Practice question

In alpha helix, periodicity 3.6 residues per turn places successive amide protons approximately 2.8 angstroms apart, enabling strong dipole-dipole cross-relaxation observed as intense sequential dNN cross-peaks in two-dimensional NOESY. Continuous chain of Ni to Ni+1 NH-NH contacts along entire sequence is hallmark of helix, often accompanied by medium-range i to i+3 and i to i+4 contacts. Beta sheets show different pattern with strong CαH-NH and long-range interstrand contacts. Recognizing continuous amide-amide NOE ladder confirms helical folding in synthetic and natural peptides.