cis and trans carbonyl and carbene carbon chemical shift were detected as the nature of the para group, Y, was varied. The authors concluded that the lack of any conjugative effect indicated that the phenyl ring was probably perpendicular to the carbene plane. 

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