CHAPTER 2 : WATER2.1 Weak Interactions in Aqueous Systems

2.2 Ionization of Water, Weak Acids, and Weak Bases

2.3 Buffering against pH Changes in Biological Systems

2.4 Water as a Reactant

2.5 The Fitness of the Aqueous Environment for Living Organisms

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GENERAL BIOCHEMISTRY

1st SEMESTER

Mervi Curie M. Belen

2.1 WEAK INTERACTIONS IN

AQUEOUS SYSTEMS

Principles of Biochemistry by Zubay, Parson, and Vance Chapter 1

Major component of living systems and interacts

with many biomolecules.

Interaction of molecules to water:

- hydrophilic or water-loving

- hydrophobic or water-abhorring

- amphipathic

IMPORTANCE OF WATER

HYDROGEN BONDS Between H2O molecules

Lehninger 4th edition

Dipole- Due to greater electronegativity of the oxygen atom over the hydrogen atoms

Hydrogen bonds- A noncovalent interaction between polar molecules

HYDROGEN BONDS Favor the ordering of water molecules (ice)

Lehninger 4th edition

SOME BIOMELCULES & THEIR

POLARITY

POLAR & NONPOLAR INTERACTIONS

POLAR BIOMOLECULES – dissolves readily in water by replacing

H2O – H2O interactions with more energetically favorable H2O

– SOLUTE interactions

NONPOLAR BIOMOLECULES – poorly soluble, interferes with

H2O-H2O interactions but are unable to form H2O - SOLUTE

interactions

WEAK INTERMOLECULAR FORCES

HYDROGEN BONDS

IONIC INTERACTIONS

VAN DER WAALS FORCES

- weak, but has significant influence on the 3D structure of

macromolecules

H2O is a DIPOLAR molecule

H2O is TETRAHEDRAL

HYDROGEN BONDING GIVES WATER ITS

UNUSUAL PROPERTIES

SOME PROPERTIES OF WATER COMPARED TO

OTHER SOLVENTS

Principles of Biochemistry by Zubay, Parson, and Vance Chapter 1

Lehninger 4th edition

Covalet C – C bond requires 348 kj/mol

HIGH MELTING POINT

H2O (solid) → H2O (liquid) ∆H = 5.9 kJ/mol

H2O (liquid) → H2O (gas) ∆H = 44.0 kJ/mol

o BOND DISSOCIATION ENERGY – energy require to break a bond

Liquid H2O = 23 kJ/mol intermolecular

Liquid H2O = 470 kJ/mol intramolecular

HYDROGEN BONDING are not unique to

water

Lehninger 4th edition

Can you give examples of POLAR

MOLECULES that form H bonds with water?

H BONDS ARE HIGHLY DIRECTIONAL

Greatest when Electrostatic Interaction between bonded

molecules are MAXIMIZED

“F” is for FORCE of IONIC

INTERACTIONS

F = Q1Q2/εr2

where:

Q1 or Q2 = magnitude of charges

ε = dielectric constant of solvent

r = distance b/w charged groups

H2O is effective in screening the

electrostatic interactions between

dissolved ions.

Ionic interactions are stronger in

less polar environments.

ε at 25°C

H2O = 78.5

Benzene = 4.6

WHEN ENTROPY (∆S) IS LARGE, WHAT IS THE

VALUE OF GIBB’S FREE ENERGY (∆G) ?

ENTROPY INCREASES AS CRYSTALLINE

SUBSTANCES DISSOLVE

NaCl dissolving in H2O ↑ freedom of motion

↑ entropy

Lehninger 4th edition

WHAT ARE THE DIFFERENT GASES WHICH ARE

BIOLOGICALLY IMPORTANT?

DRAW THEIR DIPOLE MOMENTS.

HOW DOES ORGANISMS FACILITATE THE

TRANSPORT OF NONPOLAR MOLECULES?

NONPOLAR GASES ARE POORLY SOLUBLE IN

WATER

Lehninger 4th edition

ALL SOLUTES INTERFERE WITH HYDROGEN

BONDING

NONPOLAR COMPOUNDS FORCE

ENERGETICALLY UNFAVORABLE CHANGES IN

THE STRUCTURE OF H2O

WHAT IS THE ENERGY CHANGE

WHEN DISSOLVING NONPOLAR

SOLUTE IN WATER

BENZENE, HEXANE, & OTHER NONPOLAR

COMPOUNDS MIXED WITH WATER

- formation of 2 phases

- hydrophobic, interfere with H bonding, unable

to undergo favorable interactions with water

molecules

- lower entropy

AMPHIPATHIC COMPOUNDS

Contains POLAR & NONPOLAR regions

- proteins, pigments, certain vitamins, sterols, phospholipids

o POLAR REGION – interacts favorably with solvent & dissolves

o NONPOLAR REGION – avoids contact with water

Lehninger 4th edition

MICELLES

A spherical cluster of ions in aqueous solution in

which nonpolar groups are in the interior & the

ionic (polar) groups are at the surface.

HYDROPHOBIC INTERACTIONS – the forces that

hold the nonpolar regions of the molecule in a

micelle

- thermodynamically stable

HOW DOES VAN DER WAALS FORCES WORK?

Van Der Waals INTERACTIONS are WEAK

INTERATOMIC ATTRACTIONS

COLLIGATIVE PROPERTIES ARE ALSO CALLED

“COLLECTIVE” PROPERTIES

SOLUTES AFFECT THE COLLIGATIVE PROPERTIES

OF SOLUTION

COLLIGATIVE PROPERTIES OF

SOLUTION

VAPOR-PRESSURE LOWERING

FREEZING POINT DEPRESSION

BOILING POINT ELEVATION

OSMOTIC PRESSURE

COLLIGATIVE PROPERTIES OF

SOLUTION

VAPOR-PRESSURE LOWERING

FREEZING POINT DEPRESSION

BOILING POINT ELEVATION

OSMOTIC PRESSURE

OSMOSIS

The selective passage of solvent molecules

through a porous membrane from a dilute

solution to a more concentrated one.

VAN’T HOFF EQUATION

Π = icRTwhere:

π = force necessary to resist water movement

R = gas constant

T = absolute temperature

ic = osmolarity of solution

(van’t Hoff factor x solute’s molar concentration)

VAN’T HOFF FACTOR “i”

i = actual number of particles in solution after dissociation

number of formula units initially dissolved in solution

What are the van’t Hoff factor of the following?

NaCL

Na2SO4

CaCl2

Nonelectrolytes

EQUATION FOR Π ON DILUTE

SOLUTIONS

Molality = moles solute/kg solvent

Molarity = moles solute/L solution

R = 0.0821 L.atm/K.mol

K = (°C + 273.15°C) 1K/1°C

Sample Problem:

A solution is prepared by dissolving 35.0 g of

hemoglobin (Hb) in enough water to make up 1L in

volume. If the osmotic pressure of the solution is

found to be 10 mmHg at 25°C, calculate the molar

mass of Hb.

OSMOSIS IN CELLSLehninger 4th edition

2.2 IONIZATION OF WATER, WEAK

ACIDS, & WEAK BASES

IONIZATION OF WATER

H2O ↔ H+ + OH - or H2O + H2O ↔ H3O + + OH -

Lehninger 4th edition

THE EQUILIBRIUM CONSTANT

EQUILIBRIUM CONSTANT (K) – a number equal to

the ratio of the equilibrium concentrations of the

products to the equilibrium concentrations of

reactants, raised to the power of its

stoichiometric coefficient.

Kw THE ION PRODUCT OF WATER

H2O ↔ H+ + OH – Keq = [H+][OH –] / [H2O]

[H2O] pure water at 25°C = 55.5 M

Keq = [H+][OH –] / [H2O]

[H2O] Keq = [H+][OH –] = Kw

Kw = (55.5 M) (1.8 x10-16 M) = 1.0 x 10 -14 M2

At neutral concentration of H+ and OH-

[H+] = √ Kw = 1.0 x 10 -7 M

THE POWER OF HYDROGEN

The pH Scale Designates the H+ and OH-

Concentrations

NEUTRAL, BASIC, ACIDIC

pH = - log [H+]Lehninger 4th edition

NAME SOME WEAK ACIDS AND BASES

Weak Acids and Bases have Characteristic

Dissociation Constant

Reference: General Chemistry, Raymond Chang page 553

Kw THE ION PRODUCT OF WATER

H2O ↔ H+ + OH – Keq = [H+][OH –] / [H2O]

[H2O] pure water at 25°C = 55.5 M

Keq = [H+][OH –] / [H2O]

[H2O] Keq = [H+][OH –] = Kw

Kw = (55.5 M) (1.8 x10-16 M) = 1.0 x 10 -14 M2

At neutral concentration of H+ and OH-

[H+] = √ Kw = 1.0 x 10 -7 M

Ka THE DISSOCIATION CONSTANT

HA ↔ H+ + A-

Keq = [H+][A –] / [HA] = Ka

pKa = log 1/Ka = -log Ka

LOWER pKa , THE STRONGER THE ACID

HIGHER pKa, THE WEAKER THE ACID

Lehninger 4th edition

pKa = pH

Titration Curves Reveal the pKa of Weak Acids

TITRATION CURVESLehninger 4th edition

SAMPLE PROBLEM

Calculate the pH in the titration of 25 mL of 0.10

M CH3COOH by NaOH after the addition to the

acid solution of

(a) 10 ml of 0.10 M NaOH

(b) 25 ml

(c) 35 ml

2.3 BUFFERING AGAINST pH

CHANGES IN BIOLOGICAL SYSTEMS

BUFFERS RESIST CHANGES IN pH

Buffers Are Mixtures of Weak Acids and their

Conjugate Bases

HENDERSON-HASSELBACH EQUATION

A Simple Expression Relates pH, pKa, and

Buffer Concentration

GENERAL FORM:

pH = pKA + log [CONJUGATE BASE] / [ACID]

HA ↔ H+ + A-

Ka = [H+][A –] / [HA]

[H+] = Ka [HA] / [A –]

- log [H+] = -log Ka [HA] / [A –]

- log [H+] = -log Ka -log [HA] / [A –]

- log [H+] = -log Ka + log [A –] / [HA]

pH = p Ka + log [A –] / [HA]

BUFFERS in the CELL’s CYTOPLASM

Weak Acids or Bases Buffer Cells & Tissues

against pH Changes

pH OPTIMUM

Characteristic pH at which

enzymes show maximum

catalytic activity

2.4 WATER AS A REACTANT

CONDENSATION REACTION

HYDROLYSIS REACTION

Condensation Reaction

ADP + POH ↔ ATP + H2O

Hydrolysis reaction

ATP + H2O ↔ ADP + POH

POLYMERIZATION

DEPOLYMERIZATION

Enzymatic Depolymerization – catalyzed by the

enzyme hydrolases, exergonic

Polymerization - endergonic

2.5 THE FITNESS OF THE AQUEOUS

ENVIRONMENT FOR LIVING

ORGANISMS

WATER IS AMAZING!

High Specific Heat

High Heat of Vaporization

High Degree of Internal Cohesion

Low Density of Solid Phase than

Liquid Phase