Chapter-2 (Page 22-37) Physical and Chemical Properties of Water

Chapter-2 (Page 22-37) Physical and Chemical Properties of Water

Introduction About 70% of the mass of the human body is water. Water is central to biochemistry for the following reasons: 1- Biological molecules assume their shape because of surrounding water. 2 - Medium for biochemical reactions is water. 3 Water itself actively participates in many chemical reactions that support life.

Introduction The reactivity of many functional groups on biological molecules depends on the relative concentrations of H + and OH - in the surrounding medium.

Chapter Contents 1- Physical Properties of Water: A - Water is a Polar Molecule. B - Hydrophilic Substances Dissolve in Water. C - The Hydrophobic Effect Causes Nonpolar Substances to Aggregate in Water. D Water Moves by Osmosis and Solutes Move by Diffusion. 2 Chemical Properties of Water : A Water Ionizes to Form H + and OH -. B Acids and bases alter the ph. C Buffers resist changes in ph.

Physical Properties of water Dr. Amjid Iqbal PhD (University of Cambridge,UK) 5

1- Physical Properties of Water: A - Water is a Polar Molecule A water molecule consists of two hydrogen atoms bonded to an oxygen atom. The O H bond distance is 0.958 Ǻ (1 Ǻ = 10-10 m) and the angle formed by the three atoms is 104.5 (Fig. 2 1).

1- Physical Properties of Water: A - Water is a Polar Molecule Water is a polar molecule: the oxygen atom with its unshared electrons carries a partial negative charge (δ - ) of - 0. 66e, and the hydrogen atoms each carry a partial possitive charge (δ + ) of + 0.33e, where e is the charge of electron.

Water Molecules Form Hydrogen Bonds Dr. Amjid Iqbal PhD (University of Cambridge,UK) 8

How Do Water Molecules Bond With Each Other? Water molecules orient themselves so that O-H bond of one water molecule (positive end) point towards electron pair of other water molecule (negative end). The resulting intermolecular association called hydrogen bond Dr. Amjid Iqbal PhD (University of Cambridge,UK) 9

1- Physical Properties of Water: A - Water is a Polar Molecule Figure 2-2: A hydrogen bond between two water molecules. The strength of the interaction is maximal when the O H covalent bond of one molecule points directly toward the one pair electron cloud of the other.

Representation of hydrogen bond H-bond can be represented as D-H.A D-H is donor group such as O-H, N-H or S-H Where as A is acceptor such as O, N or S Each water molecule can participate in a maximum of four hydrogen bonds with other water molecules Dr. Amjid Iqbal PhD (University of Cambridge,UK) 11

Hydrogen Bond Distance Hydrogen bonds are structurally characterized by an H, A distance that is at least 0.5 Ǻ shorter than the calculated van der Waals distance (the distance of closest approach between two nonbonded atoms). In water, O..H hydrogen bond distance is 1.8 A versus 2.6 A for the corresponding van der waals distance. Dr. Amjid Iqbal PhD (University of Cambridge,UK) 12

Strength of hydrogen bond Energy of individual hydrogen bond is 20 KJ/mol is relatively small than energy of an O-H covalent bond 460 kj/mol Dr. Amjid Iqbal PhD (University of Cambridge,UK) 13

Ice is a Crystal of Hydrogen Bonded Water Molecules: Water molecules in ice are arranged in an unusually open structure. Each water molecule is tetrahedrally surrounded by four nearest neighbors to which it is hydrogen bonded (Fig. 2 3).

Figure 2-3: The structure of ice (regular). Each water molecule interacts tetrahedrally with four other water molecules. Oxygen atoms are red and hydrogen atoms are white. Hydrogen bonds are represented by dashed lines. N.B: at 0 c, liquid water has a density of 1.0 g / ml, where ice has a density of 0.92 g / ml.

States of Water Solid (Ice) Dr. Amjid Iqbal PhD (University of Cambridge,UK) 16

Expansion of water Water expands on freezing. At 0C, liquid water has density of 1.00 g/ml, whereas ice has density of 0.92 g/ml Dr. Amjid Iqbal PhD (University of Cambridge,UK) 17

Consequences of expansion of water on life Because ice is less dense than water, it floats at surface of water and maintain the temperature and make marine life possible If water contracted on freezing, it would become more dense and ice would sink to the bottom of lakes and oceans and water would have been frozen solid and whole earth would be locked in permanent ice age. Water at even great depth has temp close to 4C) Dr. Amjid Iqbal PhD (University of Cambridge,UK) 18

Melting of Ice Melting of ice is collapse of tetrahedral orientation of hydrogen bonded water molecules Liquid water has 15% less hydrogen bonds than ice at OC Dr. Amjid Iqbal PhD (University of Cambridge,UK) 19

Physical States Vapor Dr. Amjid Iqbal PhD (University of Cambridge,UK) 20

The Structure of Liquid Water is Irregular: Three to seven membered rings of hydrogen - bonded molecules commonly occur in liquid water (Fig. 2 4), in contrast to the six - membered rings characteristic of ice (Fig. 2 3). Moreover, these networks continually break up and re-form every 2 x10-11 s or so. Liquid water therefore consists of a rapidly fluctuating, three -dimensional network of hydrogen bonded H 2 O molecules.

1- Physical Properties of Water: A - Water is a Polar Molecule Figure 2-4: Rings of water molecules (irregular). These molecules, containing three, four, five molecules, are based on theoretical predictions and spectroscopic data.

Hydrogen bonds and Other Weak Interactions Influence Biological Molecules Structures of biological molecules are determined by collective influence of many individually weak interactions These interactions include ionic interactions, hydrogen bonds, van der Waals forces Dr. Amjid Iqbal PhD (University of Cambridge,UK) 23

Ionic Interactions are association between opposite charges The noncovalent associations between neutral molecules, collectively known as van der Waals forces Dr.Amjid Iqbal PhD (University of Cambridge) 24

Bond Energies in Biomolecules Type of Bond Example Bond Strength (KJ/mol) Covalent Non-Covalent O-H C-H C-C 460 414 348 Ionic Interaction -COO-.+NH3-86 Van der Waals forces Hydrogen bond 20 Dipole-dipole interaction 9.3 London dispersion forces 0.3 Dr.Amjid Iqbal PhD (University of Cambridge) 25

Hydrophilic substances dissolve in water Solubility depends on the ability of solvent to interact with a solute more strongly than solute particles interact with each other. Water is said to be the Universal Solvent. Hydrophilic (Greek: hydro, water + philos, loving). On the other hand, nonpolar substance are virtually insoluble in water ( oil and water don t mix ) and are consequently described as hydrophobic (Greek: phobos, fear).

Why do salts such as NaCl dissolve in water? Polar solvents, such as water weaken the attractive forces between oppositely charged ions (such as Na + and Cl - ) and can therefore hold the ions apart. Such ions are said to be solvated or when water is the solvent, to be hydrated.

1- Physical Properties of Water: B Hydrophilic substances dissolve in water Figure 2-6: Solvation of ion. The dipoles of the surrounding water molecules are oriented according to the charge of the ion. Only one layer of solvent molecules is shown.

Solubility of uncharged polar molecules Solubilities of polar and ionic substances increase when they carry functional group such as hydroxyl, carbonyl, carboxylate or ammonium, as they can make hydrogen bonds with water. Dr.Amjid Iqbal PhD (University of Cambridge) 29

Biomolecules such as protein, nucleic acids and carbohydrates are soluble in water because of presence of these groups Dr.Amjid Iqbal PhD (University of Cambridge) 30

The Hydrophobic Effect Causes Nonpolar Substances to Aggregate in Water When a nonpolar substance is added to an aqueous solution, it does not dissolve but instead is excluded by the water. The Hydrophobic Effect: is the tendency of water to minimize its contact with hydrophobic molecules. A nonpolar group can neither accept nor donate hydrogen bonds.

Amphiphiles Form Micelles and Bilayers: Most biological molecules have both polar (or charged) and nonpolar segments and are therefore simultaneously hydrophilic and hydrophobic. Such molecules, for example, fatty acids (soaps) are said to be Amphiphilic or Amphipathic (Greek: amphi, both; pathos, suffering).

How do amphiphiles interact with aqueous solvent? Water hydrate hydrophilic portion and exclude hydrophobic portion.(an example of fatty acid) Non Polar hydrocarbon chain Polar carboxylate group Dr.Amjid Iqbal PhD (University of Cambridge) 33

Micelles Globules of thousand of amphiphilic molecules arranged so that hydrophilic groups at the surface and can interact with aqueous solvent while hydrophobic groups associate t center, away from the solvent. Dr.Amjid Iqbal PhD (University of Cambridge) 34

Bilayers Amphiphilic molecules may arrange themselves to form bilayered sheets in which the polar groups face the aqueous phase. Dr.Amjid Iqbal PhD (University of Cambridge) 35

Dr.Amjid Iqbal PhD (University of Cambridge) 36

Water Moves by Osmosis and Solutes Move by Diffusion Osmosis: is the movement of solvent across the membrane from a region of high concentration to a region of relatively low concentration. The Osmosis pressure of the solution: is the pressure that must be applied to the solution to prevent the inward flow of water; it is proportional to concentration of the solute (Fig. 2 13). Diffusion of the solutes: is the basis for the laboratory technique of Dialysis.

1- Physical Properties of Water: D Water Moves by Osmosis and Solutes Move by Diffusion Figure 2-13: Osmotic Pressure. (a) A water-permeable membrane separates a tube of concentrated solution from pure water. (b) As water moves into the solution by osmosis, the height of the solution in the tube increases. (c) The pressure that prevent the influx of water is the osmotic pressure (22.4 atm for a 1 M solution).

Dialysis-Treatment of Kidney failure Diffusion of solutes is the basis for dialysis Dr.Amjid Iqbal PhD (University of Cambridge) 39

1- Physical Properties of Water: D Water Moves by Osmosis and Solutes Move by Diffusion In this process, solutes smaller than the pore size of the dialysis membrane freely exchange between the sample and the bulk solution until equilibrium is reached (Fig. 2 14). Dialysis is particularly useful for separating larger molecules, such as protein or nucleic acids, from smaller molecules. Because small solutes (and water) move freely between the sample and the surrounding medium, dialysis can be repeated several times to replace the sample medium with another solution

Dialysis is used to remove waste materials such as urea and creatinine (present at high concentration in the blood) as well as excess water Dr.Amjid Iqbal PhD (University of Cambridge) 41

2 Chemical Properties of Water

Water Ionizes to Form H + and OH - Water is a neutral molecule with a very slight tendency to ionize. H 2 0 H + + OH - (free proton) (hydroxide ion) There is actually no such thing as a free proton (H + ) in solution. Rather, the proton is associated with a water molecules as a Hydronium Ion, H 3 O +.

Proton Jumping The proton of a hydronium ion can jump rapidly to another water molecule and then to another. Proton jumping is responsible for acid-base reactions Dr.Amjid Iqbal PhD (University of Cambridge) 44

Proton Jumping and Hydronium Dr.Amjid Iqbal PhD (University of Cambridge) 45

Ionization (dissociation) of water Ionization of water is described by following equilibrium expression K = [H + ][OH - ] [H 2 O] Where K is the dissociation constant Considering [H 2 O] constant yields K w = [H + ][OH - ] Dr.Amjid Iqbal PhD (University of Cambridge) 46

K w K w = [H + ][OH - ] Where K w is the ionization constant of water For pure water ionization constant is 10-14 at 25º For pure water [H + ] = [OH - ] = (K w ) 1/2 = 10-7 M Dr.Amjid Iqbal PhD (University of Cambridge) 47

For pure water (neutral) [H + ] = [OH - ] = (K w ) 1/2 = 10-7 M Solution with [H + ] = 10-7 M Neutral 10-7 M Acidic 10-7 M Basic Human blood is slightly basic with Dr.Amjid Iqbal PhD (University of Cambridge) 48

ph The values of [H+] are inconveniently small and difficult to deal, so there is another scale called ph ph = -log[h+] Higher the ph, lower is the H+ concentration, lower the ph, the higher is the H+ concentration. Dr.Amjid Iqbal PhD (University of Cambridge) 49

Dr.Amjid Iqbal PhD (University of Cambridge) 50

Relationship between ph and [H + ] / [OH - ] concentration Dr.Amjid Iqbal PhD (University of Cambridge) 51

ph: Alkalinity/Acidity The scale goes from 0 to 14 When ph = 7, solution is neutral ph < 7, it is acidic ph > 7 is alkaline (or basic) One ph unit represents a ten-fold change in H+ concentration Dr.Amjid Iqbal PhD (University of Cambridge) 52

ph values of Some Common Substances Dr.Amjid Iqbal PhD (University of Cambridge) 53

Acids and Bases Bronsted and Lowery definition: Acid is a substance that can donate a proton. Base is a substance that can accept a proton. HA + H 2 O H 3 O + + A - / Acid Base Conjugate Conjugate Acid Base or HA A - + H + Acid Conjugate Conjugate Base Acid Dr.Amjid Iqbal PhD (University of Cambridge) 54

Acid strength is specified by its dissociation constant Molar concentration K a [H O [HA][H ][A O] for: HA + H 2 O H 3 O + + A - reactants products 3 2 - ] Water concentration is constant, therefore term [H2O] combined with dissociation constant, so above expression would be modified as. Dr.Amjid Iqbal PhD (University of Cambridge) 55

[ H ][ A ] K a K [ H2 O] [ HA] From now on we will drop the a, in K a Weak acids (K<1) Strong acids (K>1) Just like [H+], it is difficult to deal with small values of acid dissociation, so it can be transformed to pk by following formula pk = -logk Dr.Amjid Iqbal PhD (University of Cambridge) 56

Classification of acids Weak acids only partially dissociate in aqueous solution, (K< 1) Dr.Amjid Iqbal PhD (University of Cambridge) 57

Strong acids rapidly transfer all of their protons to aqueous solution, (K>1), Many of mineral acids such as HClO4, HNO3 and HCl are strong acids. Dr.Amjid Iqbal PhD (University of Cambridge) 58

Buffers Resist Changes in ph Background Suppose add 0.01 ml droplet of 1M HCl in 1 L of water and it would change the ph of water from 7 to 5 Such huge change in ph is intolerable to biological systems, as small change in ph can affect the structure and functions of biological molecules So to maintain the ph is very important Dr.Amjid Iqbal PhD (University of Cambridge) 59

Buffers When [HA] = [A-], the ph of solution does not change by the addition of strong base or strong acid. Such solution is known as buffer A buffer can resist ph changes. Buffer capacity: The ability to resist ph changes on addition of acid or base Dr.Amjid Iqbal PhD (University of Cambridge) 60

Biological Applications of Buffer Biological fluids, both intracellular and extracellular are buffered. For example, the ph of the human blood is 7.4. The phosphate and bicarbonate ions in most biological fluids are important buffering gents Dr.Amjid Iqbal PhD (University of Cambridge) 61

End of Chapter Dr.Amjid Iqbal PhD (University of Cambridge) 62