2.1 properties of water 2.2 ph 2.3 buffers 2.4 water's unique role in the fitness of the...
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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
CHEM 330 Lecture 2
Water (G&G, Chapter 2)
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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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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
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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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“Flickering” H bonds in water: a series of snapshots at 5 picosecond intervals
Figure 2.3
The Dynamics of Liquid Water
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Solvent Properties of Water
• Interaction with electrolytes
• Interaction with polar, uncharged molecules
• Interaction with nonpolar molecules
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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
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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
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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
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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
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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
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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?
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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?
O-
O
CH3
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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 waterHydrophobic domains shielded from water
Variety of structures possibleWedge-shaped amphiphiles form micelles (spherical)Cylinder-shaped 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
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
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• 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
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H+
Dissociation of Water: water as a source of ions
_
Hydroxide
Proton
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:
[ H + ] [ A - ]
[HA]Ka =
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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:
[A-][HA]pH = pKa + log10
Given any two parameters, you can solve the third