Proteins carbon shifts

The transition from the bisabolane skeleton to the cuparane is not favourable energetically. Initially a secondary carbocation is formed from a tertiary, though a hydrogen shift quickly moves the positive charge back to a tertiary centre. The five-membered ring which is formed is very cluttered and, therefore, suffers from steric strain. The driving force must, therefore, come from the protein which catalyses the reaction. The 1,2-carbon shift to the chamigrane skeleton relieves some of the steric strain but this creates a spiro centre, the strain of which is relieved by the 1,2-carbon shift which creates the thujopsane skeleton. 

FIGURE 2.9 Typical chemical shifts of an Ala residue in a protein. Note that the amide proton has the highest proton shift (8 ppm) since it is directly attached to the electronegative nitrogen. The proton shifts of the H. proton (5 ppm) and the methyl protons (1 ppm) decrease systematically due to their increased distance from the electronegative atoms. The carbon shifts follow the same trend, with the carbonyl showing the highest chemical shift, followed by the alpha and then the beta carbon. Note that the three methyl protons will have the same chemical shift because they are equivalent. 

Although the natural abundance of nitrogen-15 [14390-96-6] leads to lower sensitivity than for carbon-13, this nucleus has attracted considerable interest in the area of polypeptide and protein stmcture deterrnination. Uniform enrichment of is achieved by growing protein synthesi2ing cells in media where is the only nitrogen source. reverse shift correlation via double quantum coherence permits the... 

Sex and age differences in stable isotopes of nitrogen and carbon are not pronounced. There is no evidence that males and females were eating different foods and the only evidence for age differences, higher 8 N in infants, has been explained by the trophic level shift during the time the infant derives most of its protein from, breast milk. The small amount of variation in both and 5 N values supports the historical sources, which indicate that while food was plentiful, the diet was rather monotonous. 

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... 

Solid state 13C CPMAS NMR spectra of Wheat High Molecular Weight (W.HMW) subunits show well resolved resonances identical with spectra of dry protein and peptide samples [24], Most of the amino acids side-chain resonances are found in the 0-35 ppm region followed by the alpha resonances of the most abundant amino acids glycine, glutamine and proline at chemical shifts of 42, 52 and 60 ppm, respectively, and the carbonyl carbons show a broad peak in 172-177 ppm region. The CPMAS spectra of hydrated whole HMW provides important information on the structural characteristics. 

Csaszar, A. G. 1992. Conformers of Gaseous Glycine, J. Am. Chem. Soc. 114,9568-9575. de Dios, A. C., J. G. Pearson, and E. Oldfield. 1993. Chemical Shifts in Proteins An Ab Initio Study of Carbon-13 Nuclear Magnetic Resonance Chemical Shielding in Glycine, Alanine, and Valine Residues, J. Am. Chem. Soc. 115, 9768-9773. 

The structural and functional integrity of organoclay-wrapped Mb and Hb molecules was demonstrated by retention of the secondary protein structure as well as distinctive shifts in the absorption spectra associated with oxygen or carbon monoxide binding to the heme metallocenter. The latter indicated that the wrapped... 

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