Chemistry Reference
In-Depth Information
When small molecules bind to the receptor, the NOEs undergo drastic changes
leading to the observation of transferred NOEs (trNOEs). These changes are the
basis for a variety of NMR experiments that are designed to detect and
characterize an intermolecular interaction [3]. The well-known trNOEs are
described by a short correlation time and small positive NOE values for a small
molecule (ligand), while a large molecule (receptor) presents a long correlation
time and large negative NOE values. Consequently, let us consider a protein-
ligand system, where the ligand is present in excess and is in fast chemical
exchange. In this case, the large negative NOE acquired by the ligand while it was
at the binding site overcomes the small positive NOE of the free ligand (Scheme
2). The observed response will be dependent on the fraction of bound ligand
(among some other parameters) [28].
We describe subsequently, some of the NMR techniques based on the trNOE
phenomenon that have been used to detect and characterize intermolecular
interaction in combinatorial libraries and other complex compound mixtures.
Scheme 2:
Schematic illustration of the methods using magnetization transfer. For these
experiments, protein resonances are saturated selectively without affecting resonances of the
ligand. Compounds that interact (rhombus) with the protein shows the saturation effect (parts of
the bound ligand colored in darker gray), which indicate reduced signal intensities. Non-binding
molecules (oval circles) are and behave like small molecules. (Adapted from Refs. [3, 26]).
NOE Pumping and Reverse Pumping
In the NOE pumping experiment the magnetization transferred from the receptor
to the ligand can be observed, while in its related reverse NOE pumping
experiment the reverse process of magnetization transferred from the ligand to the
receptor is monitored.
