• 2.1 Properties of Water• 2.2 pH• 2.3 Buffers• 2.4 Water's Unique Role in the Fitness of

the Environment

CHEM 330 Lecture 2

Water (G&G, Chapter 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

Bond angle 104.3°

+

+

-

Covalent Bond Length Between H and O: 0.95 Å

Dipole Moment

Water Close Up

:

:Two lone electron pairs

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

“Flickering” H bonds in water: a series of snapshots at 5 picosecond intervals

Figure 2.3

The Dynamics of Liquid Water

Solvent Properties of Water

• Interaction with electrolytes

• Interaction with polar, uncharged molecules

• Interaction with nonpolar molecules

NaCl Na+(aq) + Cl-(aq)H2SO4 2H+ (aq ) + SO4

2-(aq)NaOH Na+ (aq) + OH- (aq)

saltsstrong acids strong bases

Major biological strong electrolytes: Phosphates, KCl, NaCl, CaCl2

Electrolytes

Note that a solution containing electrolytes, though rich in ions, is electrically neutral

H2O

Compounds yielding ions when added to waterStrong electrolytes: ionization is complete, eg

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

What effect does the intervening solvent have?

in solution

+ -charge e1

charge e2

r

F e1e2

r2

Solvent Dielectric constant (D)water 78.5methanol 32.6acetone 20.7benzene 2.3

As D increases, ions in solution interact more weakly with each other & more strongly with the solvent

F = e1e2

Dr2D: the dielectric constant of the solvent

Ionic interactions

Interaction of water with ions:no naked ions

Cl-

Na+

Chloride anion

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

O

HOH

HH

H

H

OHOH

OH

OH

Water forms extensive H-bonds with molecules such as glucose, rendering it highly soluble

Life’s trouble with solutions, and life’s solution

Countermeasures1) Strong cell wall (bacteria, single-cell eukaryotes)2) Surround cells with an isotonic environment (multicellular eukaryotes)

Cell, full of solutes, whichcannot pass through membrane

Water: can pass through membrane;tendency is to dilute the cell contentscausing cell to burst

What to do?

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

• 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-

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 waterHydrophobic domains shielded from water

Variety of structures possibleWedge-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

XH X- + H+

BH+ B + H+

H2O

H2O

Simple, but cumbersome:eg “physiological” [H+] ranges from~ 0.5 M (stomach) ~ 0.00000001 M (blood)

Measure [H+] to indicate degree of acidity

• 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

The pH Scale

Low pH indicates a high proton concentration (high acidity)High pH indicates a low proton concentrationHigh pH indicates a high concentration of hydroxide -OH (high basicity)Each difference of 1 pH unit is a ten-fold difference in proton concentration

H+

Dissociation of Water: water as a source of ions

_

Hydroxide

Proton

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:

[ H + ] [ A - ]

[HA]Ka =

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