1

Water

The Solvent of Life

Importance of Water

•  Folding (structure) of biomolecules is affected by the physical and chemical properties of water

•  Water is the medium for most biochemical reactions

•  Water (or its derivatives) participate in many biochemical reactions

Physical Properties of Water

Water is a Polar Molecule

Figure 2-1a

Structure of Water

Figure 2-1b

sp3 Hybridization Permanent Dipole (Electronegativity of Oxygen)

H

O

H

2e–

2e–

!- H

O

H

!+

!+!-

2

Figure 2-2

Hydrogen Bonding

Figure 2-7

Hydrogen Bonding by Functional Groups

Characteristics of Hydrogen Bonds

•  Length = 1.8Å (versus 0.96 Å) •  Strength = ~20kJ/mol (versus 460kJ/mol) •  Most stable when linear

Example of Weak Interaction

Table 2-1

Weak Interactions

Figure 2-5

van der Waals Interactions

Figure 2-3

Structure of Ice

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Consequences of Water Structure

Cohesiveness of Water Molecules

•  High surface tension •  High boiling point •  High heat of vaporization •  High heat of fusion

Water remains liquid over a wide range of conditions of temperature

and pressure

Ice is Less Dense than Liquid Water

•  Ice floats •  Ice layer insulates water below Large bodies of water remain liquid

providing a liquid medium in which life has been able to evolve and

persist under a range of conditions on earth

Water has a High Heat Capacity

Water modulates temperature on earth within a range

compatible with life

Effect of Water on the Structure and Function of

Biomolecules

Figure 2-6

Solvation of Ions

4

Figure 2-8

Orientation of Water Molecules Around a Nonpolar Solute

(Entropic Effect)

Figure 2-9

Aggregation of Nonpolar Molelcules in Water

(Entropic Effect)

Figure 2-10

Fatty Acid Anions (Amphipathic) Monolayers

(very dilute solutions)

WaterPolar Head Groups

Hydrophobic Tails

Air

Figure 2-11

Structure of Micelles and Bilayers Liposomes

Phospholipid Bilayer

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Osmosis and Diffusion

Water moves by Osmosis

Solutes move by Diffusion

Osmosis

Movement of solvent from a region of high concentration

(e.g. pure water) to a region of relatively low concentration (e.g.

solvent plus solute)

Figure 2-13

Osmotic Pressure Cellular Resistance to Osmotic

Pressure

Iso-osmolar environment (complex organisms)

Rigid Cell Wall

(plants, bacteria, fungi)

Diffusion

Movement of solute from a region of high solute

concentration to a region of relatively low solute

concentration

Figure 2-14

Diffusion and Dialysis

Necessity for Impermeable Membranes

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Chemical Properties of Water

Water Ionizes to Form H+ and OH–

H2O + H2O H3O+ + OH–

Acid Base HydroniumIon

HydroxideIon

K1

K-1

H+ + OH–k1

k-1H2O

OR

Dissociation Constant (K)

K =[H2O]

[H+][OH-]

Ionization (Ion Product) of Water (Kw)

K[H2O] = [H+][OH–]

K = 1.8 x 10–16M

[H2O] = 55.5 M

55.5 M x 1.8 x 10–16 = [H+][OH–] = Kw

Kw = [H+][OH–] = 1 x 10–14 M2

pH

pH = –log[H+] = log[H+]1——

Kw = [H+][OH–] = 1 x 10–14 M2

Neutrality

[H+] = 1 x 10–7 M

pH = 7

Figure 2-16

Relationship of pH and [H+] & [OH–]

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Solutions

H+ pH

[H+] = [OH-] Neutral = 10-7 M 7

[H+] > [OH-] Acidic >10-7 M <7

[H+] < [OH-] Basic < 10-7 M >7

Many Reactions Generate Acid or Base

Acids and Bases Alter the pH

Ionization of a Weak Acid

HA + H2O H3O+ + A–

Acid BaseConjugate

BaseConjugate

Acid

k1

k-1

Strength of an Acid (Acid Dissociation Constant, Ka)

K =[H3O+][A–][HA][H2O]

Ka = K[H2O] = [H+][A–][HA]

Henderson-Hasselbalch Equation

pKa = –logKa

when [A–] = [HA]

pKa = pH

pH = pKa + log[A–][HA]

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Acetic Acid

0

4

8

12

EquivalencePoint

pKa

CH3COOH = CH3COO–

pH

Base

CH3COOH CH3COO–

Figure 2-18

Titration of a Polyprotic Acid

Buffers (Weak Acids) Resist Changes in pH

Buffering

[low pH] [high pH]CH3COOH

OH– H2O

H+H2O

CH3COO–

Figure 2-17

Titration Curves and Buffers

Biological Fluids are Heavily Buffered

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Blood Buffering System

H2CO3 H+ + HCO3–

CO2 + H2O H2CO3

Disturbances in the Blood Buffer System

Acidosis

Alkalosis