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Chapter 2 Water and Chemical Bonds in Biochemistry

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The pH Scale D. Acids and Bases E. Relationship between pH and pK Buffers Buffering Capacity ... in molar concentration In pure water the pH is 7 If the pH of an ... – PowerPoint PPT presentation

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Title: Chapter 2 Water and Chemical Bonds in Biochemistry


1
Chapter 2 Water and Chemical Bonds in
Biochemistry
2
2-1 Water and hydrogen bond
3
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4
Water (H2O)
5
Due to the dipolar nature,there is an
electrostatic attractionbetween the oxygen
atomof one water molecule and the hydrogen of
another. This attractionresults in ahydrogen
bond.
6
In ice each water moleculeforms 4 hydrogen
bonds. In liquid water eachwater molecule
formsabout 3.4 hydrogen bonds.
7
Certain other molecules can form hydrogen
bonds. Needed is A hydrogen acceptor (oxygen or
nitrogen with lone pair of e-). A hydrogen donor
(hydrogen atom covalently bonded to
oxygen or nitrogen)
8
Some biologically important hydrogen bonds
9
A linear hydrogen bond is the strongest
10
Intermolecular Forces
11
Table of Force Energies
Type of Force Energy (kJ/mol) Ionic
Bond 300-600 Covalent 200-400 Hydrogen
Bonding 20-40 Ion-Dipole 10-20 Dipole-Dipole
1-5 Instantaneous Dipole/ Induced
Dipole 0.05-2
12
2-2 Hydrophilic and Hydrophobic properties
13
Hydrophilic molecules Water is a polar
solvent. Many biomolecules are charged or polar
and readily dissolve in water. They are
hydrophilic.
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DNA
16
Examples of polar groups in biomolecules
17
Hydrophobic molecules Biomolecules are soluble
in nonpolar solvents such as chloroform or
benzene. They are hydrophobic. Example waxes,
hydrocarbon chains of lipids.
18
Examples of nonpolar groups in biomolecules
Beeswax
19
Amphipathic molecules Biomolecules that have
both hydrophilic and hydrophobic regions are
termed amphipathic.
20
Examples of amphipathic biomolecules
21
Lipids self-aggregatein aqueous solution
22
Lipids self-aggregate in aqueous solution
23
Lipids self-aggregatein aqueous solution
24
Lipids self-aggregatein aqueous solution
25
Enzyme substrate interaction releasesordered
water molecules and increases entropy.
26
Enzyme substrate interaction releases ordered
water molecules and increases entropy
27
Solubilities of Some Gases in Water
28
Four types of noncovalent weak interactions
among biomolecules in aqueous solvent
29
2-3 Osmosis
30
Osmoticpressure
H2Oflows in
31
Osmosis water movement across a cell membrane
driven by differences in osmotic pressure.
32
Osmotic pressure Force that results from water
entering a cell that is in a hypotonic
solution.
33
Isotonic solutions of equal osmolarity (solute
concentration). Hypertonic A cell is in a
hypertonic solution if the medium has a higher
osmolarity than the cytosol. Water flows out of
the cell, the cell shrinks. Hypotonic - A cell is
in a hypotonic solution if the medium has a
lower osmolarity than the cytosol. Water
flows into the cell, the cell swells.
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How do cells compensate for living in a hypotonic
solution?
1. Some cells have an outer cell wall that is
rigid and prevents cell rupture (lysis).
Examples Plants, bacteria. 2. Some organisms
have a special organelle called a contractile
vacuole that serves as a water pump. Example
Protozoans. 3. Cells can compensate by pumping
sodium ions (Na) out of the cell. Water will
leave the cell with the sodium to maintain
osmotic balance.
39
2-4 Ionization of water, weak acids, and weak
Bases
40
A. Pure Water Is Slightly Ionized
H3O
H
H2O
H
OH-
41
B Equilibrium Constant
H2O OH- H
OH- H Keq H2O Kw OH-
H 10-14 M2 H2O55.5 M Pure H2O H
OH- 10-7 M pH - log H -log (10-7)
7 If H gt 10-7 M then pH lt 7 (acidic) If H lt
10-7 M then pH gt 7 (basic)
Equilibrium constant

1.8 x 10-16 M
Ion product of water

42
B Equilibrium Constant
Blood H 4 x 10-8 M Blood pH 7.4
43
C. The pH Scale
44
D. Acids and Bases
Acid (HA) - something that has a proton and is
willing to give it up. Base (A-) - something that
has a place to put a proton HA H
A- HA H2O
H3O A- Strong acids completely dissolve in
H2O (HCl-). Weak acids dont completely dissolve
in H2O. A- H K HA H2PO4-
HPO42- H (acid)
(base) HPO42- H K 1.51 x 10-7
M H2PO4- pK - log (K) 6.82
O O P OH OH
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E. Relationship between pH and pK
pH pK log (A- / HA) (Henderson - Hasselbalch
equation) pH pK when The molar concentration
of acid and conjugate base are equal. H2PO4-
HPO42- pH pK 6.8
48
Henderson Hasselbalch equation
49
Buffers
Buffers are solutions that contain both the
acidic and basic forms of a weak acid. Buffers
minimize changes in pH when strong acids (H) and
bases (OH-)are added.
10-3 M HCl
H 10-3 M Cl 10-3 M pH 3.0
H2O
H 10-7 M pH 7.0
10-3 M HCl
0.1 M H2PO4- 0.1 M HPO42- H 10-6.8 M pH 6.8
0.101 M H2PO4- 0.099 M HPO42- H 10-6.79 M pH
6.79
O -O P O- OH
O -O P OH OH
10-3 M HCl
50
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51
Buffering Capacity
Buffering capacity depends on the concentration
of the buffering agent.
10-3 M HCl
0.1 M H2PO4- 0.1 M HPO42- H 10-6.8 M pH
6.8
0.101 M H2PO4- 0.099 M HPO42- H 10-6.79 M pH
6.79
10-3 M HCl
0.002 M H2PO4- 0.002 M HPO42- H 10-6.8 M pH
6.8
0.003 M H2PO4- 0.001 M HPO42- H 10-6.32 M pH
6.34
O -O P O- OH
O -O P OH OH
10-3 M HCl
52
H2PO4- HPO42- H (acid)
(base) HPO42- H Ka 1.51 x 10-7
M H2PO4- 0.001 M H
1.51 x 10-7 M 0.003 M H (1.51 x
10-7 M)(0.003 M)/(0.001 M) 4.53 x 10-7M pH
-log H -log(4.53 x 10-7) 6.34
O O P OH OH
53
HPO42- H K H2PO4- K 1.51 x
10-7 M pK - log (K) 6.8
6.8
54
O O P OH OH
H2PO4- HPO42- H (acid)
(base)
Phosphate buffers act in the cytoplasm of
cells,keeping the pH inside a cell near 7.
55
Many biomolecules inside a cell possess ionizable
groups thatare weak acids and act as
intracellular buffers. The side chain of the
amino acid histidine contains an
imidazolefunctional group with a pK 6.
56
Enzymes, protein molecules that serve as
biological catalysts,have a characteristic pH
optimum
57
Physiological pH
The pH in the human body needs to remain
7 Biological molecules are very sensitive to
pH. enzyme catalysis, protein-protein
interactions, receptor binding Acid-Base balance
of the blood is maintained using a CO2 -
bicarbonate buffer. CO2 H2O H2CO3
H HCO3- pKa 6.1 (acid)
(hydrated (bicarbonate CO2)
base) There is more than 10-fold more
base (HCO3-) than acid (CO2) so pH gt pK (pH
7.4) CO2 is exhaled by the lungs H HCO3-
CO2 H2O Breathing rate controls CO2 CO2
balance is controlled by the lungs, HCO3- by the
kidneys
58
H2CO3 H HCO3- pKa 6.1 (hydrated
CO2) (bicarbonate (acid)
base)
If acid is produced, such as lactic acid in
muscleCH3-CH(OH)-COOH H CH3-CH(OH)-COO-
HCO3- H H2CO3 If base is produced, such
as NH3 from protein degradation NH3 H
NH4 H2CO3 H HCO3-
59
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60
Question What is the pH of a solution whose
hydrogen ion concentration is 3.2 x 10-4
M? Answer pH -logH pH -log(3.2 x
10-4) pH 3.5
61
Question What is the pH of a solution whose
hydroxide ion concentration is 4.0 x 10-4
M? Answer Define pOH -log OH- Kw
HOH- 10-14 logH logOH- log
10-14 pH pOH 14 OH- 4.0 x 10-4 pOH -log
OH- -log(4.0 x 10-4) 3.4 pH pOH 14 pH
14 pOH 14 3.4 10.6
62
Work the problems in Box 2-3 page 67 Working in
Biochemistry
63
Work Through Acid Base Worksheet 1
64
CH3COOH ? CH3COO- H Acid
Base
CH3COO- H Ka CH3COOH
65
CH3COOH ? CH3COO- H Acid
Base
CH3COO- H Ka CH3COOH
Ka 1.74 x 10-5 pKa -log(1.74 x 10-5) 4.76
66
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67
Henderson Hasselbalch equation
68
The pH of a solution of acetic acid is 6.22. What
is the ratio of base to acid?
6.22 4.76 log A-/HA 6.22 4.76 log
A-/HA 1.46 log A-/HA 101.46
A-/HA 28.8 A-/HA A-/HA
CH3COO-/CH3COOH
69
The concentration of CH3COOH is 0.1 M. The
concentration of CH3COO- is 0.01 M. What is the
pH of the solution?
pH 4.76 log 0.01/0.1 pH 4.76 log
0.1 pH 4.76 (-1) pH 4.76 - 1 pH 3.76
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