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NMR Spectroscopy: Spin–Spin Coupling01:08

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The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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Spin–Spin Coupling: One-Bond Coupling01:17

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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

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In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
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Atomic Nuclei: Nuclear Spin01:08

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All atomic particles possess an intrinsic angular momentum, or 'spin'. Electrons, protons, and neutrons each have a spin value of ½, although protons and neutrons in nuclei may have higher half-integer spins owing to energetic factors.
Atomic nuclei have a net nuclear spin, , which can have an integer or half-integer value. In atomic nuclei, the spins of protons are paired against each other but not with neutrons, and vice versa. Consequently, an even number of protons does not contribute to...
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Ether-linked lipids: Spin-label EPR and spin echoes.

Maria Oranges1, Rita Guzzi1, Derek Marsh2

  • 1Department of Physics, Molecular Biophysics Laboratory, University of Calabria, 87036, Rende, CS, Italy.

Chemistry and Physics of Lipids
|February 8, 2018
PubMed
Summary

Ether-linked phospholipids exhibit altered water penetration and polarity compared to ester-linked ones. Electron spin echo envelope modulation (ESEEM) and electron paramagnetic resonance (EPR) reveal these differences in membrane structure.

Keywords:
D2O-ESEEMDihexadecyl phosphatidylcholineFlexibility profilePolarity profileSpin-label EPRWater penetration

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Area of Science:

  • Membrane biophysics
  • Supramolecular chemistry
  • Physical chemistry

Background:

  • Phospholipid membranes are crucial for cellular function.
  • Ether-linked lipids differ structurally from ester-linked lipids, potentially affecting membrane properties.
  • Understanding these differences is key to elucidating membrane behavior and function.

Purpose of the Study:

  • To investigate the impact of ether-linked chains on phospholipid membrane properties.
  • To compare water penetration, polarity, phase behavior, and chain flexibility of ether-linked vs. ester-linked phospholipids.
  • To characterize the interdigitated and fluid Lα phases of dihexadecyl phosphatidylcholine (DHPC).

Main Methods:

  • Site-specific spin labeling of phospholipids.
  • Electron spin echo envelope modulation (ESEEM) spectroscopy.
  • Conventional electron paramagnetic resonance (EPR) spectroscopy.
  • Analysis of D2O-ESEEM and 14N-hyperfine splitting.
  • X-ray diffraction data comparison.

Main Results:

  • D2O-ESEEM showed comparable water exposure in DHPC's interdigitated phase, from headgroup to terminal methyl groups.
  • A uniform transmembrane polarity profile was observed in frozen interdigitated DHPC dispersions.
  • Conventional EPR indicated interdigitated lipid chains within the intermediate gel phase of DHPC.
  • Compared to ester-linked DPPC, DHPC exhibited outward shifts in polarity and chain flexibility profiles in the Lα phase.
  • DHPC's polarity profile was shifted towards higher polarities, consistent with X-ray diffraction findings on hydrocarbon thickness and area per lipid.

Conclusions:

  • Ether-linked chains in DHPC significantly alter membrane water penetration, polarity, and chain flexibility compared to ester-linked lipids.
  • The study provides detailed insights into the structural organization and phase behavior of ether-linked phospholipids.
  • These findings contribute to a deeper understanding of lipid-lipid and lipid-water interactions in biological membranes.