NOESY

NOESY, Nuclear Overhauser Effect Spectroscopy, is a two-dimensional nuclear magnetic resonance technique that reveals through-space interactions between nuclear spins, most commonly protons. In a NOESY experiment, magnetization is allowed to mix during a short mixing time; if two nuclei are close in space (typically within a few angstroms), magnetization is transferred between them via the Nuclear Overhauser effect, producing cross-peaks in the 2D spectrum. Diagonal peaks reflect the same nucleus, while cross-peaks indicate proximity.

NOESY spectra are widely used in structure determination, conformational analysis, and assignment support. They provide distance restraints for structure calculations; intensities roughly scale with inverse sixth power of distance, though factors such as dynamics and spin diffusion complicate interpretation. Mixing times are chosen to optimize NOE buildup; typical values are 100–300 ms for small molecules, and longer times (200–600 ms) may be used for larger macromolecules, with increased risk of spin diffusion.

In proteins and nucleic acids, NOESY provides many constraints used in computational structure refinement; NOE-derived distances are combined with dihedral angles and other data to generate 3D models. The technique also supports dynamic studies and interaction analyses, and can aid resonance assignment by correlating spatially close protons.

Limitations and variants: NOE intensities depend on molecular tumbling, so signals can be weak or negative for very large systems; ROESY (rotating-frame NOE) or other experiments may be preferred in such cases. Spectral overlap, solvent effects, and the need for isotopic labeling can complicate interpretation; careful experimental design and corroborating data are often required.