Presence of a paramagnetic center in a molecule affects its NMR
spectrum (blue dots) with respect to the one of a diamagnetic analogue
(green dots). The change in relaxation rates, in chemical shifts and
signals splitting provide restraints called paramagnetic relaxation
enhancement (PRE), preudocontact shift (PCS), or contact shift (CS)
and paramagnetism-induced residual dipolar coupling (RDC), respectively.
Paramagnetic effects on nuclear relaxation and chemical shifts are
generally smaller for 13C with respect to 1H,
and protonless NMR allows to measure observables closer to the paramagnetic
center with respect to proton NMR. The approach can be applied for
structural purposes to establish the reciprocal orientation of two
interacting partners or to establish long-range crosstalk within a
protein.