Water: What makes it so good for life?
Lecture 2:
Water, pH and pKa
Structure
ice vs. water
or more technically solid vs. liquid
Solvent properties
High heat capacity
High dielectric constant
Forms H-bonds with polar solutes
Hydrophobic interaction
Solvation of ions
High heat of vaporization
High boiling point
High viscosity
High density
Margaret A. Daugherty
Fall 2003
BIOC 205
Colligative properties
vapor pressure depression
boiling point elevation
freezing point depression
osmotic pressure
BIOC 205
Water
The structure of water
• Water is a unique
solvent
• Lattice structure
“looser” in frozen form
(ice floats!)
•
Hydrogen bond donor &
acceptor with itself
– H20l:Hydrogen bonds are
non-ideal; dynamic
Key points: permanent dipole arises
from difference in electronegativity between O & H
from geometry
– H20s:Hydrogen bonds
ideal; rigid
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BIOC 205
Solvation of ions
Water is an excellent solvent
Water is highly polar,
hence excellent solvent
Dipole nature of water
allows formation of
electrostatic
interactions with salts
Hydration shells form
around ions; dynamic,
lifetimes ~10 nsec
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Hydrophobic vs Hydrophilic Interactions
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Hydrogen Bonds
• The polar nature and geometry of water allows
water molecules to form hydrogen bonds with
hydrophilic substances
hydrophilic (water loving)
– Ex: sugars, salts, amino acids, organic acids
– associate through hydrogen bonding
hydrophobic (water fearing)
– Ex: alkanes, fats and oils, sterols
– lack functional groups that allow the formation of hydrogen
bonds, thus they are excluded from the aqueous phase
– associate through hydrophobic interactions
Water is a good solvent for polar molecules
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Hydrophobic Interactions
Hydrogen Bonding
The 3D
structures of
many
biological
molecules are
determined
by hydrogen
bonding
tendency of nonpolar molecules to associate in H2O
• Water is
ordered
around
hydrophobic
molecules
R
K
Water forms a clathrate
“cage” about a nonpolar
substance
• Entropy
decreases
S
• Hyrophobes
aggregate in
H20
E
Slice through IFABP
Hydrophobic amino
acids cluster inside
the protein
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Amphiphilic Molecules
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COLLIGATIVE PROPERTIES OF WATER: influence of
solute on the physical properties of water.
• contain both hydrophobic
and hydrophilic regions
Vapor pressure depression: solute
molecules present a barrier to escaping solvent
molecules. Vapor pressure decreases.
• Ex: fatty acids, certain
amino acids, detergents
Boiling point elevation: a higher T is
now required to raise the vapor pressure to get
boiling.
Freezing point depression: At Tm,
Amphiphilic molecules form micelles in H20
the vapor pressure of liquid and solid
water are equal. If the vapor pressure is
lowered, then water must be cooled to
below 0C to freeze it.
A 1 molal solution:
Freezing pt: -1.86 oC
Boiling pt: +.53oC
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Osmotic pressure: next page!
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Acid Base Properties: Definitions
Osmotic pressure: a critical problem for cells
•
Many biological molecules (including AAs) are acids or bases;
•
Acids are proton donors, bases are proton acceptors;
•
Acids dissociate into protons and a conjugate base
HA <--> H+ + A-
More solute on side B
Water moves to equalize
concentration
isotonic
Osmotic pressure
reached when rate of
water movement balances
in each direction
hypotonic
•
Bases associate with protons to become their conjugate acids:
B + H+ <--> BH+
•
Strong electrolyte: substances that are almost completely
dissociated to their respective ions.
examples: strong acids & bases (HCl & NaOH)
salts (NaCl, K2SO4…)
•
Weak Electrolytes: substances that have a slight tendency to
dissociate to their respective ions
examples: acetic acid, lactic acid, phosphoric acid
imidazole, histidine, carbonic acid/bicarbonate
hypertonic
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pH scale
Ionization of water
H2O + H20 <--->
H30+
+
At neutral pH: [H+]=[OH-]=1 x 10-7 M (of course!)
OH-
pH = -log [H+]
Simplify the reaction:
KEQ = [H+][OH-] / [H2O]
or
KEQ = 1.8 x 10-16
Physiological
pH
H20 <---> H+ + OHAt 25C: [H +]=[OH-]=1 x 10-7 M
[H20] = 55.5 M
Since the concentration of H20 is constant (negligible dissociation)
Ion product of water: Kw,25C
Insert table 2.2
Kw = 55.5 * KEQ = [H+][OH-]
= 1 x 10-14 M
At neutral pH: [H+]=[OH-]=1 x 10-7 M
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BIOC 205
Acid Base Properties -- pKa
Henderson-Hasselbach Equation
Definition: pKa for a functional group is the pH at which the acidic or
basic group on 50% of the molecules are ionized; have equal
amounts of HA and A-
HA <-----> H+ + AKa = [H+][A-]/HA
rearrange
Ionization equilibrium of a
weak acid:
H+ = Ka[HA]/[A-]
HA <=> H+ + A-
log[H+] = logKa + log([HA]/[A-])
+] [A -]
Ka =[H
---------------[HA]
pH = pKa + log
pKa = -log Ka =1log----Ka
[A-] (conj base)
[HA] (conj acid)
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Titration Curves: How does pH change with changin!g [!0H
! -!]
Buffers
[HA]= [A-]
or
pH = pKa
• Mixtures of weak acid & their
conjugate base
HA + OH- <==> A- + H2O
Ammonium ion
In general, weak acids are the
most effective buffers
Formic acid
Beneficial thing about
buffers: Resist changes
in pH
H3PO4 --> H2PO4-
--> HPO42-
--> PO43-
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Titration curve
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Proteins are optimized to function in their environments
Digests dietary
proteins; found in
stomach
Breaks down
bacterial cell
walls; found in
tears
BUFFERS: the biological “biggies”
Intracellular pH control (6.9 - 7.4): phosphate system
HPO42-/H2PO4- pKa = 7.2
Blood pH control (7.4): bicarbonate system
H2CO3/HCO!3- with pKa = 3.8
Part of TCA
cycle - converts
malate to
fumarate; found
in mitochondria
Role of proteins - help intracellularly:
free histidine pKa ~ 6.0
histidine in proteins pKa ~7.0
BIOC 205
Review
1). Water has many chemical & physical properties that make it ideal
for life.
2). The dipole in H20 arises from the difference in electronegativity
between 0 & H and from the angle between the O-H bonds.
3). Liquid H20 is fluid; frozen water has a lattice like structure.
4). H2O has a high dielectric constant, making it an excellent solvent.
5). Water forms clathrate cages around H 20. This is entropically
unfavorable.
6). Colligative properties depend on the # of solute molecules; not
their chemical composition.
7). Colligative properties are critical to life.
8). H20 acts as a weak acid and weak base.
9). Many biological molecules are weak electrolytes.
10). pH is related to pKa through the Henderson-Hasselbach equation
11). Titration curves permit us to determine pKas
12). Buffers are most effect at +/- 1 pH unit from pKa
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13). Buffers are important for cellular viability.
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