Topic 3.4: Auto/transverse relaxation + TROSY Flashcards
1
Q
- Describe the contributions of expressions to relaxation in 15N relaxation
A
- Spectral density samples different frequencies which are related to different transitions in a spin system where dipolar coupling is present between 2 spins
- ωH – ωN relates to transition where frequency relates to difference of Larmor frequency – ZQ transition
- ωH + ωN relates to sum of Larmor frequencies – DQ transition
- ωN related to N Larmor frequency
2
Q
- Briefly how is fast dynamics in the Nitrogen example characterised?
A
- Fast (ps – ns) backbone dynamics characterised in solution by measuring 15N R1, 15N R2 and 1H-15N NOE at at least one magnetic field using the Lipari-Szabo formalism
3
Q
- What is the difference between transverse and longitudinal relaxation expressions?
A
- transverse relaxation rate expression has additional term J(0) – spectral density sampled at 0 frequency
- the larger this term, the slower the motion (hence solution NMR line widths are boarder for larger molecules)
4
Q
- How is J(0) related to the overall magnetisation of a sample
A
- Relates to oscillating field in z direction that lead to change of Larmor frequency
- This change is different for each spin and leads to a loss of coherence/dephasing
- Loss of magnetisation due to this destructive interference
5
Q
- Different Larmor frequencies can also come from sources unrelated to motion e.g. inhomogeneous filed across a sample. What is the problem with this phenomenon in experiments?
A
- Problem with simply doing a 90o pulse into a transverse plane and allowing to relax is the other processes that have an effect of magnetisation but are unrelated to relaxation
- These cause inhomogeneous broadening and are a result of variations across sample causing a variation in field, shifting Larmor frequency of spins, leading to decay
6
Q
- How is transverse relaxation - T2 measured in order to overcome dephasing?
A
- Longitudinal relaxation – T1 was measured via inversion recovery
- Here, spin-echo experiment used to overcome inhomogeneous broadening
- After a 90o transverse plane flip (1), spins fan out (2) in xy plane as some spins have higher frequency than others
- A 180o pulse (3) flips everything back meaning slow spins are now ahead and all arrive at the same time, meaning differences in frequency can be separated out.
7
Q
- Using a sketch to aid your answer, describe the difference between longitudinal and transverse relaxation vs correlation time
A
- T1 has a minimum
- 1/ ω 0 represents minimum
- T2 continues to decrease with increasing τc
- Slower motion leads to a more efficient T2
- Difference indicates T1 is the most efficient relaxation
8
Q
- Why is the Lipari-Szabo approach needed to characterise dynamics and what is it?
A
- Difficult/impossible to know form of correlation function and therefore spectral density, which is needed to calculate relaxation rates
- Assumed before G(τ) was a simple decaying function and J(ω) was a Lorentzian.
- Lipari-Szabo model free approach uses similar assumptions where the contribution of each type of motion to J(ω) is modelled as a Lorentzian function of correlation times, τ, for each motion and amplitudes of motions expressed as the generalized order parameters S2
9
Q
- Highlight the differences in the form of LS equations describing spectral density in a solution and solid
A
- Order parameter = 0 – unrestricted motion
- Order parameter = 1 – rigid
10
Q
- How will the form of LS equations change when describing the spectral density of the same molecule as before, but with more motions?
A
- Add more Lorentzians to account for more internal motion
- Internal motions can be separated on to distinct timescales (fast and slow)
11
Q
- Draw a sketch to show the differences in correlation functions of a solid and a solution sample. Explain the differences in the sketch.
A
- Solid: internal motions decay correlation functions but as not sampled in all directions, slow motions can be detected on a larger range
- Solution: decays to 0 due to overall tumbling, and cannot probe relaxation motions slower than this tumbling timescale, therefore wont contribute to correlation function and relaxation rate.
12
Q
- Describe the size limitation in solution NMR
A
- Will come a point where molecule is so large, that it reorients slowly enough for peaks to become too broad to analyse
13
Q
- Describe the types of relaxation in the following molecule
A
14
Q
- What is TROSY?
A
- Transverse-Relaxation Optimised Spectroscopy is a method of analysing the cross-correlated interactions between two different interactions in a single motion
15
Q
- Describe how 1D TROSY works
A
- NMR line width depends on T2 (transverse)
- When the same motion modulates two correlated interactions (e.g. dipolar and CSA contributions) the effect of fluctuations adds or cancel depending on the spin state
16
Q
- Give an example of a 1D TROSY spectrum
A
- 1: relaxation contribution from 15N CSA which depends on distance of electron density around nucleus
- 2: 1H-15N dipolar coupling
- 1/2 oscillation in same period with opposite phase causes destructive interference and narrower peaks
- 1/2 in sync results in larger oscillation and higher contribution to relaxation
- CSA/dipolar coupling increase one another giving more efficient relaxation and broader peaks
17
Q
- Describe the following 2D TROSY spectrum
A
18
Q
Describe the effect of decoupling and the benefit of selecting the smallest peak in a TROSY spectrum
A
- Small narrow peak selection allows study of very large species as much of the broad messy peaks removed.
