Chapter 2
Water
Weak Interactions in Aqueous Systems
Water is the Solvent for Living Cells
• The attractions between adjacent
water molecules gives liquid water
great internal cohesion and leads
to its unusual properties.
• The electrostatic interaction
between the hydrogen atom of one
water molecule and the oxygen
atom of another yields the
hydrogen bond.
• This bonds are relatively weak
with bond dissociation energies of
about 23 kJ/mol compared to 470
kJ/mol for the covalent hydroxy
bond.
Chapter 2
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Weak Interactions in Aqueous Systems
•
•
•
•
•
The tetrahedral arrangement of the water
molecule gives each molecule the potential to
form 4 hydrogen bonds (1 for each hydrogen and
2 for the oxygen)
In the liquid state, the disorganization of the
molecules yields an average of 3.4 bonds per
molecule
In the solid state, the fixed nature of the molecule
in the crystal lattice yields the full hydrogen
bonding compliment.
The number of hydrogen bonds possible in both
states results in the high melting and boiling points
for water
This hydrogen bonding pattern gives water unique
properties:
– High boiling and melting points
– High surface tension
– A lower density in the solid state compared to the
liquid state
Chapter 2
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Weak Interactions in Aqueous Systems
Though a small molecule, water is powerful…
Chapter 2
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Weak Interactions in Aqueous Systems
Colligative Properties of Water
• Colligative Properties depend ONLY
on the number of solute particles in
solution, NOT on the identity of the
solute.
– Vapor pressure lowering
– Freezing point depression
– Boiling point elevation
– Osmotic pressure
• These particles affect properties of the
solvent by lowering the effective
concentration of the solvent
• For water, the presence of a solute
disrupts the hydrogen bonding network
Chapter 2
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Weak Interactions in Aqueous Systems
Energetics of Solution Formation (∆Ssoln)
Chapter 2
∆G = ∆H – T∆S
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Weak Interactions in Aqueous Systems
Energetics of Solution Formation (∆Hsoln)
• The solute–solvent
interactions are stronger
than solute–solute or
solvent–solvent.
• Favorable process
• Negative ∆Hsoln
Exothermic ∆Hsoln
Chapter 2
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Weak Interactions in Aqueous Systems
Energetics of Solution Formation (∆Hsoln)
• The solute–solvent
interactions are weaker
than solute–solute or
solvent–solvent.
• Unfavorable process.
• Positive ∆Hsoln
Endothermic ∆Hsoln
Chapter 2
In the end, Like dissolves Like!
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Weak Interactions in Aqueous Systems
“The more complex the system, the weaker
are the forces that govern its behavior”
- J. R. Platt
Stronger
Forces:
Internuclear
Structure: Atomic nuclei
Weaker
Weakest
Atomic
Covalent
bonds
Non-covalent
bonds
Gravity
Atoms
Simple
compounds
Macromolecules
the
Universe
Simple
Complex
Most
Complex
Chapter 2
9
Weak Interactions in Aqueous Systems
• An intermolecular force is an attraction between molecules
– Intramolecular bonds occur between atoms within a molecule.
• Intermolecular forces are much weaker than intramolecular bonds
• Non-covalent interactions seen in aqueous systems:
– Ionic interactions (Charge-Charge Interactions)
– Hydrogen bonds
– van der Waals interactions (Dispersion Forces)
– Hydrophobic “bonds”
• These weak non-covalent interactions are important in:
– Stabilization of proteins and nucleic acids
– Recognition of one biopolymer by another
– Binding of reactions to enzymes
Chapter 2
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5
Weak Interactions in Aqueous Systems
Approx. energy
~ 20 kJ/mole
20-40 kJ/mole
~ 8 kJ/mole
~ 4 kJ/mole
Chapter 2
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Weak Interactions in Aqueous Systems
Ionic (or Charge-Charge) Interactions
•
Ionic interactions are about 5-10% the strength of a carbon-carbon bond (20-40 vs. 350 kJ/mole)
•
Ionically stabilized compounds (like NaCl) are readily dissolved in solvents with a high dielectric
constant (like water)
•
–
When a sodium chloride crystal is placed in water, the water molecules attack the edge of
the crystal.
–
The anions (Cl-) are surrounded by the positively charged hydrogens on water.
–
The cations (Na+) are surrounded by the negatively charged oxygen on water.
The same process applies to biomolecules containing charged functional groups like carboxcylic
acids (-COO-) and amines (-NH3+)
Chapter 2
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Weak Interactions in Aqueous Systems
Hydrogen Bonds
• Hydrogen bonds are present when a molecule has an N-H,
O-H, or F-H bond.
• They consist of ~ 5% of the strength of a carbon-carbon
bond (20 vs. 350 kJ/mole)
• They are highly directional
• They are based on the dipole moment of water
• They account for the unusual properties of water as both
substance and solvent
Chapter 2
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Weak Interactions in Aqueous Systems
Common Hydrogen Bonds
Peptides
Chapter 2
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Weak Interactions in Aqueous Systems
Hydrogen Bond Strength is Highly Directional
• Hydrogen bonds are strongest when the bonded molecules are
oriented to maximize electrostatic interactions
• This occurs with the acceptor atom is in line with the covalent bond
between the donor atom and the hydrogen
• This requirement allows hydrogen bonds to maintain a specific and
set geometric arrangement, vital to the function of biological
macromolecules
Chapter 2
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Weak Interactions in Aqueous Systems
Biologically Important Hydrogen Bonds
Chapter 2
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Weak Interactions in Aqueous Systems
Van der Waals Interactions
• Van der Waals interactions result from the formation of instantaneous,
temporary dipoles in non-polar molecules due to electron motion.
• They are ~ 1% the strength of a carbon-carbon bond (4 vs. 350 kJ/mole)
• In a molecule, electrons are constantly orbiting the
nucleus and a region may become temporarily
electron poor and slightly positive while another
region becomes slightly negative.
• This creates a temporary dipole and two molecules
with temporary dipoles are attracted to each other.
Chapter 2
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Weak Interactions in Aqueous Systems
Energy of interaction
van der Waals Interactions
van der Waals radii
+
• Strongly repulsive at short internuclear
distances, very weak at long distances
• Maximal when the two atoms are
separated by their van der Waals radii
Electron cloud overlap
Distance between
atomic centers
-
Net
Energy
Chapter 2
Atomic
attraction
rv
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Weak Interactions in Aqueous Systems
Hydrophobic Interactions
• Hydrophobic (“Water Hating”)
interactions are ~ 2% the
strength of a carbon-carbon
bond (8 vs. 350 kJ/mole)
• Water becomes highly ordered
around hydrophobic groups (∆S)
• Greasy chains stay together to
minimize entropy loss by water
• The hydrophobic effect is the exclusion of non-polar
substances by water
– Critical for stability and structure of many macromolecules
Chapter 2
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Weak Interactions in Aqueous Systems
Amphipathic Compounds
• Amphipathic compounds contain
regions that are polar and regions that
are nonpolar.
• When mixed with water, the polar region
dissolves in the water, but the nonpolar
region tends to try to escape from the
water by clustering together.
• This results in the formation of micelles
• The nonpolar regions are held together
by hydrophobic interactions.
• Same idea results in the structure of
biological membranes!
Chapter 2
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Weak Interactions in Aqueous Systems
Summary
• Covalent bonds are relatively strong
bonds that result when two atoms share
electrons
• Weak interactions, which are very
important in biochemistry, include
hydrogen bonds, ionic bonds, hydrophobic
interactions and van der Waals forces.
• Water is a solvent with unique properties
that affect all aspects of life on Earth.
– Its melting point, boiling point, vapor
pressure, surface tension, viscosity, and
dielectric properties are all affected by
extensive hydrogen bonding.
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
Ionization of Water
• Water undergoes an autoionization reaction.
H2O(l) + H2O(l) ⇄ H3O+(aq) + OH-(aq)
OR
H2O(l) ⇄ H+(aq) + OH-(aq)
• Only about 1 in 5 million water molecules is present
as ions so water is a weak electrolyte.
• The concentration of hydrogen ions, [H+], in pure
water is about 1 × 10-7 mol/L at 25°C.
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
Ionization of Water
• the energy to break hydrogen bonds requires
about 20 kJ/mole…
H-O-H •••• O-H2
H-O-H + H-O-H
• the energy to ionize requires about 460 kJ/mole…
H-O-H •••• O-H2
H3O+ + - OH
• Thus, only “two of every 109 molecules in pure
water are ionized at any instant”
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
Ionization of Water
• The molar ratio of H+ to OH- in the reaction is 1 to 1, so if
the [H+] = 1 × 10-7 mol/L at 25°C, then the [OH-] must
also be 1 × 10-7 mol/L at 25°C:
H2O (l) ⇄ H+ (aq) + OH- (aq)
[H+] • [OH-] = (1 × 10-7)(1 × 10-7) = 1.0 × 10-14
• This value is the ion product constant of water, Kw.
KW = [H+][OH-]
• We can use this value to calculate [H+] or [OH-]
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
pH
• Living cells have a VERY narrow range
of tolerance for pH, i.e. [H+].
• The [H+] concentration will be important
- either explicitly or implicitly - for many
other topics in this course
• [H+] is controlled in all biological
organisms, and in virtually all
biochemical experiments.
Acidic solutions: [H+] > 1.0 x 10–7 M,
Neutral solutions: [H+] = 1.0 x 10–7 M,
Basic solutions: [H+] < 1.0 x 10–7 M,
pH < 7.00
pH = 7.00
pH > 7.00
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
For Biological systems, protons are necessary for
numerous cellular processes, therefore, a source of
these are needed:
• Ionization of a strong acid is TOO
• Ionization of water itself is way
BIG!
TOO LITTLE!
• Ionization of a weak acid is JUST RIGHT!
[the Goldilocks Principle…]
Chapter 2
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13
Ionization of Water, Weak Acids and Weak Bases
Equilibrium Constant Ka
Similar to Kw for water, weak acids and bases
have equilibrium constants that are related to
their degree of dissociation
HA ⇆ H+ + A[H+ ][A − ]
pKa = -log Ka
Ka =
[HA]
Chapter 2
What does the Ka value tell us
about the strength of the acid?
27
Ionization of Water, Weak Acids and Weak Bases
Comparison with Water
Water:
Acetic acid:
Keq = 1.8 x 10-16
Keq = 1.7 x 10-5
A 100 billion-fold difference…
• The contribution of protons from any reasonable
concentration of a weak acid in water dwarfs that from the
water itself
• But, they exist as hydronium ions, since water itself is 55.5
M (every proton finds a water molecule to associate with)
• And, their concentration must still obey the equation for the
ionization of water
Chapter 2
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14
Ionization of Water, Weak Acids and Weak Bases
• Weak acids have only a modest tendency to shed their protons
(definition of an acid)
• When they do, the corresponding anion becomes a willing proton
acceptor, and is called the conjugate base
• Also, the properties of a buffer rely on a balance between a weak acid
and its conjugate base
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
Titration Curves for Weak Acids
• The pKa is defined as: the
pH at which the acid and its
conjugate base exist at
equal concentrations.
• On the titration curve graph,
it is the inflection point at
which:
[HA] = [A-]
• So, the larger the pKa, the
weaker the acid
Chapter 2
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15
Ionization of Water, Weak Acids and Weak Bases
Buffers
• A buffer is a solution that resists
changes in pH when an acid or a base is
added.
• A buffer always contains a combination
of an acid-base conjugate pair.
• The buffer capacity is the solution’s
ability to resist pH change
• Two important values define any buffer:
– First: the pKa of the weak acid
– Second: the total concentration of the
weak acid plus its conjugate base
• The effective buffering range is usually
at pH values equal to the pKa ± 1
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
Buffers – An Example
HAc ⇄ H+ + Ac• When base is added to a
buffer with a high buffer acid
concentration, the acid
reacts with the added base
to maintain pH
• The system resists an
increase in pH
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
Buffers – An Example
HAc ⇄ H+ + Ac• When acid is added to a
buffer with a high buffer base
concentration, the base
reacts with the added acid to
maintain pH
• The system resists a
decrease in pH
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
Buffers – An Example
HAc ⇄ H+ + Ac• When a buffer has
equivalent concentrations of
both the acid and base
components, it can resist pH
changes in either direction
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
Whoever Henderson and Hasselbalch were,
they noticed that…
• The titration curves for all of these weak
acids have the same basic shape!
• Thus there must be some common
principle involved in each case.
• Perhaps a mathematical description is
possible?
• Can you derive it yourself?
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
The Feared Henderson-Hasselbalch Equation
Learn how to derive it, how to use it
(work problems ad nauseum), and don’t
ever be afraid of it again…
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
The Bicarbonate Buffer System
•
Physiological pH is normally around 7.4.
•
Cellular and metabolic processes often result
in the formation or requirement for H+ ions
•
These ions are produced or absorbed by your
bicarbonate physiological buffer system
•
For example:
– When you exercise vigorously, your muscles
will produce lactic acid (H+), which must be
neutralized to maintain pH.
– This acid reacts with bicarbonate to produce H2CO3, which is then breaks down into
CO2 and water.
– As the CO2 builds up in your lungs, it is expelled as you exhale.
•
Your ability to regulate pH in this manner is closely linked to the partial pressure
of CO2 in your lungs.
– If P(CO2) drops, your blood pH increases causing alkalosis
– If P(CO2) increases, your blood pH decreases causing acidosis
Chapter 2
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Ionization of Water, Weak Acids and Weak Bases
pH Optimum
• Many aspects of cell structure and
function are dependent on pH
• The catalytic activity of enzymes
are especially sensitive
• Enzymes usually show a
maximum catalytic activity at a
characteristic pH called the
optimum pH
• pH values just above or below the
optimum can lead to a drastic drop
off in activity
• Biological systems must be able
to finely control the pH in order to
maintain metabolic and cellular
functions
Chapter 2
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Water as a Reactant
Condensation and Hydrolysis Reactions
• The use of water as a reactant is a
vital aspect of numerous biological
reactions:
– Depolymerization of peptides,
carbohydrates & nucleic acids
– Formation of ATP from ADP
• Hydrolysis reactions insert the H
and OH groups from water into
another reactant
– These reactions are performed by
enzymes called hydrolases
• Condensation reactions produce
water from the joining of two
reactants
Chapter 2
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