The transferred NOE

Abstract

The transferred NOE (TrNOE, also called exchange-transferred NOE, et-NOE) is an experimental modification of the NOE experiment, designed to provide information on the structure of a receptor-bound ligand that is present in excess, and is in fast exchange between free and bound states. It is normally observed using a NOESYexperiment that includes a relaxation filter to remove interfering signals from the receptor. NOEs develop on the bound ligand but are observed on the spectrum of the free ligand following dissociation from the complex. It is a useful experiment, though it is necessary to carry out suitable controls, to demonstrate that (a) the NOEs are those from specific binding and not from weak nonspecific binding; (b) the NOEs observed are genuinely from the bound ligand, not the free ligand; and (c) the NOEs do not arise from spin diffusion. Incorporation of specific labeling patterns in the ligand and receptor can improve the signal-tonoise ratio and information content. The TrNOE works best for relatively weak complexes (dissociation constant 100 nM or weaker) and for larger receptors (>30 kDa up to very large, which can therefore include membrane-bound receptors and even whole cells). The TrNOE is closely related to saturation transfer difference (STD) and chemical exchange saturation transfer (CEST).