TUMS
Azin Nowrouzi, PhD
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Importance of water
1.
The most abundant substance in living systems.
–
2.
Makes up 70% or more of the weight of most organisms.
Living organisms depend on water for their existence.
–
Physical & chemical properties of water make it fit to support life:
•
•
3.
4.
5.
6.
7.
High boiling point  water remains liquid in most seasons.
Ice less dense than water  floats on liquid water and water freezes
from top to bottom. So a good insulator: a frozen layer of ice serves as
a blanket that protects creatures below.
Ubiquitous solvent in cells.
Excellent solvent of polar and ionic substances.
Medium where most cell’s metabolic reactions take place.
Ionization of water and its acid-base reactions important
for the functions of proteins and nucleic acids.
The Shapes of proteins and nucleic acids and structure of
biological membranes are consequence of their
interaction with water.
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WATER
Microscopic
Structure
of waster
Water
molecule:
Polarity
WeakInteractions
• H-bonds
•
•
•
Crystals
Bound water
H-bonds between
•Water molecules and other molecules
•Between dissolved molecules
Ionization of
Water,
• Weak Acids
• Weak Base
• pH
Water as a solvent:
Buffering
against
pH Changes
•
•
•
Biological
buffers
Water as
a reactant
Ions
Polar substances
Non-polar substances
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“Water molecule” is polar
• Water has a simple structure.
• Oxygen has 6 electrons in the outer shell: 1s2, 2s2, 2p4.
• Sp3 hybridization  H—O—H bond angle is 104.5.
• The net charge of water molecule is zero. But O—H bond is
polar because O is more electronegative than H. Sharing of
electrons between H and O is unequal.
• The charge on O = -0.82 and on H = +0.41. This charge
separation produces permanent dipoles.
4
Strength of H-bond
• H-bond can form when a H atom that is bonded
to O or N lies within 0.27-0.30 nm of another O
or N atom that has unshared e-.
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Water molecules form H bond
• Attraction between positive H and
negative O of another H2O produces
dipole-dipole attraction called a
hydrogen bond (H bond).
• H bond is 10% covalent and 90%
electrostatic.
• The strength of H-bond is 4
kcal/mol, less than 5% as strong as a
typical covalent bond.
The strength of O—H bond is 110
kcal/mol. H-bond is a weak
interaction.
• Notice that the hydrogen bond (shown
by the dashed line) is somewhat
longer (117 pm) than the covalent
O—H bond (99 pm). This means that
it is considerably weaker.
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Some biologically important
H-bonds
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Structure of water in ice
• At any given time, most of the
molecules in water are
engaged in hydrogen bonding,
but the lifetime of each
hydrogen bond is just 1-20
picoseconds (1ps = 10-12s).
• In liquid water, molecules are
disorganized and in continuous
motion, so that each molecule
forms H-bonds with an average
of only 3.4 other molecules.
• In ice crystal each H2O is
hydrogen bonded to 4 other
H2O molecules.
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Microscopic structure of water
• Short-lived groups of water molecules interlinked
by hydrogen bonds in liquid water are called
flickering clusters.
more dense water  less dense water
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Structured water
2(H2O)4
More dense water

(H2O)8
Less dense water
Suggested by Prof. Martin Chaplin
of the London South Bank University
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Structured Water Is Changing Models
(H2O)8
(H2O)14
(H2O)17
(H2O)21
(H2O)196
(H2O)280
(H2O)more
Bennett Daviss
The Scientist 2004, 18(21):14.
(H2O)280
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• At any given time, most of the molecules in liquid water
are engaged in hydrogen bonding, but the lifetime of
each hydrogen bond is just 1 to 20 picoseconds (1 ps =
10-12 s).
• Upon breakage of one hydrogen bond, another hydrogen
bond forms, with the same partner or a new one, within
0.1 ps.
• The apt phrase “flickering clusters” has been applied to
the short-lived groups of water molecules interlinked by
hydrogen bonds in liquid water.
• The sum of all the hydrogen bonds between H2O
molecules confers great internal cohesion on liquid
water.
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‫كريستالهاي منجمدشده آب‬
The messages from water
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‫سالم‬
‫‪14‬‬
‫(كاواچي ) رقص سنتي ژاپني‬
‫“‪Goldberg Variations‬‬
‫آهنگ باخ‬
water can have highly organized
local structures when it interacts
with molecules capable of imposing
these structures on the water.
William Royer Jr.
U. of Mass. Medical school
Organized water
molecules
India, 2003
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Stabilization by “bound water” molecules
a
b
• Water binding in hemoglobin
The crystal structure of hemoglobin, shown
(a) with bound water molecules (red spheres) and
(b) without the water molecules
B-DNA with a spine
of water molecules
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‘Bound water’ in biological systems
• Intracellular water very close to any
membrane or organelle (sometimes called
vicinal water)
• Organized very differently from bulk water
• This structured water plays a significant
role in governing the shape (and thus
biological activity) of large folded
biopolymers.
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Water chain in cytochrome f
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Proton hopping
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Water molecules form H-bonds with
polar solutes
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Electrostatic interaction with
charged solutes
• When NaCl is mixed with water,
a shell of water surrounds each
Na+ and Cl- ion.
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Ions change structure of liquid water
• Ionic substances are soluble
because the net attraction of the
+ and – ions for water is greater
than the attraction of oppositely
charged ions for each other.
• Formation of the Hydration
shell.
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Structured water
2(H2O)4
More dense water
(H2O)8

Less dense water
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Types of ions
• Structure-breaking ion 'chaotrope'
(disorder-maker) (Na+)
• structure-forming ion 'kosmotrope'
(order-maker) (K+)
• Kosmotropes shift the local equilibrium to the right.
• Chaotropes shift it to the left.
more dense (condensed) water  less dense water
Water
preference
Ion
Surface charge
density
Intra-cellular
Extra-cellular
Ca2+
2.11
0.1 mM
2.5 mM
High density
Na+
1.00
10 mM
150 mM
High density
K+
0.56
159 mM
4 mM
Low density 24
Nonpolar gases
are poorly soluble in water
25
Hydrocarbons in water
•
Hydrocarbons and nonpolar molecules are
insoluble because water-water interactions
are stronger than water-hydrocarbon
interactions. So water molecules force
nonpolar molecules together and surround
them.
•
This phenomenon is called hydrophobic
effect or hydrophobic interaction.
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27
Water as a solvent
Water is a polar solvent:
• Compounds that dissolve easily in water are
hydrophilic (Greek, “water-loving).
• Nonpolar molecules that are not dissolved in
water can be dissolved in nonpolar solvents
(chloroform, and benzene). These nonpolar
molecules are called hydrophobic (waterfearing).
• Compounds that contain regions that are polar
(or charged) and regions that are nonpolar are
called amphipatic or amphiphilic.
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Water undergoes ionization
• Water is the biological solvent.
• Pure water contains not only H2O, but also
equal concentrations of hydrated protons,
hydronium ions (H3O+), and hydroxide ions.
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Ionization of water
Keq = 1.8 x 10-16
[H2O]=55.5 M
because (1000g/L)/(18.015 g/mol)
(55.6)Keq = [H+][OH-]
(55.6)(1.8 x 10-16) = [H+][OH-] = Kw
Kw = 1.0 x 10-14 = [H+][OH-]= [H+]2
[H+] = [OH-] = = 1.0 x 10-7 M
[H+] = [OH-] Neutral solution
[H+] > [OH-] Acidic solution
[H+] < [OH-] Basic (alkaline) solution
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The pH of some
aqueous fluids.
31
Dissociation of weak acids
• An acid is a chemical species that can donate a proton (H+), and a
base is a species that can accept (gain) a proton, according to the
common Brnstead-Lowry definition.
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Functions of [H+]
• [H+] and therefore pH is important for many
biochemical processes:
– Transport of oxygen in blood
– Enzymatic reactions
– Generation of metabolic energy during respiration and
photosynthesis
– Metabolism and its regulation.
– Measurement of pH of blood and urine are commonly
used in medical diagnoses.
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Types of acids
34
Titration curve
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Buffers & Buffer Systems
• Buffers are aqueous
systems that tend to resist
changes in pH when small
amounts of acid (H+) or
base (OH-) are added.
• Buffers include weak acids
that can donate hydrogen
ions and weak bases that
can absorb them.
• For example, a mixture of
equal concentrations of
acetic acid and acetate ion
found at the midpoint of the
titration curve:
acetic acid-acetate buffer
pair
Here the buffering power of the system
is maximal
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Example: Cpmparison of buffered and
unbuffered solutions
Add to each 1mmol
of solid NaOH.
• [OH-] = 1.0 mmol/100ml
= 0.01M
pOH = 2.00
pH = 12.0
• The change of pH is from
7.0 to 12.0 or 5 units.
• 1.0 mmol of OH- reacts with 1.0 mmol
of HA.
 mmol of HA decreases to 9.0.
 mmol of A- increases to 11.0.
pH = 7.0 + log 11/9
pH = 7.09
• The change of pH is only from 7.00 to
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7.09 or 0.09 units.
How does a buffer system work?
• Buffering results from two reversible reaction equilibria.
• It occurs in a solution of nearly equal concentrations of a proton
donor and its conjugate proton acceptor.
• The decrease in concentration of one component of the system is
balance exactly by an increase in the other.
39
The Henderson-Hasselbalch equation
• Important for
understanding
buffer action and
acid-base balance
in the blood and
tissues.
• Calculation of pH,
pKa and the molar
ratio of proton
donor and
acceptor.
• Shows why
pH=pKa at the
midpoint of
titration.
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Body buffer systems
A. Blood buffers
1. plasma buffers
1. Bicarbonate buffer
2. Hemoglobin, HCO3-, and phosphate in erythrocytes.
B. Tissue buffers
A. Muscle
B. Bone
C. Intracellular buffer system
A. Phosphate buffer system
B. Protein buffer system
D. Urine buffers
A. Phosphate buffer system
B. Ammonia buffer
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Important buffers at physiologic [H+]
42
Bicarbonate/carbonic acid buffer system
CO2 as an acid
1.
The greatest source of H+.
2.
End product of oxidation of glucose and triglyceride during aerobic metabolism.
3.
Most important weak acid in body fluids.
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Hemoglobin in RBCs
• Hemoglobin protein can reversibly bind either H+ (to the protein) or O2
(to the Fe of the heme group), but that when one of these substances
is bound, the other is released (as explained by the Bohr effect).
• During exercise, hemoglobin helps to control the pH of the blood by
binding some of the excess protons that are generated in the muscles.
• At the same time, molecular oxygen is released for use by the
muscles.
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2. Phosphate/phosphoric acid buffer sytem
• Phosphate concentration in the blood is so low that it is
quantitatively unimportant.
• Phosphates are important buffers intracellularly and in urine where
their concentration is higher.
• Phosphoric acid is triprotic acid that exists in four forms.
Phosphoric acid has a pKa value for each of the three dissociations
with three pK values of 2.1, 6.8, and 12.7:
H3PO4 ↔ H2PO4- ↔ HPO4-2 ↔ PO4-3
• When a solution of phosphoric acid (H3PO4) is titrated completely,
there will exist three buffering regions.
• Physiologically, we only need to deal with the pH range between pH
4.5 (acidic urine) and 8.0 (alkaline urine).
• For this reason, only two forms of phosphate (H2PO4- and HPO4-2)
and one pK value (6.8).
• H2PO4- is the acid form, and HPO4-2 is the salt form.
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Acid-base disturbances
•
•
•
•
The condition called respiratory acidosis occurs when blood pH falls as a
result of decreased respiration. When respiration is restricted, the
concentration of dissolved carbon dioxide in the blood increases, making
the blood too acidic. Such a condition can be produced by asthma,
pneumonia, emphysema, or inhaling smoke.
Metabolic acidosis is the decrease in blood pH that results when
excessive amounts of acidic substances are released into the blood. This
can happen through prolonged physical exertion, by diabetes, or restricted
food intake. The normal body response to this condition is increases
breathing to reduce the amount of dissolved carbon dioxide in the blood.
This is why we breathe more heavily after climbing several flights of stairs.
Respiratory alkalosis results from excessive breathing that produces
an increase in blood pH. Hyperventilation causes too much dissolved
carbon dioxide to be removed from the blood, which decreases the carbonic
acid concentration, which raises the blood pH. Often, the body of a
hyperventilating person will react by fainting, which slows the breathing.
Metabolic alkalosis is an increase in blood pH resulting from the
release of alkaline materials into the blood. This can result from the
ingestion of alkaline materials, and through overuse of diuretics. Again, the
body usually responds to this condition by slowing breathing, possibly
through fainting.
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Water as a reactant (metabolic water)
47
• Cybotactic region: That part of a solution
in the vicinity of a solute molecule in which
the ordering of the solvent molecules is
modified by the presence of the solute
molecule. The term solvent "cosphere" of
the solute has also been used.
KOSOWER (1968); STEWART and
MORROW (1927)
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