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Related Concept Videos

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

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

992
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...
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¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

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

1.4K
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.
1.4K
Chair Conformation of Cyclohexane02:02

Chair Conformation of Cyclohexane

16.2K
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...
16.2K
Stereoisomerism of Cyclic Compounds02:33

Stereoisomerism of Cyclic Compounds

9.2K
In this lesson, we delve into the role of ring conformation and its stability, which determines the spatial arrangement and, consequently, the molecular symmetry and stereoisomerism of cyclic compounds. 1,2-Dimethylcyclohexane is used as a case study to evaluate the possible number of stereoisomers. Here, given the multiple (n = 2) chiral centers, there are 2n = 4 possible configurations that lack a plane of symmetry, as the ring skeleton exists in a non-planar chair conformation. In addition,...
9.2K
Conformations of Cyclohexane02:11

Conformations of Cyclohexane

12.2K
Cyclohexane does not exist in a planar form due to the high angle and torsional strain it would experience in the planar structure. Instead, it adopts non-planar chair and boat conformations.
The chair form is the most stable and derives its name from its resemblance to the “easy chair.” In the chair conformation, two carbon atoms are arranged out-of-plane — one above and one below, minimizing the torsional strain. In the chair form, the bond angle is very close to the ideal...
12.2K
Stability of Substituted Cyclohexanes02:30

Stability of Substituted Cyclohexanes

13.3K
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...
13.3K

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Solvent-induced conformational changes in cyclic peptides: a vibrational circular dichroism study.

Christian Merten1, Fee Li, Kenny Bravo-Rodriguez

  • 1Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G2G2, Canada. yunjie.xu@ualberta.ca.

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This study investigated cyclic tetrapeptides, revealing how solvent environment influences their three-dimensional structure and β-turn conformations. Peptide structure and solvent interactions were explored using vibrational circular dichroism spectroscopy.

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Area of Science:

  • Biochemistry
  • Chemical Physics
  • Structural Biology

Background:

  • Peptide three-dimensional structure is significantly affected by the surrounding solvent.
  • Cyclic tetrapeptides serve as valuable model systems for studying β-turns.

Purpose of the Study:

  • To investigate the conformational preferences of cyclic tetrapeptides in different solvent environments.
  • To elucidate the role of solvent polarity in stabilizing specific β-turn structures.

Main Methods:

  • Vibrational Circular Dichroism (VCD) spectroscopy was employed to analyze peptide structures.
  • Nuclear Magnetic Resonance (NMR) data was used for comparison and confirmation.

Main Results:

  • D-leucine containing tetrapeptide (D-2) predominantly adopts a βII turn in both apolar and polar solvents.
  • L-leucine containing tetrapeptide (L-2) shows a preference for βI over βII turns.
  • Glycine containing tetrapeptide (1) shifts from a βII to a βI turn preference with increasing solvent polarity.

Conclusions:

  • Solvent polarity plays a crucial role in dictating the conformational landscape of cyclic tetrapeptides.
  • Conformational changes involve the breaking of intramolecular hydrogen bonds and the formation of solvent-mediated hydrogen bonds.
  • These findings enhance our understanding of peptide structure-solvent interactions and β-turn stabilization.