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#coupling constant

2 public questions tagged with this topic.

NMRSpectroscopy

The coupling constant in ¹³CH₃Cl proton NMR is:

One-bond carbon-proton coupling in 13CH3Cl arises from Fermi contact interaction between directly bonded magnetic nuclei, magnitude proportional to gyromagnetic ratios and s-character, typically 125 to 160 Hz for sp3 carbon, much larger than vicinal proton-proton couplings near 7 Hz. Observed value around 140 Hz reflects strong interaction in chlorinated alkane, producing widely spaced doublet satellites in proton spectrum when decoupling absent. This large constant enables polarization transfer in INEPT and DEPT experiments, underpins HSQC sensitivity enhancement, and is exploited for carbon editing in metabolomics and protein-labeled samples.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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NMRSpectroscopy

What is the typical J-coupling constant for an α-helix?

Vicinal coupling constant 3JHN-Hα reflects phi dihedral angle via Karplus equation. Alpha helix constrains phi near -57 degrees, orienting amide proton and alpha proton in gauche arrangement with limited orbital overlap, producing small couplings between 3.9 and 5.5 Hz, often averaged around 4.8 Hz. Extended beta strands with phi near -120 degrees give trans-like overlap yielding larger couplings 8 to 10 Hz. Measuring 3J therefore supports helical conformation, aiding backbone assignment, validation of peptide design, and analysis of intrinsically disordered regions in solution NMR.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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