Download Lecture 31 More on Water: Ionization and the Hydrophobic Effect

Physical Principles in Biology
Biology 3550
Fall 2016
Lecture 31
More on Water:
Ionization and the Hydrophobic Effect
Friday, 11 November
c
David
P. Goldenberg
University of Utah
[email protected]
Hydrogen Bonds Between Water Molecules
Each water molecule can act as a donor for two hydrogen bonds and
an acceptor for two hydrogen bonds.
In ice, each water molecule is hydrogen bonded to four others. Same
geometry as carbon atoms in a diamond.
What about in liquid water?
A Simulation of 1,000 Water Molecules
(Water Movie)
4.5×10−12 s (4.5 ps) total time.
0.5 ps/frame
Simulation courtesy of Prof. Valeria Molinero, University of Utah
Ionization of Water
+
–
−*
H2 O −
)
−− H + OH
Equilibrium constant for ionization:
+
Keq =
−
[H ]eq [OH ]eq
[H2 O]eq
= 1.8×10−16 M
Ignoring H2 O concentration:
Kwat = [H+ ]eq [OH− ]eq = 10−14 M2
Logarithmic representations of H+ and OH– :
pH = − log [H+ ] (molar concentration)
pOH = − log [OH− ] (molar concentration)
Logarithmic representation of equilibrium expression:
pH + pOH = 14
Ionization of Other Molecules
Other polar groups can ionize in water.
+
+
+
Ionized groups are stabilized by interactions with water.
Electric charge has huge effect on chemical and physical properties of
molecules.
Extent of ionization is determined by thermodynamics!
Ionization equilibrium constants (pKa ) and H+ concentration (pH)
+
Dynamics of H Ion Diffusion in Water
H+ ions diffuse through water much more rapidly than other ions, and
much more rapidly than predicted by Stokes–Einstein equation.
+
H
H
H
H
H
H
O
O
O
H
H+
H
H
O
H
H
H
O
O
O
H
H
H
H
H
O
+
H
H
H
H
H
O
O
O
O
H
H
H
H+
O
H
H
O
H
H
O
H
H
O
H
H
H
H+ exchange reactions require only about 1 microsecond.
Relay mechanism proposed by Theodor Grotthaus in 1806.
Basic mechanism is still thought to be correct, but details are still being
studied and debated.
Mechanism is found in some H+ pores in membrane proteins.
The Hydrophobic Effect
The basic observation: Water and oil don’t mix!
A confusing and still controversial subject, partly
because of terminology.
Non-polar molecules are poorly soluble in water.
Are non-polar molecules afraid of water?
What happens when a non-polar molecule does
dissolve in water?
Clicker Question #1
Why don’t oil and water mix?
1
Water and oil molecules repel one another.
2
Oil molecules stick together very tightly.
3
Water molecules stick together very tightly.
4
All of the above.
5
None of the above.
All answers count for now!
What Happens to Water Hydrogen Bonds
When a Non-polar Molecule is Introduced?
Non-polar molecules do not form hydrogen bonds with water!
Do water hydrogen bonds break to allow non-polar molecule in?
That would be unfavorable, and would explain low solubility.
What we know (or think we know) about this comes from
thermodynamic measurements.
Thermodynamics of Transfer of a Non-polar Molecule to Water
CH4
CH4
Methane in a
non-polar solvent
Methane in
Water
∆G is measured by comparing solubilities of the same molecule in
non-polar solvent and water.
The positive ∆G confirms that transfer to water is unfavorable.
Thermodynamics of Transfer of a Non-polar Molecule to Water
CH4
CH4
Methane in a
non-polar solvent
Methane in
Water
A negative ∆H is not what we expect for breaking hydrogen bonds!
Suggests that favorable interactions are being formed when non-polar
molecule transfered!
Does this happen when molecule is removed from non-polar solvent or
when it is added to water?
Thermodynamics of Transfer of a Non-polar Molecule to Water
A path through the gas phase from the non-polar solvent to water
Gas Phase
CH4
CH4
CH4
Methane in a
non-polar solvent
Methane in
Water
Removing molecule from non-polar solvent breaks some favorable
interactions.
Favorable interactions are being formed when non-polar molecule is
introduced into water!
Clicker Question #2
CH4
CH4
Methane in a
non-polar solvent
Methane in
Water
What is the entropy change?
1
≈ 80 kJ/(mol · K)
2
≈ 80 J/(mol · K)
3
0
4
≈ −80 J/(mol · K)
5
≈ −80 kJ/(mol · K)
Thermodynamics of Transfer of a Non-polar Molecule to Water
CH4
CH4
Methane in a
non-polar solvent
Methane in
Water
∆G = ∆H − T ∆S
∆G is positive because ∆S is negative!
(an “entropically driven” process).
Something becomes more ordered when non-polar molecule is
introduced into water.
The “Iceberg Model”
Introduction of a non-polar molecule causes water molecules to
become more ordered.
Either new hydrogen bonds form or existing ones become stronger,
leading to negative ∆H.
Why do water molecules do this, instead of just allowing hydrogen
bonds to break?
I don’t know! (And, I don’t think that anyone else really does either.)
Transfer Free Energy versus Molecular Size
Hydrocarbons
Nonpolar
amino acid
sidechains
Polar amino acid side chains
F. M. Richards. Areas, volumes, packing and protein structure. Annu. Rev. Biophys.
Bioeng., 6:151–176, 1977.
Bimolecular Association can be Entropically Favorable!
+
Associating molecules lose entropy.
Water molecules near hydrophobic surfaces become less ordered.
Ions bound to charged groups can have similar effect.