Derivation of equations for the temperature-dependent I_ab model

Simple intramolecular isomerization; temperature dependence

1. Considerations and Definitions

2. Derivation of a main equation

3. Define functions for equilibrium concentrations

4. Test if solution is meaningful

5. Check whether the solution satisfies all initial equation and conditions

6. Save results on disk for future use

Here, I am introducing temperature dependence into I_ab model.

1. Considerations and Definitions

clean up workspace

reset()

Set path to save results into:

ProjectName:="I_ab_T";

CurrentPath:="/Users/kovrigin/Documents/Workspace/Global Analysis/IDAP/Mathematical_models/NMR_line_shape_models/1D/I_ab_T/";

Temperature dependent parameters in the model

1. equilibrium constant, K_A;

2. rate constants of forward and reverse transitions, k_1_A and k_2_A;

3. chemical shifts of the species, w_Ra and w_Rb

4. line widths (transverse relaxation rate constants) of the species, FWHH_Ra and FWHH_Rb.

Let's consider them one-by-one.

Equilibrium constant, K_A

The K_A is dependent on temperature via the van't Hoff equation:

eq1_1:= log(K)= -dH/(R*T)+dS/R

Assumption 1: We will neglect temperature dependence of dH and dS in this treatment (dCp=0)

Rate constants

Assumption 2: We assume that Arrhenius's equation and Eyring's transition state theory are applicable.

The Arrhenius' law connects activation energy, Ea, temperature and the rate constant:

eq1_2:= k=A*exp(-E_a/(R*T))

The activation energy, Ea, is dH of the transition from Ra species to the transition state. The pre-exponential multiplier, A, includes the entropy change of the activation transition and the rate constant for conversion of the transition state into the Rb species, k_t.

eq1_3:= A=k_t*exp(dSa/R)

Assumption 3: Enthalpy and entropy of activation are not dependent on temperature.

Chemical shifts of the individual species

Chemical shift dependence on temperature may be modelled using empirical linear relationship

eq1_4:= w=a+b*T

where a and b coefficients are determined from fitting or fixed based on additional assumptions.

Line widths of the individual species

Line widths of the peaks will contain contributions from both the exchange dynamics and overall tumbling of the protein. To accurately fit the exchange contribution we need to estimate the base relaxation rate constant for the specific peak.

Considerations:

1. Base R2 is dependent on the overall tumbling of the molecule through a linear combination of spectral density functions.

2. Spectral density function may be modeled using Lipari-Szabo approach (Cavanaugh et al, 2006, p.368). In this treatment the R2 is roughly a linear function of the rotational correlation time.

eq1_5:= R2=a1+b1*tau_c

3.The rotational correlation time reduces with temperature according to eq 1.44 (Cavanaugh et al, 2006, p.21)

eq1_6:= tau_c= 4 * pi *eta_w*r_H^3/(3*k_B*T)

That is of the form:

eq1_7:= tau_c = b2*eta_w/(T)

4. Sovent viscosity in this equation, eta_w, is also inversely temperature-dependent.

// temperature, C, viscosity, microPa*s from CRC Handbook

data:=[

[0, 1793],

[10,1307],

[20, 1002],

[30, 798],

[40, 653]];

p1:=plot::PointList2d(data);

plot(p1);

The values for pure water from CRC Handbook, p. 6-2 indicate hyperbolic dependence of the kind

eq1_8:= eta_w= b3/T

Assumption 4: Changes in viscosity of the NMR sample maybe approximated by changes in pure water viscosity reasonably well.

As a result, the R2 may be approximated by a quadratic hyperbolic function of temperature

eq1_9:= eq1_5 | eq1_7 | eq1_8;

% | a1=a4 | b1=b4/(b2*b3)

This is the assumed functional dependency of R2 on temperature.

A semi-empirical approach could rely on analysis of linewidths of other resonances from similar protons in the spectrum, which do not undergo conformational exchange. We could fit these peaks with a lorentzian function where FWHH=2*R2 to determine coefficients a4 and b4. By observing large number of peaks we may conclude whether these parameters may be used as global parameters. I expect that b4 will be global parameter while a4 might be individual---reflecting unique R2 of each site.

References

1. Cavanaugh, Fairbrother, Palmer, Rance, Skelton (2006) Protein NMR Spectroscopy: Principles and Practice, Second Edition, Academic Press.

3. CRC Handbook of Chemistry and Physics 87th edition 2006-2007, Ed. Lide DR, Taylor and Francis,

TEMPLATE

Isomerization constant

These relationships serve as restraints for solve(), but not restrict these values in calculations!

K_A

K_A ;

assume(K_A > 0):

assumeAlso(K_A , R_):

Total concentrations

Rtot - total concentration of the molecule

Rtot;

assumeAlso(Rtot>0):

assumeAlso(Rtot,R_):

Common equilibrium concentrations

Raeq - equilibrium concentration of Ra form

Raeq;

assumeAlso(Raeq>0):

assumeAlso(Raeq<Rtot):

assumeAlso(Raeq,R_):

Rbeq - equilibrium concentration of Rb form

Rbeq;

assumeAlso(Rbeq>0):

assumeAlso(Rbeq<Rtot):

assumeAlso(Rbeq,R_):

anames(Properties,User);

2. Derivation of a main equation

Total concentrations of the molecule

eq2_1:= Rtot = Raeq + Rbeq

Write equilibrium thermodynamics equations

eq2_2:= K_A = Rbeq / Raeq

Express Raeq as a function of all constants and total concentrations

Express Rbeq

eq2_2;

solve(%,Rbeq);

%[1][1];

eq2_3:= Rbeq= %

Substitute to the mass balance equation

eq2_1;

% | eq2_3;

eq2_4:= %:

Express Raeq

eq2_4;

solve(%, Raeq);

eq2_5:= Raeq=%[1]

Express Rbeq

eq2_3;

eq2_6:= % | eq2_5;

Summary of equations for all species

Independent parameters:

Rtot, K_A

[Ra]

eq2_5

[Rb]

eq2_6

Generate functions

fRaeq:= (Rtot, K_A) --> eq2_5[2]

fRbeq:= (Rtot, K_A) --> eq2_6[2]

4. Test if solution is meaningful

Set some realistic values for constants:

Total_R:=1;

Ka:=0.5;

Test that all equilibrium concentrations are positive values:

fRaeq(Total_R, Ka);

fRbeq(Total_R, Ka);

if (fRaeq(Total_R, Ka)>0 and

fRbeq(Total_R, Ka)>0

)

then

print(Unquoted,"Solution is meaningful.");

else

print(Unquoted,"WARNING!!!!");

print(Unquoted,"Solution is NOT meaningful: some concentrations become negative!");

end_if

Solution is meaningful.

5. Check whether the solution satisfies all initial equation and conditions

Here are all original independent

equations:

eq2_1;eq2_2;

Test eq2_1

eq2_1;

% | eq2_5 | eq2_6;

normal(%);

bool(%)

The found solution satisfies all original equations

5. Save results on disk for future use

(you can retrieve them later by executing: fread(filename,Quiet))

ProjectName

Eq_Raeq_I_ab:= eq2_5;

Eq_Rbeq_I_ab:= eq2_6;

Reassign function names

fRaeq_I_ab:=fRaeq:

fRbeq_I_ab:=fRbeq:

filename:=CurrentPath.ProjectName.".mb";

write(filename,Eq_Raeq_I_ab,Eq_Rbeq_I_ab, fRaeq_I_ab, fRbeq_I_ab)

Conclusions

1. I derived analytical solutions for the I_ab system