Difference between revisions of "DNA Melting Thermodynamics"

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[[Category:20.309]]
 
[[Category:20.309]]
 
[[Category:DNA Melting Lab]]
 
[[Category:DNA Melting Lab]]
==DNA solution==
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{{Template:20.309}}
 +
 
 +
==DNA in solution==
  
 
{{LecturePoint|Consider a solution containing equal quantities of complementary single stranded DNA (ssDNA) oligonucleotides <math>\left . A \right .</math> and <math>\left . A' \right .</math>.}}
 
{{LecturePoint|Consider a solution containing equal quantities of complementary single stranded DNA (ssDNA) oligonucleotides <math>\left . A \right .</math> and <math>\left . A' \right .</math>.}}
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[[Image:DNA_strands_in_solution.gif]]
 
[[Image:DNA_strands_in_solution.gif]]
  
{{LecturePoint|The forward reaction where two ssDNA oligos combine to form dsDNA is called annealing. The reverse process is called thermal denaturation or melting.}}
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{{LecturePoint|The forward reaction in which two ssDNA oligos combine to form dsDNA is called annealing. The reverse process is called thermal denaturation or melting.}}
  
{{LecturePoint|At low temperatures, dsDNA is favored. As the temperature rises, dsDNA increasingly separates into its component ssDNA oligos.}}
+
{{LecturePoint|At low temperatures, dsDNA is favored. As the temperature rises, dsDNA increasingly separates into its component ssDNA oligos. (Think about why with respect to enthalpic and entropic considerations.)}}
  
 
{{LecturePoint|The melting temperature, <math>\left . T_m \right .</math>, is defined to be the point where half of the dsDNA is denatured.}}
 
{{LecturePoint|The melting temperature, <math>\left . T_m \right .</math>, is defined to be the point where half of the dsDNA is denatured.}}
  
{{LecturePoint|Short sequences of about 10-40 base pairs (such as those used in the DNA Melting lab) tend to denature all at once, while longer sequences may melt in segments.}}
+
{{LecturePoint|Short sequences of about 10-40 base pairs (such as those used in the DNA Melting lab) tend to denature all at once. Longer sequences may melt in segments.}}
  
{{LecturePoint|Less energy is required to split the double hydrogen bond of A-T pairs than the triple bond of G-C pairs. Thus, A-T rich sequences tend to melt at lower temperatures than G-C rich ones.<ref>Breslauer et al., [http://www.pnas.org/content/83/11/3746.full.pdf Predicting DNA duplex stability from the base sequence] PNAS 83: 3746, 1986</ref> Several web tools are available to predict the melting temprature. (See, for example, [http://mfold.rna.albany.edu/?q=DINAMelt/Hybrid2 DINA Melt] or [http://www.basic.northwestern.edu/biotools/oligocalc.html Oligocalc].)}}
+
{{LecturePoint|Less energy is required to split the double hydrogen bond of A-T pairs than the triple bond of G-C pairs. Thus, A-T rich sequences tend to melt at lower temperatures than G-C rich ones.<ref>Breslauer et al., [http://www.pnas.org/content/83/11/3746.full.pdf Predicting DNA duplex stability from the base sequence] PNAS 83: 3746, 1986</ref>}}
 +
 
 +
Several web tools are available to predict the melting temprature. (See, for example, [http://mfold.rna.albany.edu/?q=DINAMelt/Hybrid2 DINA Melt] or [http://www.basic.northwestern.edu/biotools/oligocalc.html Oligocalc].)
  
 
[[Image:AT_Pairing.png]][[Image:GC_Pairing.png]]
 
[[Image:AT_Pairing.png]][[Image:GC_Pairing.png]]
  
==Equilibrium concentrations of ssDNA and dsDNA==
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==Fundamental equilibrium relationships==
  
 
{{LecturePoint|The concentrations of the reaction products are related by the equilibrium constant: <math>K_{eq} = \frac{\left [ A \cdot A' \right ]}{\left [ A \right ] \left [ A' \right ]}</math>}}
 
{{LecturePoint|The concentrations of the reaction products are related by the equilibrium constant: <math>K_{eq} = \frac{\left [ A \cdot A' \right ]}{\left [ A \right ] \left [ A' \right ]}</math>}}
  
{{LecturePoint|The value of <math>\left . K_{eq} \right .</math> is a function of temperature. According to the van't Hoff equation:}}
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{{LecturePoint|The value of <math>\left . K_{eq} \right .</math> is a function of temperature. We can equate the fundamental definition of the standard free energy change with its relationship to the equilibrium constant in solution:}}
 
:<math>
 
:<math>
 
\begin{align}
 
\begin{align}
 
\Delta G^{\circ} & = \Delta H^{\circ} - T \Delta S^{\circ}\\
 
\Delta G^{\circ} & = \Delta H^{\circ} - T \Delta S^{\circ}\\
& = -R T \ln K\\
+
& = -R T \ln K_{eq}\\
 
\end{align}
 
\end{align}
 
</math>
 
</math>
 
:where
 
:where
::<math>\Delta G^{\circ}</math> is the change in free energy
+
::<math>\Delta G^{\circ}</math> is the standard change in free energy
::<math>\Delta H^{\circ}</math> is the enthalpy change
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::<math>\Delta H^{\circ}</math> is the standard enthalpy change
 
::<math>\left . T \right .</math> is the temperature
 
::<math>\left . T \right .</math> is the temperature
::<math>\Delta S^{\circ}</math> is the entropy change
+
::<math>\Delta S^{\circ}</math> is the standard entropy change
::<math>\left . R \right .</math>is the [http://en.wikipedia.org/wiki/Gas_constant gas constant]
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::<math>\left . R \right .</math> is the [http://en.wikipedia.org/wiki/Gas_constant gas constant]
  
 
{{LecturePoint|Solving for <math>\left . K \right .</math>:}}
 
{{LecturePoint|Solving for <math>\left . K \right .</math>:}}
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</math>
 
</math>
  
==Equation for dsDNA fraction==
+
Note that the above equation can be differentiated with respect to temperature to yield the (perhaps once!) familiar van't Hoff equation.
 +
 
 +
==Theoretical relation of dsDNA fraction and thermodynamic parameters==
  
 
{{LecturePoint|In the lab, the fraction of dsDNA will be measured with a fluorescent dye that preferentially binds to dsDNA. As such, it will be useful to derive an equation that relates the fraction of dsDNA to temperature and the thermodynamic parameters.}}
 
{{LecturePoint|In the lab, the fraction of dsDNA will be measured with a fluorescent dye that preferentially binds to dsDNA. As such, it will be useful to derive an equation that relates the fraction of dsDNA to temperature and the thermodynamic parameters.}}
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{{LecturePoint|Similarly, let <math>\left . C_{DS} \right .</math> be the concentration of double stranded DNA: <math>C_{DS} = {\left [ A \cdot A' \right ]}</math>}}
 
{{LecturePoint|Similarly, let <math>\left . C_{DS} \right .</math> be the concentration of double stranded DNA: <math>C_{DS} = {\left [ A \cdot A' \right ]}</math>}}
  
{{LecturePoint|<math>\left . C_T \right .</math> is the total concentration of DNA. <math>\left . C_T = 2 C_{SS} + 2 C_{DS}\right .</math>}}
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{{LecturePoint|<math>\left . C_T \right .</math> is the total concentration of DNA strands. <math>\left . C_T = 2 C_{SS} + 2 C_{DS}\right .</math>}}
  
 
{{LecturePoint|Let <math>\left . f \right .</math> be the fraction of total DNA that is double stranded}}
 
{{LecturePoint|Let <math>\left . f \right .</math> be the fraction of total DNA that is double stranded}}
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</math>
 
</math>
  
{{LecturePoint|At the melting point, <math>f = \frac{1}{2}</math> and <math>K_{eq} = \frac {4}{C_T}</math>.}}
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{{LecturePoint|At the melting point, <math>f = \frac{1}{2}</math> by definition and thus <math>K_{eq} = \frac {4}{C_T}</math>.}}
  
{{LecturePoint|Substituting from equation 1:}}
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{{LecturePoint|Substituting for ''K<sub>eq</sub>'' from equation 1:}}
  
 
:<math>
 
:<math>
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</math>
 
</math>
  
{{LecturePoint|Taking the log of both sides and solving for <math>\left . T \right .</math>,}}
+
{{LecturePoint|Taking the log of both sides and solving for <math>\left . T \right .</math>:}}
 
:<math>
 
:<math>
 
T(f) = \frac{\Delta H^{\circ}}{\Delta S^{\circ}-R \ln(2f/C_T(1-f)^2)}
 
T(f) = \frac{\Delta H^{\circ}}{\Delta S^{\circ}-R \ln(2f/C_T(1-f)^2)}
 
</math>
 
</math>
  
==Simulating DNA melting==
+
We now know temperature as a function of dsDNA fraction for a given total DNA strand concentration and DNA identity (therefore'' &Delta;H'', etc.).
 +
 
 +
==Simulating DNA melting for tractable calculation of dsDNA fraction==
  
 
{{LecturePoint|For simulating DNA melting experiments, it will be convenient to have an expression for <math>\left . f \right .</math> in terms of <math>\left . T \right .</math>. Unfortunately, this gets pretty yucky. On the bright side, Matlab and Python are good at calculating yuck.}}
 
{{LecturePoint|For simulating DNA melting experiments, it will be convenient to have an expression for <math>\left . f \right .</math> in terms of <math>\left . T \right .</math>. Unfortunately, this gets pretty yucky. On the bright side, Matlab and Python are good at calculating yuck.}}
  
{{LecturePoint|Taking the log of both sides of equation 2 and using the quadratic formula (eliminating the nonphysical root):}}
+
{{LecturePoint|Taking the log of both sides of equation 2 (after re-substiting in equation 1 for simplicity) and using the quadratic formula (eliminating the nonphysical root):}}
 
:<math>
 
:<math>
 
f = \frac{1 + C_T K_{eq} - \sqrt{1 + 2 C_T K_{eq}}}{C_T K_{eq}}
 
f = \frac{1 + C_T K_{eq} - \sqrt{1 + 2 C_T K_{eq}}}{C_T K_{eq}}
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</math>
 
</math>
  
See the pages [[Matlab:Simulating DNA melting]] or [[Python:Simulating DNA Melting]]
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See the pages [[DNA Melting Part 1: Simulating DNA Melting - Basics]]. And if you're interested in a Python implementation see [[Python:Simulating DNA Melting]]
  
 
==References==
 
==References==
 
<References />
 
<References />
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 +
{{Template:20.309 bottom}}

Latest revision as of 21:43, 1 November 2013

20.309: Biological Instrumentation and Measurement

ImageBar 774.jpg


DNA in solution

$ \bullet $ Consider a solution containing equal quantities of complementary single stranded DNA (ssDNA) oligonucleotides $ \left . A \right . $ and $ \left . A' \right . $.
$ \bullet $ Complementary ssDNA strands bond to form double stranded DNA (dsDNA). The reaction is governed by the equation $ 1 A + 1 A' \Leftrightarrow 1 A \cdot A' $

DNA strands in solution.gif

$ \bullet $ The forward reaction in which two ssDNA oligos combine to form dsDNA is called annealing. The reverse process is called thermal denaturation or melting.
$ \bullet $ At low temperatures, dsDNA is favored. As the temperature rises, dsDNA increasingly separates into its component ssDNA oligos. (Think about why with respect to enthalpic and entropic considerations.)
$ \bullet $ The melting temperature, $ \left . T_m \right . $, is defined to be the point where half of the dsDNA is denatured.
$ \bullet $ Short sequences of about 10-40 base pairs (such as those used in the DNA Melting lab) tend to denature all at once. Longer sequences may melt in segments.
$ \bullet $ Less energy is required to split the double hydrogen bond of A-T pairs than the triple bond of G-C pairs. Thus, A-T rich sequences tend to melt at lower temperatures than G-C rich ones.[1]

Several web tools are available to predict the melting temprature. (See, for example, DINA Melt or Oligocalc.)

AT Pairing.pngGC Pairing.png

Fundamental equilibrium relationships

$ \bullet $ The concentrations of the reaction products are related by the equilibrium constant: $ K_{eq} = \frac{\left [ A \cdot A' \right ]}{\left [ A \right ] \left [ A' \right ]} $
$ \bullet $ The value of $ \left . K_{eq} \right . $ is a function of temperature. We can equate the fundamental definition of the standard free energy change with its relationship to the equilibrium constant in solution:
$ \begin{align} \Delta G^{\circ} & = \Delta H^{\circ} - T \Delta S^{\circ}\\ & = -R T \ln K_{eq}\\ \end{align} $
where
$ \Delta G^{\circ} $ is the standard change in free energy
$ \Delta H^{\circ} $ is the standard enthalpy change
$ \left . T \right . $ is the temperature
$ \Delta S^{\circ} $ is the standard entropy change
$ \left . R \right . $ is the gas constant
$ \bullet $ Solving for $ \left . K \right . $:
$ K_{eq} = e^\left [\frac{\Delta S^{\circ}}{R} - \frac{\Delta H^{\circ}}{R T} \right ] \quad (1) $

Note that the above equation can be differentiated with respect to temperature to yield the (perhaps once!) familiar van't Hoff equation.

Theoretical relation of dsDNA fraction and thermodynamic parameters

$ \bullet $ In the lab, the fraction of dsDNA will be measured with a fluorescent dye that preferentially binds to dsDNA. As such, it will be useful to derive an equation that relates the fraction of dsDNA to temperature and the thermodynamic parameters.
$ \bullet $ Let $ \left . C_{SS} \right . $ represent the concentration of either single stranded oligonucleotide: $ C_{SS} = {\left [ A \right ] = \left [ A' \right ]} $.
$ \bullet $ Similarly, let $ \left . C_{DS} \right . $ be the concentration of double stranded DNA: $ C_{DS} = {\left [ A \cdot A' \right ]} $
$ \bullet $ $ \left . C_T \right . $ is the total concentration of DNA strands. $ \left . C_T = 2 C_{SS} + 2 C_{DS}\right . $
$ \bullet $ Let $ \left . f \right . $ be the fraction of total DNA that is double stranded
$ f = \frac{2 C_{DS}}{C_T} = \frac{C_T - 2 C_{SS}}{C_T} = 1 - 2 \frac{C_{SS}}{C_T} $
$ \bullet $ Therefore, $ C_{SS} = \frac{(1 - f)C_T}{2} $
$ \bullet $ Now we can solve for $ \left . K \right . $ in terms of $ \left . f \right . $ and $ \left . C_T \right . $:
$ K_{eq} = \frac{C_{DS}}{C_{SS}^2} = \frac{f C_T / 2}{ [(1 - f) C_T / 2] ^ 2} = \frac{2 f}{(1 - f)^2 C_T} $
$ \bullet $ At the melting point, $ f = \frac{1}{2} $ by definition and thus $ K_{eq} = \frac {4}{C_T} $.
$ \bullet $ Substituting for Keq from equation 1:
$ e^\left [\frac{\Delta S}{R} - \frac{\Delta H}{R T} \right ] = \frac{2 f}{(1 - f)^2 C_T} \quad (2) $
$ \bullet $ Taking the log of both sides and solving for $ \left . T \right . $:
$ T(f) = \frac{\Delta H^{\circ}}{\Delta S^{\circ}-R \ln(2f/C_T(1-f)^2)} $

We now know temperature as a function of dsDNA fraction for a given total DNA strand concentration and DNA identity (therefore ΔH, etc.).

Simulating DNA melting for tractable calculation of dsDNA fraction

$ \bullet $ For simulating DNA melting experiments, it will be convenient to have an expression for $ \left . f \right . $ in terms of $ \left . T \right . $. Unfortunately, this gets pretty yucky. On the bright side, Matlab and Python are good at calculating yuck.
$ \bullet $ Taking the log of both sides of equation 2 (after re-substiting in equation 1 for simplicity) and using the quadratic formula (eliminating the nonphysical root):
$ f = \frac{1 + C_T K_{eq} - \sqrt{1 + 2 C_T K_{eq}}}{C_T K_{eq}} $
$ \bullet $ Substituting from equation 1 gives the desired result.
$ f = \frac{1 + C_T e^\left [\frac{\Delta S^{\circ}}{R} - \frac{\Delta H^{\circ}}{R T} \right ] - \sqrt{1 + 2 C_T e^\left [\frac{\Delta S^{\circ}}{R} - \frac{\Delta H^{\circ}}{R T} \right ]}}{C_T e^\left [\frac{\Delta S^{\circ}}{R} - \frac{\Delta H^{\circ}}{R T} \right ]} $

See the pages DNA Melting Part 1: Simulating DNA Melting - Basics. And if you're interested in a Python implementation see Python:Simulating DNA Melting

References

  1. Breslauer et al., Predicting DNA duplex stability from the base sequence PNAS 83: 3746, 1986