EXSY

Exchange Spectroscopy (EXSY) is a two-dimensional nuclear magnetic resonance (NMR) experiment used to study chemical exchange processes between chemically or conformationally distinct states. In EXSY, magnetization can be transferred between exchanging sites during a mixing period, generating cross-peaks that connect resonances of sites that interconvert. Diagonal peaks correspond to sites that do not exchange during the mixing time, while off-diagonal cross-peaks indicate exchange between sites. In contrast to NOESY/ROESY cross-peaks, EXSY cross-peaks arise from chemical exchange rather than through-space dipolar coupling, enabling the measurement of exchange rates and populations.

Experimentally, EXSY is implemented as a 2D NMR experiment (often 1H, but applicable to other nuclei) with a controlled mixing time during which exchange can occur. By recording spectra at different mixing times, the buildup and decay of exchange cross-peaks can be analyzed to extract kinetic parameters k_AB and k_BA and the populations of the states, under appropriate relaxation conditions. Variants include selective EXSY, where only specific resonances are excited, and EXSY-based methods integrated with TOCSY for larger networks of exchanging species.

Applications of EXSY include characterizing tautomerism, conformational exchange in flexible molecules, ligand binding and unbinding kinetics, solvent exchange processes, and proton exchange rates. It is widely used in organic chemistry, biochemistry, and materials science to quantify exchange rate constants and to identify exchanging species that are closely related in chemical shift.

Limitations include spectral overlap, relaxation losses during the mixing time, and the need that exchange occur on a timescale accessible to NMR during t_m. Proper interpretation requires careful control experiments to distinguish exchange from non-exchange cross-peaks.