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Related Concept Videos

¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule01:10

Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule

In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the others.
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons00:58

¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

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...
¹H NMR: Pople Notation01:09

¹H NMR: Pople Notation

The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. When the coupled nuclei have well-separated chemical shifts, they are assigned letters that are far apart in the alphabet, such as A and X. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet (AB, MN, or XY).
A proton...
¹H NMR Signal Multiplicity: Splitting Patterns01:13

¹H NMR Signal Multiplicity: Splitting Patterns

When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become anti-parallel to spin X. Consequently, there are two possible excitation frequencies for A (A1 and A2), depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2, indicated...
Mass Spectrometry of Amines01:15

Mass Spectrometry of Amines

In mass spectroscopy, amines undergo fragmentation to give parent ions with odd molecule weights. This observed mass spectrum follows the nitrogen rule; a molecule with an odd number of nitrogen atoms produces a molecular ion with an odd molecular weight. Amines undergo fragmentation through α cleavage, producing nitrogen-containing cations—iminium ions—and alkyl radicals. Mass spectra of aromatic and cyclic aliphatic amines exhibit strong molecular ion peaks, but acyclic aliphatic amines show...

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Residue-Specific Exchange of Proline by Proline Analogs in Fluorescent Proteins: How "Molecular Surgery" of the Backbone Affects Folding and Stability
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C(alpha)-methyl proline: a unique example of split personality.

Alessandro Moretto1, Francesco Terrenzani, Marco Crisma

  • 1Department of Chemistry, University of Padova, Institute of Biomolecular Chemistry, 35131 Padova, Italy.

Biopolymers
|September 7, 2007
PubMed
Summary
This summary is machine-generated.

Methylation of proline residues ((alphaMe)Pro) influences peptide structures. X-ray diffraction reveals (alphaMe)Pro allows for helical and extended conformations, enhancing beta-turn formation.

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A Mass Spectrometry-Based Proteomics Approach for Global and High-Confidence Protein R-Methylation Analysis

Published on: April 28, 2022

Area of Science:

  • Structural Biology
  • Peptide Chemistry
  • Biochemistry

Background:

  • Proline residues are unique alpha-amino acids with cyclic side chains, influencing peptide backbone conformation.
  • Tertiary amides and proline's cyclic nature typically restrict conformational flexibility.
  • C(alpha)-methylation of proline ((alphaMe)Pro) introduces steric bulk, potentially further altering conformational preferences.

Purpose of the Study:

  • To investigate the conformational effects of C(alpha)-methylation on proline residues in peptide structures.
  • To determine the preferred conformational states and flexibility of (alphaMe)Pro-containing peptides.
  • To assess the impact of (alphaMe)Pro on beta-turn formation propensity.

Main Methods:

  • X-ray diffraction analysis of four N(alpha)-blocked, (alphaMe)Pro-containing, dipeptide N'-alkylamides.
  • Analysis of torsion angles (omega, phi, psi) to map conformational space.
  • Comparison of observed conformations with known helical and turn structures.

Main Results:

  • X-ray diffraction confirmed that (alphaMe)Pro prefers conformations typical of 3(10)/alpha-helices.
  • Unexpectedly, (alphaMe)Pro also allows exploration of the semi-extended [type-II poly(Pro)(n) helix] region.
  • The propensity for beta-turn formation is significantly enhanced in peptides containing (alphaMe)Pro compared to proline.

Conclusions:

  • C(alpha)-methylation of proline introduces significant conformational plasticity, enabling access to both helical and extended structures.
  • (alphaMe)Pro enhances the inherent beta-turn-forming tendency of proline, offering new possibilities for peptide design.
  • This sterically demanding residue expands the conformational repertoire accessible to peptides, impacting their structural and functional properties.