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MCB653/21/16 1
Lecture 20
Chemical Potential
Reading:
Lecture 20, today: Chapter 10, sections A and BLecture 21, Wednesday: Chapter 10: 10‐17 – end
MCB653/21/16 2
Pop Question 7 – Boltzmann DistributionTwo systems with lowest energy at 0 kBT. Energy levels separated by 3 kBT in system A, and 5 kBT in B. Calculate the partition coefficients for each system, then the probabilities of finding molecules in each energy level (use the energy levels from 0 to 15 kBT). System A: System B: U = 15kBT U = 12kBT U = 15kBTU = 9kBT U = 10kBTU = 6kBT U = 5kBTU = 3kBT U = 0kBTU = 0kBT
Simply apply the Boltzmann distribution:System A: System B: Q = e0+e‐3+e‐6+e‐9+e‐12+e‐15=1.0524 Q = e0+e‐5+e‐10+e‐15=1.0068Relationship between Q and the overall distribution of particles?Smaller steps between levels → larger Q → molecules spread more. Less likely to find molecules at any given levelWhy limit to <15 kBT? Because the contributions to the distribution (and the Q) of higher energy levels becomes negligible – i.e. the higher energy levels are unpopulated for all practical purposes.
p=0.000000291p=0.00000584p=0.000117p=0.0024p=0.0473p=0.9501
p=0.00000030384p=0.00004509330p=0.006692438p=0.993245928
MCB653/21/16 3
F0F1 ATP synthase performs an unfavorable reaction
Credits: John Walkerhttp://www.mrc-mbu.cam.ac.uk/research/atp-synthase
~50 kJ/mol stored per ATP ~15,000 kJ/mol per second
MCB653/21/16 4
Today’s goals
• Explore the link between concentration and the free energy change
• Chemical potential Molar free energy G˚i
• Equilibrium constant K:
• Reaction quotient Q and mass action ratio Q/K:
• Using G and K to look at biological systems:• ATP hydrolysis • Wednesday: acid‐base equilibria, protein folding
i Gni
T ,P ,n ji
G RT lnQK
Go RT lnK
MCB653/21/16 5
Chemical Potential
• The chemical potential (i) is defined as the rate of change of the free energy with respect to the number of molecules:
• Often in chemical reactions, we use moles (n):
• Molar free energy, G°i
i GNi
T ,P ,N ji
GNi
G(for one molecule of i)
i Gni
T ,P ,n ji
Gi
Units of energy(e.g. J)
Units of energy per mole(e.g. J mol-1)
MCB653/21/16 6
Chemical potential at play ‐ diffusion
ΔG <, >, or = 0? equilibrium: ΔG = ?
Figure from The Molecules of Life (© Garland Science 2008)
dG indNin outdNout 0indNin outdNin 0(in out )dNin 0dG dN
spontaneousdNout = -dNin
What happens to μin and μout at equilibrium?
in out
inout
[ ]in = [ ]out
inout
i GNi
T ,P ,N ji
MCB653/21/16 7
Direction of spontaneous change and
• System changing towards equilibrium:
• For a spontaneous change, dNin and (in‐out) should have opposite signs• If Nin decreases, dNin < 0• (in‐out) > 0 and in > out
• Molecules move spontaneously from regions of high chemical potential to low chemical potential
dG (in out )dNin 0
MCB653/21/16 8
Chemical potential and concentration
• In an ideal dilute solution,molecules do not influence each other and the enthalpy is independent of concentration.
• Assumption commonly used in biochemistry
• For an ideal dilute solution, we’ll show that the difference in chemical potential is related to the ratio of concentrations:
• Where C1 and C2 are the concentrations of molecules
2 1 kBT lnC2
C1
MCB653/21/16 9Figure from The Molecules of Life (© Garland Science 2008)
G1 H1 TS1 G2 H2 TS2
H1
NA1
T ,P ,NB
TS1
NA1
T ,P ,NB
A1 G1
NA1
T ,P ,NB
H2
NA 2
T ,P ,NB
TS2
NA 2
T ,P ,NB
A 2 G2
NA 2
T ,P ,NB
MCB653/21/16 10
2 1
H2
NA 2
T ,P ,NB
TS2
NA 2
T ,P ,NB
H1
NA1
T ,P ,NB
TS1
NA1
T ,P ,NB
2 1 TS2
NA 2
T ,P ,NB
TS1
NA1
T ,P ,NB
For an ideal solution, depends on entropy
ideal solution enthalpy changes are the same H2
NA 2
T ,P ,NB
H1
NA1
T ,P,NB
MCB653/21/16 11
Calculating the entropy:
• We can use the probabilistic definition of entropy, with three states:
Figure from The Molecules of Life (© Garland Science 2008)
p2 NB
M (B molecules)
p1 NA1
M (A molecules)
p0 M (NA1 NB )
M (empty gridboxes)
2
1
MCB653/21/16 12
S1 MkB pi ln pii0
2
S1
NA1
T ,P ,NB
MkB
NA1
p0 ln p0 p1 ln p1 p2 ln p2 p2 does not depend on NA1 d/dNA1 = 0
MkB
NA1
M (NA1 NB )M
lnM (NA1 NB )
M
NA1
Mln
NA1
M
MCB653/21/16 13
1 TS1
NA1
T ,P ,NB
kBT lnNA1
M
ln
M (NA1 NB )M
Applying simple rules for derivatives (chain rule, product rule, etc..) we get:
NB (solvent) >> NA1 (solute)
2 1 kBT lnNA 2
M
ln
M NB
M
ln
NA1
M
ln
M NB
M
2 TS2
NA1
T ,P ,NB
kBT lnNA 2
M
ln
M (NA 2 NB )M
kBT lnNA 2
M
ln
NA1
M
kBT ln
C2
C1
C2 C1
MCB653/21/16 14
Molecular diffusion decreases chemical potential
• If C2 > C1 then ln(C2/C1) > 0 and is positive• The A molecules in Region 2 have a higher chemical potential• Makes sense – molecules will move spontaneously from Region 2 (high concentration) to Region 1 (low concentration)
2 1 kBT lnC2
C1
Figure from The Molecules of Life (© Garland Science 2008)
MCB653/21/16 15
Chemical potentials
• Switching to molar units:
(multiply Boltzmann constant by Avogadro’s number R = NAkB)
• Calculating the chemical potential relative to standard state:
• ***C/C° (and therefore C/1M) is unitless
RT lnC2
C1
o RT ln
CCo
o RT lnC1
MCB653/21/16 16
What is chemical potential?
• Chemical potential is proportional to the logarithm of concentration:
• Comparing two solutions:
• Comparing to standard state:
• In mechanics, the direction of spontaneous change is always towards a reduction in potential energy
• Similarly, in thermodynamics, the direction of spontaneous change is always towards a reduction in Gibbs free energy• The partial molar Gibbs free energy (G˚i) of a type of molecule (i) is its “chemical potential” (˚i )
2 1 kBT lnC2
C1
o RT lnCCo
o RT lnC1
MCB653/21/16 18
Defining a “reaction progress variable”, ξ• Reaction:
• Change in free energy as the reaction progresses:
• These terms are NOT independent. To account for their coupling, we define the reaction progress variable (ξ or “xi”) which is a measure of how far the reaction has progressed
aA bB cC dD
dG A dnA B dnB C dnC DdnD
MCB653/21/16 19
Defining a “reaction progress variable”, ξ• E.g.
• 0 < < 1
Figure from The Molecules of Life (© Garland Science 2008)
2A 1B 1C 2D
MCB653/21/16 20
Defining a “reaction progress variable”, ξ• Reaction:
• Change in free energy as the reaction progresses:
• These terms are NOT independent. To account for their coupling, we define the reaction progress variable (ξ) which is a measure of how far the reaction has progressed
aA bB cC dD
dG A dnA B dnB C dnC DdnD
nA nA (0) a
dnB b(d)dnC c(d)dnD d(d)
For a small step in the reaction:
nB nB (0) bnC nC (0) cnD nD (0) d
dnA a(d)
MCB653/21/16 21
Reaching equilibrium• Substituting into the equation for dG:
• At equilibrium, dG = 0 and d can be non‐zero
• Products of chemical potential and stoichiometriccoefficients are balanced
dG A dnA B dnB C dnC DdnD
aA bB cC dD 0aA bB cC dD
A a(d) Bb(d) Cc(d) Dd(d)
aA bB cC dD d
dni ( /)id
MCB653/21/16 22
cCo dD
o aAo bB
o RT ln[C]c[D]d
[A]a[B]b Go
Equilibrium concentrations
[A], etc, refer to the equilibrium concentrations
[A]/1M is dimensionlessaA bB cC dD
A Ao RT ln
[A]1
B Bo RT ln
[B]1
C Co RT ln [C]
1
D Do RT ln
[D]1
aAo aRT ln
[A]1
bB
o bRT ln[B]1
cCo cRT ln [C]
1
dD
o dRT ln [D]1
aA bB
cc dD
MCB653/21/16 23
Defining the equilibrium constant
• We then define the equilibrium constant, K as:
• Keq is measurable
• Keq is unitless• G° is in J/mol and RT is also in J/mol
Keq [C]c[D]d
[A]a[B]b
Go RT lnKeq
Keq eG oRT
Equilibrium constant provides a way to determine the concentrations, the extent of reaction, at equilibrium
MCB653/21/16 24
Extent of reactions at equilibrium• Hydrolysis of ATP:
• G° = ‐28.7 kJ mol‐1 at pH 7.0 and 10 mMMg2+
• [Pi] in cells is maintained at ~10‐2 M, which means at equilibrium
ATP + H2O ADP + Pi + energy
K [ADP][Pi]
[ATP][H2O]/[H2O]° ~ 1
K eG oRT e28 / 2.478 105
[ADP][ATP]
107
Extent of reaction for reaction with a smaller G°?
MCB653/21/16 25
G – in a situation not at equilibrium
dG aA bB cC dD d 0
dGd
aA bB cC dD G
Figure from The Molecules of Life(© Garland Science 2008)
MCB653/21/16 27
• Combining:
• We obtain:
• Substituting: We get:
Reaction quotient, Q, describes observed conditions
G cC
o dDo aA
o bBo RT ln
[C]obsc [D]obs
d
[A]obsa [B]obs
b
Observed, non-equilibriumG Go RT lnQ
Go Q
Go RT lnK G RT lnQK 2.3RT log
QK
G aA bB cC dD o RT ln
CCo
Reaction quotient
MCB653/21/16 28
Q/K is the mass action ratio
• The ratio Q/K is the mass action ratio
• The mass action ratio determines whether a reaction goes forward or backward:
G < 0, reaction will go forward
G > 0, reaction will go backward
Go RT lnK G RT lnQK 2.3RT log
QK
QK1
QK1
5.8 kJ/mol
MCB653/21/16 29
Mass action ratio – Q/K
Figure from The Molecules of Life (© Garland Science 2008)
A B G RT lnQK 2.3RT log
QK
mol-1
MCB653/21/16 30
G for ATP hydrolysis in cells
• G° = ‐28.7 kJ mol‐1 and at equilibrium:
• In cells, [ADP]/[ATP] = 10‐3
ATP ADP + Pi + energy
K = [ADP][Pi][ATP]
[ADP][ATP]
107
G RT lnQK
~ RT ln103
107 ~ RT ln1010
~ 57 kJ mol1
MCB653/21/16 31
ATP synthesis is not spontaneous
• G° = ‐28.7 kJ mol‐1 and at equilibrium:
• How to drive ATP synthesis?• From the chemical potential concept
• Increasing the concentration of ADP and Pi could lead to ATP synthesis
• Impractical for the cell
ATP ADP + Pi + energy
K = [ADP][Pi][ATP]
[ADP][ATP]
107
MCB653/21/16 32
An example of non‐expansion work
• Using a gradient of molecules across a membrane to synthesize ATP
Figure from The Molecules of Life (© Garland Science 2008)
(ADP+Pi)•F (ATP) •F*
B(high) B(low)
Unfavorable reaction:
Favorable reaction: (ADP+Pi)•F + B (ATP) •F*•B
MCB653/21/16 33
F0F1 ATP synthase uses proton chemical potential to synthesize ATP
Credits: John Walkerhttp://www.mrc-mbu.cam.ac.uk/research/atp-synthase
MCB653/21/16 34
Oxidative phosphorylation in mitochondria
Figures from The Molecules of Life (© Garland Science 2008)
MCB653/21/16 35
• U = q + w, and q ~ 0 (under idealized conditions)
• U = w• U is positive because ATP is produced
• w is positive, corresponding to work done on the system by the surroundings
• Chemical work as a consequence of transferring B molecules from high to low concentration, decreasing the free energy of B, and storing this energy into synthesized ATP
(ADP+Pi)•F + H+ (ATP) •F*•H+
K = [(ATP) •F*• H+][(ADP•Pi) •F][H+]
F0F1 ATP synthase couples ATP synthesis to a transmembrane proton gradient
Figure from The Molecules of Life (© Garland Science 2008)
MCB653/21/16 36
Some concepts to remember
• The chemical potential, , describes the rate of change of the free energy with respect to concentrations
• The equilibrium constant, Keq, provides a link between free energy and the concentrations at equilibrium
• The mass action ratio, Q/K, is related to the reaction free energy change, G, determining the driving force for the reaction