Biochemistry
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Transcript of Biochemistry
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Biochemistry: A Short CourseSecond Edition
Biochemistry: A Short CourseSecond Edition
Tymoczko • Berg • Stryer
© 2013 W. H. Freeman and Company
CHAPTER 2Water, Weak Bonds, and the
Generation of Order Out of Chaos
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2
Water Properties
• Polar
• H-bonding
• Dissolve many molecules
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2.2 Biochemical Interactions Take Place in an Aqueous Solution
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Weak Interactions
• Electrostatic
• H-bonds
• Van der Waals
• Hydrophobic
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H-Bonds Form Between an Electronegative Atom and Hydrogen
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Water disrupts H-bonds between 2 molecules by competing for the hydrogen bonding capability.
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“Stability in numbers”
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Phospholipids have hydrophilic & hydrophobic properties. When exposed to water, phospholipids form membranes.
Membrane Formation is Powered by the Hydrophobic Effect
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Functional Groups
• What are they?
• Key groups in biochemistry?
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pH
• What is it?
• How is it quantitatively expressed?
• What is the effect of acids & bases?
• How cells maintain a constant pH?
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The equilibrium constant Keq for the dissociation of water is:
Kw, the ionization constant of water, is given by:
This can be simplified to:
Water Ionizes to a Small Extent
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The pH of any solution is defined as:
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Acids ionize to form a proton and a base.
An Acid Is a Proton Donor, Whereas a Base Is a Proton Acceptor
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The ionization equilibrium of a weak acid is given by:
The larger Ka, the stronger the acid.
Acids Have Differing Tendencies to Ionize
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Taking the logarithm of both sides gives:
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When [A-] = [HA]
log ([A-]/[HA]) equals 0, and pH =pKa.
For any acid,
at pH > pKa, A- predominates.
At pH < pKa, HA predominates.
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Buffers
• What are they?
• Titration curves
• How do buffers work?
• Examples
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The titration curve for acetic acid
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Buffer action: adding a strong acid, 1 M HCl
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The titration curves of three important weak acids