Patterns of Chemical Shifts

This pattern of chemical-shift nonequivalence is also manifested by the nonanomeric carbon atoms. 

Using DQF-COSY and TOCSY we can link all of the protons within a single spin system, which corresponds to a single amino acid residue. We can classify each spin system as a pattern of chemical shifts unique to one amino acid or as a member of a class AMX or five spin. In order to get sequence-specific assignments, however, we have to have some way to correlate protons in one residue to protons in the next residue in the sequence. For unlabeled proteins this is done by NOE interactions certain protons in one residue are constrained by the peptide bond to be close in space to certain protons in the next residue. These NOE correlations are called sequential or z, i + 1 because they correlate a proton in residue z with a proton in the next residue in the sequence, residue z + 1. Specifically, we expect to see NOE correlations between Ha of residue z and Hn of residue z + 1 (Fig. 12.15) and sometimes between the protons of residue z and the Hn of the next residue. Because the DQF-COSY and TOCSY spectra correlate protons within a residue, we can move from... 

The pattern of chemical shift differences found for phenyl-substituted silyl anions is typical for 7i-polarization effects, whereas resonance effects are responsible for electronic effects in carbanions. UV spectroscopic results support this explanation, as carbanions show bathochromic shifts compared to silyl anions [5]. 

Far-uv circular dichroism is probably the most appropriate way to determine this structure. A helical protein exhibits double minima at wavelengths of 208 and 222 nm. However, this would only show that on average the protein contains some helical structure. To determine which residues contained helical structure, it would be necessary to use NMR spectroscopy to assign spectral peaks to residues and search for patterns of chemical shifts of peaks in the spectra, that are typical of a-helical structure. The first approach is rapid, the second takes longer but provides more information overall. 

Step 4 Pattern of chemical shifts. Examine the NMR spectrum for signals characteristic... 

Pattern of chemical shifts. The signal at S 1.07 is in the alkyl region and, based on its chemical shift, most probably represents a methyl group. No signal occurs at S 4.6 to 5.7 thus, there are no vinylic hydrogens. (If a carbon-carbon double bond is in the molecule, no hydrogens are on it that is, it is tetrasubstituted.)... 

STEP 4 Pattern of chemical shifts. The singlet at S 1.01 is characteristic of a methyl group adjacent to an sp hybridized carbon. The singlets at S 2.11 and 2.32 are characteristic of alkyl groups adjacent to a carbonyl group. 

Step 3 Pattern of chemical shifts. Examine the NMR spectrum for signals characteristic of the most common types of equivalent carbons (see the general rules of thumb for C-NMR chemical shifts in Section 11.12). Keep in mind that these ranges are broad and that carbons of each type may be shifted either farther upheld or farther downfield, depending on details of the molecular structure in question. 

STEP 3 Pattern of chemical shifts. The signal (e) at S 23 is characteristic of an sp hybridized carbon.The four signals (a-d) between S 120 and 140 are characteristic of sp hybridized carbons. Because it would be unlikely for a molecule with only seven carbon atoms to have 4 pi bonds (due to IHD = 4), it is likely that these signals represent the carbons of a benzene ring. 

Returning to Figure 11.5, let us go through the ID spectrum first, and then we will see why the COSY spectrum is so important in characterising this molecule. The three peaks in the region 0.8-1.8 ppm correspond to the fatty alkyl chain, exactly as they do in Figure 11.4. In fact, this pattern of chemical shifts is characteristic of the H-NMR spectra of all surfactants with a linear fatty alkyl chain. 

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