Carbon-13 substituent chemical shifts in the side-chain carbons of aromatic systems: the importance of π-polarization in determining chemical shifts

Bromilow, John; Brownlee, Robert T. C.; Craik, David J.; FISKE, P. R.; Rowe, Jeffrey E.; Sadek, Maruse

doi:10.1039/p29810000753

Cited by 109 publications

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“…As the phenyl substituent becomes more electron-withdrawing, forms B and C become disfavored 8 due to π-polarization. 9 The diminished delocalization increases the energy of the C i ′=O i π orbital and decreases the energy of the C i ′=O i π * orbital, making C i ′ =O i more electrophilic. The decreased delocalization is also manifested in the increased contribution from resonance form A, which results in greater double-bond character and higher C i ′=O i vibrational frequency.…”

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confidence: 99%

Energetics of an n → π* Interaction that Impacts Protein Structure

2006

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The trans/cis ratio of the amide bond in N-formylproline phenylesters correlates with electronwithdrawal by a para substituent. The slope of the Hammett plot (ρ = 0.26) is indicative of a substantial effect. This effect arises from a favorable n→π * interaction between the amide oxygen and ester carbonyl. In a polypeptide chain, an analogous interaction can stabilize the conformation of trans peptide bonds, α-helices, and polyproline type-II helices.Certain non-covalent interactions are known to direct a polypeptide chain to assume a folded structure. Those interactions include the hydrophobic effect, hydrogen bonding, Coloumbic forces, van der Waals forces, and cation-π interactions. 1 Recently, we suggested that n→π * interactions should be added to this list. 2,3 In an n→π * interaction (which is not to be confused with an n→π * electronic transition) the oxygen of a peptide bond (O i−1 ) donates electron density from one of its lone pairs into the antibonding orbital of the carbon in the subsequent peptide bond (C i ′ =O i ). This O i−1 ···C i ′ =O i interaction, which mimics the approach of a nucleophile to the electrophilic carbon of an acyl group, is strongest when O i−1 is positioned proximally and along the Bürgi-Dunitz trajectory to C i ′ =O i . In polypeptides, this geometry occurs in the α-helix and polyproline type-II (PPII) helix, 2c which is the structure assumed by the strands of the prevalent collagen triple helix. 3,4The n→π * interaction can occur only if the peptide bond containing O i−1 is trans (i.e., Z), as opposed to cis (E). Accordingly, the trans/cis ratio of an NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript present a simple model system that uses the trans/cis ratio of a peptide bond to report on the energetics of an n→π * interaction.We reasoned that the strength of an n→π * interaction could be altered by using p-substituted phenyl esters of N-formyl-L-proline to modulate the electrophilicity of the carbonyl (C i ′ =O i ).Our expectation was that the trans/cis ratio would correlate with that electrophilicity. We chose N-formylproline because its trans/cis ratio is close to unity and can thus serve as the basis for a highly sensitive assay. 2c The esters were synthesized by condensation of N-formylproline with the corresponding phenol using PyBOP activation, and were purified by flash chromatography.We used 1 H NMR spectroscopy to measure the trans/cis ratio of each ester (90−170 mM) in CDCl 3 . We chose chloroform as the solvent to minimize effects from differential solvation of the trans and cis isomers. Because the isomerization of the peptide bond is slow on the NMR timescale, two sets of signals were observed in the spectrum of each ester. Though most of the resonances exist as multiplets and overlap with those from their isomer, the resonance of the formyl proton appears as two singlets-one for each iso mer. The larger, upfield resonance arises from the trans isomer. Integration of the two signals provides the trans/cis ratio at the amide bond...

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confidence: 99%

Energetics of an n → π* Interaction that Impacts Protein Structure

2006

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“…2 to the data of Solaniova, et al, Brownlee, 15 They also reported the negative ρI values for β-C (-1.1) and 1-C (-0.8) and the positive ρI values for α-C (2.7) and 4-C (0.3). Although the signs of the values may be unchanged, the calculations were done with different sets of four resonance scales (σR-, σR o , σR BA , and σR+) so that the best fit could be obtained.…”

Section: Resultsmentioning

confidence: 52%

Substituent Chemical Shifts of (E)-1-Aryl-3-thienylpropen-1-ones

Lee¹,

Jeon²,

Yu³

et al. 2010

Bulletin of the Korean Chemical Society

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Substituent chemical shifts were examined for the 2-and 3-thiophene derivatives of chalcone and compared to the thiophene series of derivatives with the phenyl series. The chemical shift values for the α-carbons of the enones showed and inverse correlation with the Hammett σ values, but the correlation coefficients were moderate (r = 0.836 -0.878). On the other hand, the β-carbons showed a normal correlation with excellent correlation coefficients (r = 0.994). The absolute magnitude of the ρ values for the α-carbon are about half of those of the β-carbon. The observation may be the result of a through-space transition of the electronic effect of the substituents in addition to the through bond transition.

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“…This is understandable because π polarization of a conjugate system induces an alternating partial positive and negative charge at each end of the π unit. 1,13 The magnitude of ρ 2-C , however, is about nine times larger than that of ρ 7a-C . This may be evidence of the conformation Ia being more favorable than Ib.…”

Section: Resultsmentioning

confidence: 96%

“…The assignments for carbons bearing a proton were quite obvious, but the carbons and the protons of the ester groups were assigned by examination of HMBC spectra. The 3a-COOCH 3 of 2 is bonded to an sp 3 -hybridized carbon atom and its 13 C chemical shift of the carbonyl carbon is downfield by about 10 ppm compared to those of 2-, 3-and 4-COOCH 3 which are bonded to an sp 2 -hybridized carbons. The methoxy carbon of 3a-COOCH 3 group is also further downfield than other ester groups.…”

Section: Resultsmentioning

confidence: 99%

“…(1) has been used to correlate the chemical shift of m-and p-substituted compound to unsubstituted compound using the Hammett σ m and σ p values. Values of 13 σ were reported for the system of m-and p-substituted cinnamic acids in order to have a better correlation of 13 C chemical shift. 2 The dual substituent parameter (DSP) approach, on the other hand, divides the effect of substituent effect into inductive (σ I ) and resonance (σ R ) parameters as represented in Eq.…”

Section: Introductionmentioning

confidence: 99%

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Correlation of Chemical Shifts with Substituent Parameters in N-Benzyl Derivatives of Pyrrole,3a,7a-Dihydroindole,and Indole Esters

Jeon¹,

Yu²,

Lee

2002

Bulletin of the Korean Chemical Society

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Series of m-and p-substituted benzyl derivatives of pyrrole, tetramethyl 1-benzyl-3a,7a-dihydroindole-2,3,3a,4-tetracarboxylate, and trimethyl 1-benzylindole-2,3,4-tricarboxylate were prepared and their 13 C NMR spectra were obtained in 0.1 M solutions of chloroform-d. Both single substituent parameter and dual substituent parameter analyses were carried out to correlate the substituent chemical shifts. The β carbon of the indole series showed the most profound substituent effect dependence as well as the best correlation. The results are explained by the hyperconjugation of the benzyl methylene group.

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Carbon-13 substituent chemical shifts in the side-chain carbons of aromatic systems: the importance of π-polarization in determining chemical shifts

Cited by 109 publications

References 0 publications

Energetics of an n → π* Interaction that Impacts Protein Structure

Energetics of an n → π* Interaction that Impacts Protein Structure

Substituent Chemical Shifts of (E)-1-Aryl-3-thienylpropen-1-ones

Correlation of Chemical Shifts with Substituent Parameters in N-Benzyl Derivatives of Pyrrole,3a,7a-Dihydroindole,and Indole Esters

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