Which 2D-NMR method reveals through-space interactions?
Two-dimensional NOESY relies on dipole-dipole cross-relaxation transferring magnetization between nuclei close in space, typically within 0.5 nanometer, regardless of covalent bonding. COSY and TOCSY instead transfer coherence via scalar J-coupling through bonds, mapping covalent spin systems. HSQC correlates directly bonded heteronuclei like proton to carbon 13 or nitrogen 15. For biomolecular three-dimensional architecture, through-space contacts are essential because tertiary folding brings distant residues into proximity. NOESY distance restraints enable calculation of protein and RNA solution structures, distinguishing folded from unfolded states reliably.
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.