NMR Flashcards
In 1D FT NMR
What is D1?
Relaxation delay
In 1D FT NMR
What is T1?
Spin-lattice relaxation time
In 1D FT NMR
What is T2?
Spin-spin relaxation time
In 1D FT NMR
What is AQ?
Acquisition time
In 1D FT NMR
What is PW?
Size of the flip angle
In 1D FT NMR
What is TR?
Repetition time
In 1D FT NMR
What does TR equal to?
D1+AQ
If a quantitative spectra is to be acquired with a 90 deg flip angle WHAT should be set 5-7 times larger than WHAT
TR more than T1
of the slowest relaxing spin
What does reducing the flip angle lead to?
2 points
- Reduction in TR
- Reduced sensitivity
What is Earnst angle?
Optimal flip anfle for TR/T1 ratio
Why is using a smaller value of PW benifitial to shorter relaxation time?
Not all the magnetisation is tipped into the xy plane, therefore it takes shorter time for the spins to relax back to equilibrium
What happens to integrals if TR is too short
Wrong integral values
As not all the nuclei have relaxed back
For spin-spin relaxation
Why is there no point to aquire the signal after 3T2
Noise dominates the signal
What will happen to the resolution of NMR lines if T2 is too long?
Decreases, as noise domitates the signal, so fine structure of the lines is lost
For applying weighted functions prior to FT
What does Lorentz to Gauss converstion do?
To peaks and S/N
2 points
- Narrower signals
- Better S/N than regular FID, worse than exponential curve
For applying weighted functions prior to FT
What does exponential curve do?
To peaks and S/N
2 points
- Exponential line broadening
- Better S/N than regular FID and Gaussian
EXSY
What does the exchange regime depend on?
2 points
- rate of chemical exchange
- chemical shift separation in Hz ,between the two peaks
EXSY
What is rate of chemical exchange affected by?
Temperature
EXSY
What is the chemical shift separation in Hz ,between the two peaks affected by?
Spectrometer frequency
Coalescence temperature
Temperature when the 2 signals merge into 1 peak of rotamers
Why does COSY/HSQC make it easy to determine rotamers?
Cross peaks are only observed between the same isomer
EXSY
Why does NOESY show peaks even for different rotamers?
Due to mixing time, the configuration of the molecule changes
EXSY
In 2D NOESY what do the positive peaks show when negative x-peaks indicate chemical exchange?
Proximity in space
What is the basis of EXSY experiments in ligand-proteins
The basis of these experiments is the fact that the NH chemical shift is different in the free protein and in the complex, but only for amino acids that are part of the binding site or are in its vicinity
What is NOE and what does it stand for?
The nuclear Overhauser effect (NOE) is the transfer of nuclear spin polarization from one population of spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via cross-relaxation
Describe the properties of a small molecule
5 points
- < 100 D
- Short rotation correlation times
- Long relaxation times (T1, T2)
- Fast translational diffusion
- Weak positive NOE
Desribe the properties of biomacromolecule
5 points
- > 5 kD
- Long rotational correlation times, slow tumbling
- Short relaxation times (T2)
- Slow translational diffusion
- Strong negative NOE
How is binding of a ligand is detected?
2 points
- A small molecule behaving like a large molecule
- Transfer of magnetisation from a macromolecule to a ligand
What is the purpose of the T2 filter?
Keep the spins in the transverse plane where they relax
Desctibe the process of obtaining a T2 relaxation edited spectra
3 points
- Apply T2 filter to small molecules only, all signals will be preserved
- Then apply the filter to protein + small molecules, some signals will be lost
- Subtract 2 from 1, the binder becomes visible
Advantages of selective excitation
2 points
- Fast acquisition for 1D/2D spectra, as only area of interest is sampled
- High digital resolution, allows to extract coupling constants
Disadvantages of selective excitation
4 points
- Automation is problematic
- If too many signals need inspecting, it may be more efficient to acquire 2D spectra
- If signals are not separated, selective excitation is more complex
- Selective 1D or nonselective 2D 1H NMR experiments become problematic when significant signal overlap is present
