1. Water CHEM 330 Lecture 2 (G&G, Chapter 2) 2.1 Properties of Water
2.2 pH
2.3 Buffers
2.4 Water's Unique Role in the Fitness of the Environment

2. 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

3. : : Water Close Up d + d - Dipole Moment Two lone Bond angle 104.3°
electron pairs
Bond angle 104.3° :
Covalent Bond Length
Between H and O: 0.95 Å
Potential to form four H-bonds per water molecule

4. 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 ~ sec

5. The Dynamics of Liquid Water
“Flickering” H bonds in water: a series of snapshots at
5 picosecond intervals
Figure 2.3

6. Solvent Properties of Water
Interaction with electrolytes
Interaction with polar, uncharged molecules
Interaction with nonpolar molecules

7. Electrolytes Compounds yielding ions when added to water
Strong electrolytes: ionization is complete, eg
H2O
salts
strong acids
strong bases
NaCl Na+(aq) + Cl-(aq)
H2SO H+ (aq ) + SO42-(aq)
NaOH 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

8. Weak electrolytes: ionization* is incomplete:
organic acids
organic bases
CH3COOH+ H2O CH3COO- + H3O+
CH3-NH2+ H2O CH3-NH3+ + OH-
Major weak electrolytes in biology: Amines, imines, carboxylic acids
* another term for ionization is dissociation

9. What effect does the intervening solvent have?
Ionic interactions
What effect does the intervening solvent have?
in solution r
F 
e1e2 r2 + -
charge e1
charge e2
F =
e1e2
Dr2
D: the dielectric constant of the solvent
Solvent Dielectric constant (D)
water
methanol 32.6
acetone 20.7
benzene 2.3
As D increases, ions in solution interact more weakly with each other & more strongly with the solvent

10. - Interaction of water with ions:no naked ions Cl- Chloride 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

11. Water & polar neutral molecules: hydrogen bonding
Water forms extensive H-bonds with molecules such as glucose, rendering it highly soluble

12. Life’s trouble with solutions, and life’s solution
Water: can pass through membrane;
tendency is to dilute the cell contents
causing cell to burst
What to do?
Cell, full of solutes, which
cannot pass through membrane
Countermeasures
1) Strong cell wall (bacteria, single-cell eukaryotes)
2) Surround cells with an isotonic environment (multicellular eukaryotes)

13. 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)

14. Solvent Properties of Water- Recap
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?

15. 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?

16. 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)

17. 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

18. Acid-base Equilibria: Dissociation of protons from molecules in aqueous solution
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)
~ M (blood)

19. The pH Scale
A convenient means of writing low concentrations of protons:
pH = -log10 [H+]
If [H+] = 1 x M ( 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

20. _ Dissociation of Water: water as a source of ions Proton Hydroxide
Little tendency to dissociate under neutral conditions

21. No Naked Protons!
H+ in aqueous solution exists as H3O+

22. Proton movement through water: faster than any other ions

23. Dissociation of Weak Electrolytes
Consider a weak acid, HA:
HA H+ + A-
The acid dissociation constant, Ka, is given by:
[ H + ] [ A - ]
[HA]
Ka =

24. 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:
[A-]
[HA]
pH = pKa + log10
Given any two parameters, you can solve the third