Kern-Overhauser-Effect

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Energy level diagrams for a system consisting of two nuclear spins (AX spin system) a) (left) The state of equilibrium, determined by the Boltzmann distribution, b) (center) The energy levels of one nucleus ( A ) are equalized by saturation, the dipolar coupling with surrounding nuclei enables the transitions W 0 and W 2 , c) (right) changing the intensity of the other nucleus (X)

The Nuclear Overhauser Effect (engl. Nuclear Overhauser effect , NOE), named after Albert Overhauser , is an effect in nuclear magnetic resonance spectroscopy (NMR). It was discovered in 1965 by Frank Anet and Anthony Bourn , who kept the resonance of a proton or a proton group permanently in saturation during the NMR measurement with a second transmitter . In this decoupling experiment it could be observed that the resonance intensity of spatially adjacent protons increases considerably.

The nuclear Overhauser effect must be distinguished from the Overhauser effect, which acts between nuclei and unpaired electrons and was discovered by Albert Overhauser in 1951–1953.

The NOE experiment can be illustrated using a dipolar coupled two-state system of two nuclei A and X ( AX spin system). These should be spatially close. The J coupling is unaffected by this view, so J = 0. In this system there are the four states that are characterized in the order of increasing energy by | α A α X >, | α A β X >, | β A α X > and | β A β X >. These four nuclear spin states can also be found in the figure on the right.

For example, an RF pulse induces all transitions from spins of the A nuclei. Absorption takes place until the population of the lower and upper levels of the two states involved in the absorption is the same ( saturation ) and there is no longer a state of equilibrium . These transitions are: | α A α X > → | β A α X > and | α A β X > → | β A β X >.

In the figure above, the line thicknesses of the states | β A α X > and | β A β X > increase (increase in population), those of | α A α X > and | α A β X > , according to the changing occupation ratios however, since they are relatively depopulated. As a reminder: in the state of equilibrium, i.e. before the first RF pulse, the occupation of all four spin levels is different (cf. Figure a)). In equilibrium, it is determined by the Boltzmann distribution . It is also important to note that in an NOE experiment, core A is excited (i.e. saturated), but core X is detected.

The intensity of a second, time-delayed pulse, for example at the resonance frequency of X , that is ω (X), is largely determined by how the two-spin system, saturated in the previous step, relaxes. The dipolar coupling is the main driving force for relaxation . The dipolar coupling (also referred to as direct coupling ) represents the second coupling mechanism of NMR spectroscopy in addition to the scalar coupling (also J coupling, indirect (spin-spin) coupling). The relaxation rate resulting from the dipolar coupling and thus the intensity I (NOE) depends on the distance r between the two dipoles ( here : atomic nuclei A and X )

The relaxation behavior of the second core can only be influenced sufficiently if the distances between the cores AX (through space) are small enough (≤5.5  Å ). The dipolar coupling between the nearby nuclei enables different relaxation times for the | β A > or | β X > populations and thus the transition probability | α A > → | β A > or | α X > → | β X > for the second pulse (ω (A) or ω (X)). If the relaxation dynamics results in a lower population of | β A α X > and a higher population of | α A α X > than that in equilibrium, the intensity of the transition | α A > → | β A > is increased.

If all other relaxation mechanisms are neglected and only the dipolar relaxation is taken into account, the theory of NOE according to the Solomon equation gives a maximum (Kern-Overhauser) gain factor of

.

According to the formula for the total intensity

or

the maximum gain in the homonuclear 1 H experiment can thus be 50%.

The NOE is used today for many NMR experiments to determine the structure and conformation of large and small biomolecules and their interaction. This includes the structure determination of peptides and proteins . The aim of an NOE experiment is still a semiquantitative distance measurement but also signal amplification by means of NOE in order to be able to measure insensitive nuclei (e.g. 13 C) with reasonable effort ( 1 H broadband decoupling in 13 C NMR spectroscopy).

Individual evidence

  1. ↑ a b Russel S. Drago: Physical methods for chemists. Surfside Scientific Publishers, Gainesville 1992, 2nd Edition, ISBN 0-03-075176-4 , pp. 306-309.
  2. ↑ a b Harald Günther: NMR spectroscopy. Thieme, Stuttgart 1992, 3rd edition, ISBN 3-13-487503-9 , pp. 355-364
  3. ↑ Joseph B. Lambert, Scott Gronert, Herbert F. Shurvell, David A. Lighner: Spectroscopy. Pearson, Munich 2012, 2nd edition, ISBN 978-3-86894-146-3 , pp. 209-217.
  4. ↑ H. Duddeck, W. Dietrich, G. Tóth: Structure Elucidation by Modern NMR. Springer, Berlin Heidelberg 1998, 3rd edition, ISBN 978-3-7985-1111-8 , p. 42.

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