Crosspeak

A crosspeak is a term used in nuclear magnetic resonance (NMR) spectroscopy, particularly in two-dimensional (2D) NMR experiments. It refers to a signal that appears at a frequency correlation between two different nuclei or spins in a molecule. Crosspeaks provide valuable information about the spatial proximity, through-space interactions, or scalar couplings between nuclei, which is crucial for determining the structure and dynamics of molecules.

In 2D NMR experiments, such as correlation spectroscopy (COSY), total correlation spectroscopy (TOCSY), nuclear Overhauser effect spectroscopy (NOESY), and rotating-frame Overhauser effect spectroscopy (ROESY), crosspeaks are observed along with diagonal peaks. Diagonal peaks represent the auto-correlation of a nucleus with itself, while crosspeaks indicate the correlation between different nuclei. The position of a crosspeak is defined by two frequency coordinates, each corresponding to the resonance frequency of one of the correlated nuclei.

The presence and intensity of crosspeaks depend on the specific NMR experiment and the physical interactions between the nuclei. For instance, in NOESY and ROESY experiments, crosspeaks arise from dipole-dipole interactions between nuclei that are close in space, typically within 5 angstroms. In COSY and TOCSY experiments, crosspeaks result from scalar couplings (J-couplings) between nuclei that are bonded or have a common bonding partner.

Crosspeaks are essential for assigning resonances, elucidating connectivity, and determining the 3D structure of molecules, including proteins, nucleic acids, and small organic compounds. They play a significant role in various fields, such as chemistry, biochemistry, structural biology, and materials science.