Main chain carbon chemical shifts

Main Chain Carbon Chemical Shifts in Styrene-SO>> Copolymers... 

For PGs with n > 14, the and CO(amide) carbons give intense peaks at the chemical shift position for the a-helix form. Also, a large decrease in T2S of the side chains and the main chain carbons occurs at n = 14 as seen from Figure 7-20(a) and (b), respectively. As mentioned above, in PG-n(n >14) side-chain interactions are very strong enough to restrict the molecular motion and induce the formation of the crystallites. The existence of peaks for the and CO(amide) carbons and the behavior of Hl2S of the side chain and the main chain carbons result from formation of the side-chain crystallites. 

For the main-chain carbons, peaks do not disappear even at 80° C. As the amide carbonyl chemical shift appears at about 176 ppm independent of temperature, the main chain takes the right-handed a-helix conformation. Therefore, the main chain is in the rigid state and the molecular mobility should be slower than that of the side chains. The main-chain mobility of PG-2 is much lower than that of PG-6. The longer side chains can gain higher freedom of molecular motion. 

An important first step in interpreting the C-13 spectra is to distinguish a-carbons from 3-carbons, i.e. methine from methylene. Observation of multiplicity when the proton decoupler is off is one way, but this is not always easy if the lines are broadened by chemical shift multiplicity. Measurement of has been used for this purpose since the 3-carbon with two bonded protons relaxes about twice as fast as the a-carbon with only one. A very positive way is by deuterium labelling. In Fig. 3 is shown the main-chain 25 MHz carbon spectrum of two styrene-S02 copolymers containing 58 mol% styrene, or a ratio of styrene to SO2 of 1.38 (7 ). In the bottom one, 3,3-d2-styrene has been used, cind all the 3-carbon resonances are distinguishable from the a-carbon resonances since the presence of deuterium has eliminated their nuclear Overhauser effect because of this eind the deuterium J coupling ( 20 Hz), they are markedly smaller eind broader than the a-carbon resonances. 

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

As mentioned above, the principal values of chemical shift tensor give information about three dimensional electronic state of a molecule. However, in order to understand behavior of the principal values, one should obtain information about the orientation of the principal axis system of a chemical shift tensor with respect to the molecular fixed frame. The orientations of the principal axis systems of the chemical shift tensors of L-alanine Cp -carbons in some peptides were calculated, whose L-alanine moieties have different main-chain dihedral-angles, (( >,v /H-57.40,-47.50)[aR-helix], (-138.8°,134.7°)[ pA-sheet], (-66.3°,-... 

The calculated 13C shielding contour map reproduce well the experimental results. This means that the change of the main-chain dihedral angles dominates the 13C isotropic chemical shift behavior of the L-alanine residue Cp-carbon. 

Table 1 Observed 13C NMR chemical shifts of the L-alanine residue Cp-carbons for peptides and polypeptides containing the L-alanine residues from l3C CP/MAS NMR measurements, and their main chain dihedral angles and hydrogen-bond length for the L-alanine residues determined by X-ray diffraction studies.

For simple poly(a-olefins) of practical importance, the chain stereostructure can be quantitatively determined at least at the stereotriad level [1], Longer configurational sequences can be observed by nuclear magnetic resonance, particularly using larger magnetic fields or 13C NMR spectroscopy or both. The chemical shifts due to CH3,CH2 and CH carbons are widely spaced. It is mainly the chemical shift of the pendant CH3 group carbon that is utilised for sequential analysis. 

These experiments can be recorded in four dimensions with the side chain carbons comprising the fourth dimension (i.e., HCNH-TOCSY and HC(CO)NH-TOCSY),99 in order to provide 13C chemical shifts and a potential increase in resolution if ambiguities still remain. Note also that the H(CCO)NH—TOCSY is very similar in concept to the HBHA(CBCACO)NH described above. The main difference is that transfer from Cg to Ca in the latter case is through a COSY-type step. This excludes magnetization that may have originated on C7 from being observed, in contrast to the isotropic mixing sequence used in the two TOCSY-type experiments. 

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