Practice question

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.