NMR Lecture 4

Question 1

What is the definition of the NOE enhancement factor? Know the equations for transient NOE enhancement. Know the relationship between NOE enhancement and the distance of nuclei. What factors influence the observation of transient NOE?

Answer 1

NOE enhancement factor is a measure of the increase in signal intensity observed in a NMR experiment due to the presence of the NOE.
Equation:
NOE = (Intensity observed with saturation - intensity observed without saturation)/(intensity observed without saturation)
Transient NOE enhancement factor is inversely proportional to the distance of nuclei (Dipolar interaction strength decreases rapidly with increasing distance)
Factors:
- proximity of nuclei
- spectral overlap
- relaxation times
- saturation time
- molecular motion
- experimental parameters

Question 2

What does pulse sequence mean? List three components of a pulse sequence.

Answer 2

Pulse sequence describes a set of radio frequency pulses applied to a sample to produce a specific form of NMR signal.
1) Pulse: excite, invert or decouple the magnetization
2) Delay: time intervals between pulses allow the spin magnetization to evolve
3) Data acquisition: time for receiver to record FID signal.

Question 3

Be able to describe the pulse sequence for spin-echo experiment.

Answer 3

The pulse sequence is useful for mitigating the effects of magnetic field inhomogeneities and providing a more accurate measurement of NMR signals.
1) 90 degree pulse tips the nuclear spins from their equilibrium along the z-axis to the xy plane.
2) FID: nuclear spins precess around the magnetic field to produce FID, leading to phase dispersion and signal decay.
3) 180 degree pulse: applied midway through the evolution period to reverse the phase dispersion introduced during the FID.
4) Echo formation: results in the generation of a spin echo, occurring at twice the time between the 90 degree and 180 degree pulses.
5) Signal detection: spin echo is detected in the presence of inhomogeneities, resulting in an improved and refocused NMR signal.

Question 4

Explain how the magnetization of different nuclei become refocused in the end.

Answer 4

1) Inhomogeneous broadening: spread of frequencies, resulting in line broadening in the NMR spectrum
2) 180 degree pulse: swaps the positions of nuclei experiencing positive and negative frequency offsets due to inhomogeneities
3) Spin echo formation: as inverted magnetization evolves, the inhomogeneous contributions partially cancel each other out during the formation of the spin echo

Question 5

Know the definition of B0 and B1 field.

Answer 5

B0: static magnetic field
B1: magnetic field generated by an RF pulse

Question 6

What does 90 degree pulse mean? What does a 180 degree pulse mean?

Answer 6

90 degree pulse: burst of radiofrequency radiation applied perpendicular to the B0 that causes the nuclear spins to move from equilibrium position along the z-axis to the xy-plane
180 degree pulse: RF pulse applied opposite to the B0 direction that causes the nuclear spins to flip by 180 degrees, effectively reversing the transverse magnetization.

Question 7

What is NMR relaxation?

Answer 7

NMR relaxation refers to the process by which the nuclear spins in a sample return to their equilibrium state after being perturbed by a radiofrequency pulse.
- sensitive to slow molecular motions and act as a probe for accessing such information

Question 8

What are the definitions of NMR T1 and T2 relaxations? What is the major mechanism for NMR T1 and T2 relaxation?

Answer 8

T1 relaxation: excited spins return to the ground state
- interactions between nuclear spins and their immediate environment
- transfer of energy from the nuclear spins to the surrounding environment, allowing the nuclear spins to return to their equilibrium alignment along the z-axis
T2 relaxation: loss of spin coherence in the xy plane
- decay of the transverse magnetization in the xy-plane due to interactions that cause dephasing of nuclear spins
- inhomogeneous broadening of the magnetic field cause nuclear spins to experience different resonance frequencies, leading to dephasing of the spins and decay of transverse magnetization

Question 9

Know the dependence of T1 and T2 relaxations over molecular tumbling time, τc. What kind of molecular information can be derived from T1 and T2 relaxation?

Answer 9

T1 and T2 relaxation is influenced by τc.
- τc&raquo_space; T1, the relaxation rate is proportional to 1/τc. Fast molecular motion leads to efficient relaxation and T1 is sensitive to the rotational correlation time of the molecule
- T1 is most efficient when rate of tumbling motion is near NMR frequency

• τc&raquo_space; T2, the relaxation rate is proportional to 1/τc. Fast molecular motion leads to efficient dephasing of nuclear spins in the transverse plane
• T2 increases as tumbling gets slower (NMR peaks become broader)
• molecular size and shape
  molecular flexibility and dynamics
• protein dynamics
• interaction studies