Chemistry 501 Handout 2 Water Chapter 2

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Chemistry 501 Handout 2WaterChapter 2
Lehninger. Principles of Biochemistry. by Nelson
and Cox, 5th Edition W.H. Freeman and Company
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Hydrogen bonding gives water its unusual
properties
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Hydrogen bonding in ice
Common hydrogen bonds in biological systems
Ice 4 H-bonds per molecule of water. Liquid at
RT, 3.4 H-bonds per water molecule
on average.
Flickering cluster
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Some biologically important hydrogen bonds
oriented to maximize electrostatic interaction
can hold two H-bonded molecules or groups in a
specific geometric arrangement
Directionality of the hydrogen bond
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Water interacts electrostatically with charged
solutes
Entropy increases as crystalline substances
dissolve
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Nonpolar compounds force energetically
unfavorable changes in the structure of water
Amphipatic compounds in aqueous solution
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Release of ordered water favors formation of an
enzyme-substrate complex
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Weak interactions are crucial to macromolecular
structure and function
van der Walls interactions are weak interatomic
attractions
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Water binding in hemoglobin
Water chain in cytochrome f
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Solutes affect the Colligative Properties of
aqueous solutions (vapor pressure, boiling
point,freezing point, and osmotic pressure)
P iC RT
Venus flytrap
Mimosa pudica
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H2O lt--gt H OH- Keq H OH- / H2O
1.8 x 10-16 M Keq H2O H OH- Kw (ion
product of water) H2O 55.5 M --gt Kw H
OH- 1.0 x 10-14 M2 H OH- 1.0 x 10-7
M pH -log H pOH -log OH- --gt pH
pOH 14
Ionization of water
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pH of some aqueous fluids
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HA lt--gt H A- Keq H A- / HA Ka
Weak acids and bases have characteristic
dissociation constants
Conjugated acid-base pairs consist of a proton
donor and a proton acceptor
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H2O lt--gt H OH- HAc lt--gt H Ac- Kw
H OH- 1.0 x 10-14 M2 Ka H Ac- /
HAc 1.74 x 10-5 M
Titration curves reveal the pKa of weak acids
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Buffers are mixtures of weak acids and their
conjugate bases
The Henderson-Hasselbalch Equation pH pKa
log proton acceptor proton donor
The pH optima of some enzymes
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Weak acids or bases buffer cells and tissues
against pH changes
Two particularly important biological buffers
Contribution from proteins e.g.
Phosphate H2PO4- H HPO42- Bicarbonate
H2CO3 H HCO3-
histidine pKa 6.0
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Water as a reactant