Lecture 3: Spin-Spin Coupling and the Nuclear Overhauser Effect Flashcards
What is the mechanism of spin spin (J) coupling? what is the difference between the origins of spin spin coupling and dipolar coupling?
- Nuclei that are close to each other exert an influence on one another’s magnetic field and shows up as an NMR signal when the nuclei are non equivalent.
- Dipolar coupling is a through-space coupling of two NMR active nuclei. Simply put, the dipolar coupling is the magnetic effect on nucleus 1 due to the magnetic field generated by nucleus 2. This is different from J coupling which are through-bond couplings.
What is the roof effect?
-For the case of two doublets due to two STRONGLY coupled protons, the inner lines are larger than the outer ones ( “roof effect” ).
Under what conditions will strong coupling be considered?
-Strong coupling will be considered when the value of ◇d < 10
Know how to draw the peak splitting patterns for nuclei with strong couplings.
See graph
What is the Karplus Curve? What structural information can be derived from the measurement of 3JHNH(alpha)??
- The Karplus curve is a graph of the karplus relation, which describes the correlation between J-coupling constants and dihedral torsion angles.
- The coupling constant of three bond couplings, are found to vary with the dihedral angles between the two H-C-C planes.
-The two major elements of secondary structure in proteins (helices and sheets) have characteristic H-C-Ca-H dihedral angles:
~Thus, amid-Ca proton-proton coupling constants smaller than 6Hz indicated a helical structure.
~Couplings of 7 Hz indicates Beta sheet structure.
What interaction can 3HJNC coupling be used to identify?
-This coupling constant identifies a through N-H - - -O hydrogen bond.
~The presence and strength of a N-H - - - O hydrogen bond can be characterized by the measured coupling constant.
~OPEN CIRCLES correspond to side chain carboxyl groups, H bonded to backbone amides.
Know the Definiton of spin decoupling. What is homonuclear coupling and heteronuclear coupling? What advantages does this technique have?
-Spin Decoupling: the one pulse experiment gives a doublet for each of the spins when there is J coupling between them. Spin 1 multiplet frequencies depend on the state of spin 2 in alpha or beta state. If selective RF irradiation (applied during detection) rapidly mixed the a and b states of spin 2, the spin multiplet will collapse and this process is decoupling.
~Decoupling: spin averaging of some nuclei cease their spin-coupling influence on adjacent nuclei, therefore causing the observed spin-spin splitting to be reduced.
~Homonuclear: the nuclei being decoupled and observed are of the same kind.
~Heterobhclear: the nuclei being decoupled and observed are of different kinds.
What sample condition is normally used to measure residual dipolar coupling?
-Weakly aligned mediums
What is the Nuclear Overhauser Effect (NOE)?
- The Nuclear Overhauser Effect (NOE): is the transfer of nuclear spin polarization from one population of spin-active nuclei to another via cross relaxation.
What is T1 NMR Relaxation?
T1 relaxation: spin-lattice relaxation time (T1), which is the decay constant for the recovery of the z component of the nuclear spin magnetization M towards equilibrium. T1 relaxation time involves redistributing the populations of the nuclear spin states in order to reach thermal equilibrium distribution.
What is T2 NMR Relaxation?
-T2 relaxation: Spin-spin relaxation time (T2), which is the decay constant for the component of M perpendicular to Bo.
~T2 relaxation corresponds to a decoherence of the transverse nuclear spin magnetization. Random fluctuations of the local magnetic field lead to random variations in the instantaneous NMR precession frequency of different spins. As a result, the initial phase coherence of the nuclear spins is lost, until eventually the phases are disordered and there is no net XY magnetization.
