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¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

1.3K
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
1.3K
¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

1.7K
The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
1.7K
Chair Conformation of Cyclohexane02:02

Chair Conformation of Cyclohexane

19.6K
The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.
The hydrogen atoms linked to carbons are arranged in two different axial and equatorial orientations to achieve this...
19.6K
Stability of Substituted Cyclohexanes02:30

Stability of Substituted Cyclohexanes

16.0K
This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...
16.0K
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

1.9K
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...
1.9K
Basicity of Heterocyclic Aromatic Amines01:25

Basicity of Heterocyclic Aromatic Amines

7.0K
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).
7.0K

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Privileged hydration sites in aromatic side chains: effect on conformational equilibrium.

Belén Hernández1, Fernando Pflüger, Manuel Dauchez

  • 1Laboratoire Matrice Extracellulaire et Dynamique Cellulaire (MEDyC), UMR 7369, Université de Reims, Pôle Sciences, Faculté des Sciences, Moulin de la Housse, 51687 Reims Cedex 2, France. mahmoud.ghomi@univ-paris13.fr.

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Water molecules interact with aromatic amino acid side chains, influencing peptide structure. New findings reveal weaker pi-electron cloud interactions, crucial for understanding protein stability in aqueous environments.

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Area of Science:

  • Biochemistry
  • Computational Chemistry
  • Physical Chemistry

Background:

  • Water is crucial for stabilizing peptide and protein structures in aqueous environments.
  • Water interacts with polar groups via electrostatic forces and hydrogen bonding.
  • Aromatic side chains (phenylalanine, tyrosine, tryptophan, histidine) play significant roles in protein structure and function.

Purpose of the Study:

  • To identify energetically favorable interaction sites between water molecules and aromatic side chains.
  • To investigate the role of weaker interactions, termed Hwπ interactions, between water and aromatic rings.
  • To compare hydration models and their impact on peptide rotamer energies.

Main Methods:

  • Density Functional Theory (DFT) calculations were employed.
  • Both implicit (polarized solvent continuum) and explicit hydration models were utilized.
  • Analysis focused on interactions with neutral and protonated histidine, phenylalanine, tyrosine, and tryptophan.

Main Results:

  • Water interacts strongly with O-H, N-H, and -N= groups on aromatic side chains.
  • Weaker Hwπ interactions, involving water hydrogen atoms and aromatic pi-electron clouds, were identified.
  • Explicit water binding to aromatic rings influences the relative energies of side chain rotamers (χ1 and χ2).

Conclusions:

  • Water-aromatic side chain interactions are complex, involving both strong polar and weaker pi-electron cloud interactions.
  • The inclusion of explicit water molecules is essential for accurately modeling the conformational preferences of aromatic residues.
  • These findings enhance our understanding of protein hydration and its contribution to structural stability.