Nuclear Overhauser effect
The nuclear Overhauser effect ( engl. nuclear Overhauser effect, NOE ), named after Albert Overhauser, is an effect in nuclear magnetic resonance spectroscopy ( NMR). He was discovered in 1965 by Frank Anet and Bourn Anthony, in a decoupling experiment in which they saturated the resonance of a proton, observed that the intensity of the resonance of a spatially adjacent proton increased by 45%.
The NOE experiment can be illustrated by a coupled two-state system of two cores A and X ( AX spin systems ). These should be spatially close. The J-coupling is not affected by this perception. This coupled system, the states exist | α (A) α (x)>, | α (A) β (X) > | β (A), α (x)> and | β (A), β (x)>. Through a wide RF pulse, all transitions of the AX- two-spin system are induced to the population of all states is the same ( saturation), and there is no longer a state of equilibrium. The intensity of a second time-delayed pulse to the resonant frequency of A or X ( ω (A) and ω (X)) is largely determined by how this two-spin system relaxes saturated. For the relaxation of the dipolar coupling is the main driving force. Resulting from this coupling has a relaxation rate as a function of r (AX ) -6. The dipolar coupling between the nearby cores allows different relaxation time of | β (A) > and | β (X) > populations and thus the transition probability | α (A) > → | β (A) > and | α ( X) > → | β (X) > for the second pulse ( ω (A ) or ω ( x)). The relaxation behavior of the second core can only be influenced strongly enough when the distances between the nuclei AX ( through space, through space ) are small enough ( ≤ 5.5 Å). Gives the relaxation lower population of | β (A), α (x)> and a higher population of | α (A) α (x)> than that in the balance, the intensity of the transition | α (A) > → | β (A) > strengthened. Neglecting all other relaxation mechanisms and takes into account only the dipolar relaxation, the theory of the NOE results in a maximum gain of
According to the equation for the total intensity
Can thus be 50% of the maximum gain at the homonuclear 1H experiment.
The NOE is used today for many NMR experiments for the structure and Konformationsaufklärung large and small biomolecules and their interaction.