Topic 2: Chemical change

Question 1

• What is chemical exchange?

Answer 1

• Rate of exchange between environment A and B represents the transition probability per unit time between the two.
• Molecule A/B has a certain interaction with a chemical shift associated with it

Question 2

• Give examples of processes that can be probed using exchange methods

Answer 2

• Chemical reactions (A ⇌ B)
• Conformational change (e.g. helix ⇌ coil)
• Side chain flip

Question 3

• Sketch the chemical exchange regimes of a system of equal populations and characterise each regime

Answer 3

Question 4

• How would chemical exchange regimes change for a system with unequal populations

Answer 4

• Populations are a weighted average causing overall shift in fast exchange
• Slow exchange results in larger intensity of higher populated state

Question 5

• What is the position of the line under fast exchange in the following cases?
  
  • i) p_A = 0.5, p_B = 0.5
  • ii) p_A = 0.2, p_B = 0.8

Answer 5

• Fast exchange therefore |ω_A – ω_B| < k_ex; single line observed
• ω_obs = pAω_A + pBω_B
• i) ω_obs = 0.5ω_A + 0.5ω_B
• = 0.5*0 + 0.5Δω (can set one to 0 as change in)
• = 0.5Δω
• ii) ω_obs = 0.2ω_A + 0.8ω_B
• = 0.2*0 + 0.8Δω
• = 0.8Δω
• Peak shifts to the right as higher weight

Question 6

• (?)Describe the rate of exchange, k_exin terms of chemical shift (dont think kex is deltav)

Answer 6

Question 7

• Describe the different exchange regimes and what experimental techniques are used to analyse them

Answer 7

Question 8

• Very Slow exchange characteristics; k_ex << Δω
• Two …
• … proportional to the populations in the states: K_eq = I_B/I_B where I_X is the … of peak in environment X
• Exchange may have … /… effect on line width (Δv?)
• Exchange rate found via … cross peak intensity

Answer 8

• Very Slow exchange characteristics; k_ex << Δω
• Two resonances
• Intensity proportional to the populations in the states: K_eq = I_B/I_B where I_X is the intensity of peak in environment X
• Exchange may have little/no effect on line width (Δv?)
• Exchange rate found via EXSY cross peak intensity

Question 9

• Slow exchange characteristics; k_ex < Δω
  
  • Two slightly … resonances
  • … constant and exchange rates obtainable (1)
  • Exchange rates obtained from additional … (2)

Answer 9

• Slow exchange characteristics; k_ex < Δω
  
  • Two slightly broader resonances
  • Equilibrium constant and exchange rates obtainable (1)
  • Exchange rates obtained from additional broadening (2)

Question 10

• Slow to intermediate exchange characteristics; k_ex ≈ Δω
  
  • Two … to … broad resonance(s)
  • … … … used to extract exchange parameters
  • Very difficult to extract

Answer 10

• Slow to intermediate exchange characteristics; k_ex ≈ Δω
  
  • Two broad to one broad resonance(s)
  • Line shape fitting used to extract exchange parameters
  • Very difficult to extract

Question 11

• Fast exchange characteristics; k_ex > Δω
  
  • … resonance at frequency given by … average of chemical shifts in individual environments
  • … due to exchange proportional to square Δω; means R₂ will change quadratically with linear change of magnetic field
  • Exchange contributes to … … and causes broadening

Answer 11

• Fast exchange characteristics; k_ex > Δω
  
  • Single resonance at frequency given by weighted average of chemical shifts in individual environments
  • Broadening due to exchange proportional to square Δω; means R₂ will change quadratically with linear change of magnetic field
  • Exchange contributes to R₂ relaxation and causes broadening

Question 12

• How does exchange affect the transverse relaxation rate?

Answer 12

• Exchange process results in additional contribution to transverse relaxation rate
• Contribution can be removed through use of transverse field (pulses)

Question 13

• How is the exchange contribution to transverse relaxation rate measured?

Answer 13

• Using relaxation dispersion, k_ex can be extracted by measuring R₂ as a function of the amplitude of the transverse field.

Question 14

• A single observed line could indicate fast exchange, or slow exchange with one very small population. How can these be distinguished further?

Answer 14

• Temperature: slow exchange resonances broaden at higher temperatures
• However, reverse not necessarily true i.e. narrowing line at high T doesn’t always indicate fast exchange
• Magnetic field: slow exchange, line width contribution for exchange independent of magnetic field (1)
• Fast exchange, line width contribution for exchange grows as square of magnetic field (2)