Transferred NOE technique

In the transferred NOE (trNOE) experiment, a ID or 2D NOESY experiment is recorded [31]. The intermolecular NOE build-up to be used as a parameter in Eq. (5) arises from the bound state but is observed via the free ligand, requiring rapid exchange between bound and free states. The technique is described in more detail in Chapt. 16. 

In the isotope edited/ filtered spectra of a protein-ligand complex, the species actually observed is generally the complex itself. This is an important difference from transferred NOE or saturation difference techniques, where the existence of an equilibrium between free and bound species - and a certain rate of exchange between them - is essential (Chapts. 13 and 16). The general conditions for isotope filtering/editing are therefore identical to those required for standard protein NMR sample concentrations are usually limited by availability and solubility of the components to the order of 1 mM. Considerably lower concentrations will reduce the sensitivity of the experiments to unacceptable levels,... 

In the situation where the interaction is weak, one of the traditional methods that can be applied to obtain structm-al information (internuclear distances) of the bound ligand is the so-called transferred NOE (trNOE) method. Recently, it became possible to use transferred cross-correlated relaxation (trCCR) to directly measure dihedral angles. The combined use of these two techniques significantly improves the precision of the structure determination of ligands weakly bound to macromolecules. 

Additional information on molecular conformation can be obtained by NOE, transfer NOE or 2-D homonuclear correlated NMR spectroscopy (COSY), which is used to measure the distance between nuclei. For detailed information on the various techniques see refs. 86 and 87. 

The necessity for labeled proteins and the size limitation can be overcome by techniques that monitor not protein NMR signals but ligand resonances, either through line-broadening experiments, transferred NOE measurements, or relaxa-... 

X-ray crystallography, docking modes can be validated by various NMR techniques NOEs may be observed between the ligand and the receptor protein by heteronuclear-filtered NOE spectroscopy [51], chemical shift changes of protein resonances upon binding can be analyzed by simulation of shifts caused by ring currents and electrostatic effects [52], and saturation transfer difference measurements indicate which part of the ligand is in direct contact with the protein [52]. 

Chemical shift correlated NMR experiments are the most valuable amongst the variety of high resolution NMR techniques designed to date. In the family of homonuclear techniques, four basic experiments are applied routinely to the structure elucidation of molecules of all sizes. The first two, COSY [1, 2] and TOCSY [3, 4], provide through bond connectivity information based on the coherent (J-couplings) transfer of polarization between spins. The other two, NOESY [5] and ROESY [6] reveal proximity of spins in space by making use of the incoherent polarization transfer (nuclear Overhauser effect, NOE). These two different polarization transfer mechanisms can be looked at as two complementary vehicles which allow us to move from one proton atom of a molecule to another proton atom this is the essence of a structure determination by the H NMR spectroscopy. 

The pioneering work in this field, a two-dimensional relayed-NOE experiment proposed by Wagner [7], was quickly followed by the appearance of several related NMR techniques [8-17]. Application of isotropic mixing during the J-transfer period yielded the 2D TOCSY-NOESY [11, 15] and NOESY-TOCSY [12, 14] experiments. When spin-lock sequences were applied to both J and NOE-transfers, the 2D TOCSY-ROESY and ROESY-TOCSY experiments [10, 16, 17] emerged. 

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