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Related Concept Videos

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In...
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous overlap of p...
π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0, resulting in...
Basicity of Heterocyclic Aromatic Amines01:25

Basicity of Heterocyclic Aromatic Amines

Heterocyclic amines, where the N atom is a part of an alicyclic system, are similar in basicity to alkylamines. Interestingly, the heterocyclic amine having a nitrogen atom as part of an aromatic ring has much less basicity than its corresponding alicyclic counterpart. For this reason, as presented in Figure 1, piperidine (pKb = 2.8) is significantly more basic than pyridine (pKb = 8.8).
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group with both...

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Aromatic-proline interactions: electronically tunable CH/π interactions.

Neal J Zondlo1

  • 1Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States.

Accounts of Chemical Research
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Aromatic-proline interactions, driven by CH/π forces, influence protein structure by tuning cis-amide bond formation. This study demonstrates that aromatic electronics control the degree of cis-prolyl bonds, impacting protein folding and function.

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Area of Science:

  • Biochemistry
  • Structural Biology
  • Protein Chemistry

Background:

  • Proline residues are crucial for protein folding, structure, and function due to their unique cyclic nature.
  • Aromatic amino acids and proline residues engage in favorable interactions, including hydrophobic effects and CH/π interactions.
  • These interactions can stabilize local structures, such as cis-amide bonds, and influence tertiary structures and protein-protein interactions.

Purpose of the Study:

  • To investigate the nature of local aromatic-proline interactions in peptides.
  • To determine how aromatic electronics influence the formation of cis-amide bonds.
  • To quantify the contribution of aromatic-proline interactions to protein structure and stability.

Main Methods:

  • Synthesis of 26 peptide models (TXPN) with varying aromatic amino acids (X).
  • Analysis of cis-amide bond populations (Ktrans/cis) using spectroscopic methods.
  • Correlation of aromatic electronic properties with cis-trans isomerism using Hammett analysis.

Main Results:

  • The population of cis-amide bonds is tunable by the electronic properties of aromatic residues.
  • A Hammett correlation was observed, indicating electronic control over cis-trans isomerism.
  • Tryptophan exhibited the strongest aromatic-proline interaction, stabilizing cis-amide bonds.
  • Proline stereoelectronic effects further modulate cis-trans isomerism.

Conclusions:

  • Aromatic-proline interactions are significantly CH/π in nature and are tunable by aromatic electronics.
  • These interactions play a key role in stabilizing local peptide structures, particularly cis-amide bonds.
  • Understanding these interactions provides insights into protein folding, protein-protein interactions, and the design of novel peptides and proteins.