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Related Concept Videos

Prochirality02:05

Prochirality

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The concept of prochirality leads to the nomenclature of the individual faces of a molecule and plays a crucial role in the enantioselective reaction. It is a concept where two or more achiral molecules react to produce chiral products. A typical process is the reaction of an achiral ketone to generate a chiral alcohol. Here, the achiral reactant reacts with an achiral reducing agent, sodium borohydride, to generate an equimolar mixture of the chiral enantiomers of the product. For example, an...
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¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)01:20

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When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
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NMR Spectroscopy Of Amines01:19

NMR Spectroscopy Of Amines

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In proton NMR spectroscopy, primary amines and secondary amines showcase their N–H protons as a broad signal in the chemical shift range between δ 0.5 and 5 ppm. The exact position in this range depends on several factors, including sample concentration, hydrogen bonding, and the type of solvent used. Since amine protons undergo fast proton exchange in solution, the protons are labile and therefore do not participate in any splitting with adjacent protons. Thus, the observed peak is...
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Structure of Amines01:19

Structure of Amines

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The hybridized nitrogen atom in amines possesses a lone pair of electrons and is bound to three substituents with a bond angle of around 108°, which is less than the tetrahedral angle of 109.5°. However, the C–N–H bond angle is slightly larger at 112°, with a carbon–nitrogen bond length of 147 pm. This carbon–nitrogen bond length of of amines is longer than the carbon–oxygen bond of alcohols (143 pm) but shorter than alkanes’...
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Preparation of Amides01:29

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Amides are synthesized by treating carboxylic acids with amines in the presence of dehydrating agents like dicyclohexylcarbodiimide (DCC).
The DCC-promoted synthesis of amides begins with the protonation of DCC by carboxylic acid. The protonation makes it a better acceptor. Next, the addition of carboxylate to the protonated carbodiimide gives a reactive acylating agent.
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¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons00:58

¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

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Replacing each alpha-hydrogen in chloroethane by bromine (or a different functional group) yields a pair of enantiomers. Such protons are called prochiral or enantiotopic and are related by a mirror plane. Enantiotopic protons are chemically equivalent in an achiral environment. Because most proton NMR spectra are recorded using achiral solvents, enantiotopic hydrogens yield a single signal.
In chiral compounds such as 2-butanol, replacing the methylene hydrogens at C3 produces a pair of...
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Synthesis of pH Dependent Pyrazole, Imidazole, and Isoindolone Dipyrrinone Fluorophores using a Claisen-Schmidt Condensation Approach
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Comprehensive chiroptical study of proline-containing diamide compounds.

Krisztina Knapp1, Marcin Górecki, Jadwiga Frelek

  • 1Laboratory for Chiroptical Structure Analysis, Institute of Chemistry, Eötvös Loránd University, Budapest, Hungary.

Chirality
|March 11, 2014
PubMed
Summary

This study investigated peptide folding using N-acetyl-amino acid diamide models. Ac-β-HPro-NHMe exhibits distinct solid- and solution-phase behaviors, favoring cis amide conformations in various solvents, unlike its alpha-proline analog.

Keywords:
ECDIRROARaman optical activityVCDconformational analysiselectronic circular dichroisminfrared spectroscopyvibrational circular dichroism

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Area of Science:

  • Chemical Physics
  • Biophysical Chemistry
  • Computational Chemistry

Background:

  • Peptide folding is crucial for biological function.
  • N-acetyl-amino acid methylamides serve as valuable diamide models for studying peptide conformations.
  • Understanding conformational preferences is key to peptide structure-function relationships.

Purpose of the Study:

  • To perform a detailed conformational analysis of Ac-Pro-NHMe and Ac-β-HPro-NHMe diamides.
  • To investigate the influence of the environment (isolated, solvated, solid-state) on their conformational distributions.
  • To compare the conformational behavior of proline and beta-hydroxyproline derivatives.

Main Methods:

  • Density Functional Theory (DFT) calculations for conformational analysis.
  • Chiroptical spectroscopy: Vibrational Circular Dichroism (VCD), Electronic Circular Dichroism (ECD), Raman Optical Activity (ROA).
  • Single crystal X-ray diffraction for solid-state structure determination.

Main Results:

  • DFT calculations provided conformational distributions for isolated and solvated diamides.
  • Chiroptical spectroscopy and X-ray diffraction revealed distinct conformational preferences influenced by the environment.
  • Ac-β-HPro-NHMe demonstrated greater conformational freedom, favoring cis amide conformations in solution (water, DCM, ACN), contrasting with Ac-Pro-NHMe.
  • The crystal structure of Ac-β-HPro-NHMe did not directly correlate with calculated or solution conformers, while Ac-Pro-NHMe's crystal structure matched a predominant trans conformer (tα(L)).

Conclusions:

  • Environmental factors significantly impact the conformational behavior of proline-containing diamides.
  • Ac-β-HPro-NHMe exhibits unique conformational adaptability, forming cis amide structures in solution, unlike its alpha-proline counterpart.
  • Discrepancies between solid-state and solution structures highlight the complexity of conformational analysis in different phases.