2.1 Properties of Water

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CHEM 330 Lecture 2 Water (GG, 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

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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

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Water Close Up
Dipole Moment

Two lone 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
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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

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The Dynamics of Liquid Water
Flickering H bonds in water a series of
snapshots at 5 picosecond intervals
Figure 2.3
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Solvent Properties of Water

• Interaction with electrolytes
• Interaction with polar, uncharged molecules
• Interaction with nonpolar molecules

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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) H2SO4
2H (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
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Weak electrolytes ionization is incomplete
CH3COOH H2O CH3COO- H3O
organic acids organic bases
CH3-NH2 H2O CH3-NH3 OH-
Major weak electrolytes in biology Amines,
imines, carboxylic acids
another term for ionization is dissociation
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Ionic interactions
r
-

charge e2
charge e1
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Interaction of water with ionsno naked ions
Cl-
Chloride anion
Na
Sodium cation
water
Dipoles of water screen the charges of the ions
so they dont sense one another- water has a high
dielectric constant
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Water polar neutral molecules hydrogen bonding
Water forms extensive H-bonds with molecules such
as glucose, rendering it highly soluble
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Lifes trouble with solutions, and lifes 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)
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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)
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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?
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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?
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Amphiphiles in water
Hydrophilic domains face water Hydrophobic
domains shielded from water
Variety of structures possible Wedge-shaped
amphiphiles form micelles (spherical) Cylinder-sha
ped amphiphiles form bilayers (planar)
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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
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Acid-base Equilibria Dissociation of protons
from molecules in aqueous solution
Measure H to indicate degree of acidity
Simple, but cumbersome eg physiological H
ranges from 0.5 M (stomach) 0.00000001 M
(blood)
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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
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Dissociation of Water water as a source of ions
H
Proton
Hydroxide
Little tendency to dissociate under neutral
conditions
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No Naked Protons!
H in aqueous solution exists as H3O
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Proton movement through water faster than any
other ions
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Dissociation of Weak Electrolytes

• Consider a weak acid, HA
• HA H A-
• The acid dissociation constant, Ka, is given by

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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

Given any two parameters, you can solve the third