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Water and Weak Bonds
Stryer Short Course
Chapter 2
Water
• Polarity
• Medium for Brownian motion
• Solvent
Intermolecular Forces
• Electrostatic: Ionic
– Permanent dipole
• H‐bonding
• Van der Walls
Not true categories…
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H‐bonding
• 1/10 the strength of some covalent bonds
• Donor/acceptor
• ~2.3 H‐bonds/ water molecule
Transient Interactions
• Intermediate strength of H‐bonds key to function
• Changes in structure, association
Solvation Of Ions
• Dielectric constant
• Solvation shell
• Solvent/solute
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Hydrophobic Effect
• Exclusion of nonpolar substances from aqueous solution
• G = H – TS
• Cage‐like structure of water minimized upon aggregation
• Powerful structural determination
Structure of Biomolecules
• Increased order in protein
• Decreased order overall
• How?
Amphipathic Compounds
• Structures determined by hydrophobic effect
– Micelles
– Bilayer
– Vesicle
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Functional Groups
Functional Groups
Autoionization of Water
Proton jumping: faster than diffusion limit
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Kw, The Ion Product of Water
Reciprocal Relationship
pH of Neutral Water
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pH of Solutions
• If acid is added to water, the concentration of hydronium increases and pH decreases
• If base is added to water, the concentration of hydronium decreases (ion product of water) and the pH increases
• Addition of MORE acid vs. addition of a STRONGER acid
Strong Acid
• Complete dissociation
• What is the pH of a 0.01 M HCl
solution?
• You add a drop of HCl to make a 1 x 10‐8 M solution. What is the pH? • What is your assumption?
Weak Acids
Strong
Weak
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Weak Acid Dissociation Constants
• Weak acids have low [pdts], therefore low Ka
• Low Ka = high pKa
• Weaker acids have __________ Ka values and __________ pKas
Henderson‐Hasselbalch
Proton Acceptor
Proton Donor
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Qualitative Understanding
• Relationship of
– Solution pH
– Strength of acid
– Ratio of CB to CA
• Solve quantitatively, but understand qualitatively
• What is/are the major ionization state(s) for succinic acid at pH 3.2, 4.2, 5.2, and 6.2?
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Understand Figures
Be able to explain what is happening as you trace the line from left to right in this figure.
Key Tool in biochemistry:
Buffer
Buffers
Cl-
H
N
H
N
NH
NH
H+
H
N
NH
H
N
H
N
N
HCl
N
N
H
N
NH
H
N
HCl
H
N
HCl
NH
H
N
N
NH
H
N
NH
H+
Cl-
H
N
N
H
N
NH
NH
Cl-
ClH
N
H
N
NH
H
N
NH
H+
H
N
HCl
HCl
H+
H
N
H
N
NH
N
H
N
NH
H+
H
N
NH
Cl-
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Buffer Capacity
• Depends on pKa
of CA/CB mix
• Depends on concentration of CA/CB
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Polyprotic Acids
• Must be able to match the pKa with the appropriate proton
• Assumptions are legitimate if the pKa
values are more than ~3 units from each other
Blood Buffer
• Physiologic
al pH 7.4
• Closed vs. Open systems
• Acidosis
Kidney Function
• Kidneys fight acidosis caused by common metabolic processes
• Reclaims excreted bicarbonate by excreting acid
• Forms new bicarb
by CO2 producing metabolism
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