Practice question

Three-bond coupling constant between amide proton and alpha proton correlates with phi angle via Karplus curve. Beta-strand geometry places these protons trans to each other, phi near -120 to -140 degrees, maximizing coupling orbital overlap, resulting in large 3J values typically 8 to 10 Hz, often quoted around 8.5 Hz for antiparallel sheet. Helical phi near -60 degrees yields smaller values near 4 to 5 Hz. Observing 8.5 Hz along extended segment supports beta conformation, helping characterize amyloid fibrils, silk proteins, and enzyme active site strands.