Question

What regular protein structural units are associated with NH-(alpha)H coupling constants of 4 Hz and 9 Hz, and why?

Answer 1

The region comprising the NH-αH-Region (g) is called the fingerprint-region. Spectral resolution and appearance in that part serves as anindicator whether the peptide/protein can be successfully investigated by NMR. For each amino acid (expect Pro) a single peak is observed in that region. The N-terminal amino proton cannot be observed due to very rapid exchange with the solvent. Scalar couplings can only be observed between protons separated by not more than 3 bonds. Moreover, the protons need to have a different resonance frequency. However, signal overlap, exchange broadening, small values of the JNH-aH-coupling constants (which in stable a-Helices is < 4 Hz), resulting from dihedral angles close to 90°, may lead to the fact that less than the expected number of signals are observed.