Transcript CHEM 330 Lecture 1 - University of Waterloo

CHEM 330 Lecture 2
Water
(G&G, Chapter 2)
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2.1 Properties of Water
2.2 pH
2.3 Buffers
2.4 Water's Unique Role in the Fitness of
the Environment
For a small molecule, water is weird
Bulk Properties
• Abnormally high b.p., m.p.
• Abnormally high surface tension
The Molecular Explanation
• H-bond donor and acceptor
• ~ tetrahedral bond angles
• Potential to form four H-bonds
per water molecule
• Bent structure makes it polar
Water Close Up
Dipole Moment
d+
d
Bond angle 104.3°
-
Two lone
electron pairs
d+
Covalent Bond Length
Between H and O: 0.95 Å
Potential to form four H-bonds per water molecule
Comparison of Ice and Water
(or: what separates the frozen from the fluid?)
Number of H-bonds
• Ice: 4 H-bonds per water molecule
• Water: 2.3 H-bonds per water
molecule (on average)
Lifetime of H-bonds
• Ice: H-bond lifetime ~ 10-5 sec
• Water: H-bond lifetime ~ 10-11 sec
The Dynamics of Liquid Water
“Flickering” H bonds in water:
a series of snapshots at
5 picosecond intervals
Figure 2.3
Solvent Properties of Water
• Interaction with electrolytes
• Interaction with polar, uncharged molecules
• Interaction with nonpolar molecules
Electrolytes
Compounds yielding ions when added to water
Strong electrolytes: ionization is complete, eg
H2O
salts NaCl
strong acids H2SO4
strong bases NaOH
Na+(aq) + Cl-(aq)
2H+ (aq ) + SO42-(aq)
Na+ (aq) + OH- (aq)
Major biological strong electrolytes: Phosphates, KCl, NaCl, CaCl2
Note that a solution containing electrolytes, though rich in
ions, is electrically neutral
Weak electrolytes: ionization* is incomplete:
organic acids CH3COOH+ H2O
CH3COO- + H3O+
organic bases CH3-NH2+ H2O
CH3-NH3+ + OH-
Major weak electrolytes in biology: Amines, imines, carboxylic acids
* another term for ionization is dissociation
Ionic interactions in solution
+
charge e1
r
-
F
charge e2
e 1e 2
r2
What effect does the intervening solvent have?
D: the dielectric constant of the solvent
Solvent
water
methanol
acetone
benzene
Dielectric constant (D)
78.5
32.6
20.7
2.3
F=
e 1e 2
Dr2
As D increases, ions in solution
interact more weakly with each
other & more strongly with the
solvent
Interaction of water with ions:no naked ions
ClChloride anion
Na+
Sodium cation
+
+
water
Dipoles of water screen the charges of the ions so
they don’t sense one another- water has a high
dielectric constant
Water & polar neutral molecules: hydrogen bonding
OH
H
O
HO
H
H
HO
H
OH
OH
H
Water forms extensive H-bonds with molecules such as
glucose, rendering it highly soluble
Life’s trouble with solutions, and life’s solution
Cell, full of solutes, which
cannot pass through membrane
Water: can pass through membrane;
tendency is to dilute the cell contents
causing cell to burst
What to do?
Countermeasures
1) Strong cell wall (bacteria, single-cell eukaryotes)
2) Surround cells with an isotonic environment (multicellular
eukaryotes)
Water & nonpolar molecules: Hydrophobic Interactions
• H-bond network of
water reorganizes to
accommodate the
nonpolar solute
• This is an increase in
"order" of water (a
decrease in entropy)
• number of ordered
water molecules is
minimized by herding
nonpolar solutes
together
Yellow blob: nonpolar solute (eg oil)
Solvent Properties of Water- Recap
O
• Water forms H-bonds with polar solutes
• Ions in water are always surrounded by a hydration
shell (no naked ions)
• Hydrophilic (polar): water-soluble molecules
• Hydrophobic (nonpolar): water insoluble (greasy)
• Hydrophobic interaction: fewer water molecules are
needed to corral one large aggregate than many
small aggregates of a hydrophobic molecule
Hydrophilic, hydrophobic - anything else?
-
O
CH3
Amphiphilic Molecules
Also called "amphipathic"
• Contain both polar and nonpolar groups
• Attracted to both polar and nonpolar environments
• Eg - fatty acids
Polar head (carboxylic acid)
Nonpolar hydrocarbon tail
What happens in water?
Amphiphiles in water
Hydrophilic domains face water
Hydrophobic domains shielded from water
Variety of structures possible
Wedge-shaped amphiphiles form micelles (spherical)
Cylinder-shaped amphiphiles form bilayers (planar)
Protons in solution - why are they so important ?
• Most biomolecules bear groups that can undergo reversible
protonation/deprotonation reaction
•The conformation and functions of these biomolecules may
depend on their protonation state:
-Active sites of hydrolytic enzymes
-Overall fold of proteins
• Establishment of proton concentration gradients across
biological membranes is central to an understanding of cell
energetics
The study of acid-base equilibria lets us quantify these effects
Acid-base Equilibria:
Dissociation of protons from molecules in aqueous solution
H2O
XH  X- + H+
BH+  B + H+
H2O
Measure [H+] to indicate degree of acidity
Simple, but cumbersome:
eg “physiological” [H+] ranges from
~ 0.5 M (stomach)
~ 0.00000001 M (blood)
The pH Scale
• A convenient means of writing low concentrations of
protons:
• pH = -log10 [H+]
• If [H+] = 1 x 10 -7 M (0.0000001 M)
• Then pH = 7
Low pH indicates a high proton concentration (high acidity)
High pH indicates a low proton concentration
High pH indicates a high concentration of hydroxide -OH (high basicity)
Each difference of 1 pH unit is a ten-fold difference in proton concentration
Dissociation of Water: water as a source of ions
_
H+
Proton
Hydroxide
Little tendency to dissociate under neutral conditions
No Naked Protons!
H+ in aqueous solution exists as H3O+
Proton movement through water:
faster than any other ions
Dissociation of Weak Electrolytes
Consider a weak acid, HA:
HA
H+ + A-
The acid dissociation constant, Ka, is given by:
Ka =
[ H +] [A-]
[HA]
The Henderson-Hasselbalch Equation
For any acid HA, the relationship between its pKa, the
concentrations of HA and A- existing at equilibrium,
and the solution pH is given by:
pH = pKa + log10
[A-]
[HA]
Given any two parameters, you can solve the third