Lecture 3: Water (the solvent of life)

Principle: Properties of Water

The solvent properties of water shape the sevolution of living things

Principle: Ionization of Water

The Ionization behavior of water and of weak acids and bases dissolved in water can be represented by one or more equilibrium constants

Principle 3: Buffers

An aqueous solution of a weak acids and its salt makes a buffer that resists change in pH in response to added acid or base

Weak acid plus its conjugate base makes a buffer

Principle 4: Enzymes

Enzymes which catalyze all of the processes inside a cell have evolved to function optimally at near neutral physiological pH. However, enzymes that function in intracellular compartments of low or high pH show their greatest activity and stability at those pH values

Hydrogen Bonding

Electrostatic attraction between oxygen atom of one water molecule and hydrogen of another

Give water its unusual properties

High melting and boiling point and heat of vaporization than other solvents

Strength of Hydrogen Bonds

Hydrogen bonds are relatively weak

Bond dissociation energy is about 23kJ/mol in liquid H2O

~10% covalent and ~90% electrostatic

Hydrogen bonds are fleeting, their lifetimes are short, high turnover, when one breaks another forms

Number of Hydrogen Bonds Formed

In liquid, each molecule of water forms hydrogen bonds with 3.4 other molecules

In ice, each forms 4 hydrogen bonds

Phase of Matter Changes

During melting or evaporation heat is taken up by the system and entropy increases

At room temperature melting and evaporation occur spontaneous

Other ways to form Hydrogen bonds

Bonding with polar molecules

FON: Fluorine, Oxygen, Nitrogen

H Bonds form between an electronegative atom and a hydrogen

Hydrogen bonds covalently bonded to carbon do not hydrogen bond

Biological Importance of Hydrogen Bonding

Alcohols, aldehydes, ketones, and compounds containing N-H bonds all form hydrogen bonds with water

Waters interaction with charged solutes

Water interacts electrostatically with charged solutes

Hydrophilic: describes compounds that dissolve easily in H2O, generally charged or polar compounds

Hydrophobic: nonpolar molecules such as lipids and waxes

Amphipathic: contains regions that are polar/charged and regions that are nonpolar

Water as a Solvent

H2O dissolves salts and charged biomolecules by screening electrostatic interactions

The increase in entropy of the system is largely responsible for the ease of dissolving salts in water

Nonpolar Gases and Water

Nonpolar gases are poorly soluble in water

CO2 O2 and N2 are biologically important gases that are nonpolar

Their movement into aqueous solution decreases entropy by constraining their motion

Nonpolar Compounds and Water

Nonpolar compounds force energetically unfavorable changes in the structure of water

Nonpolar compounds interfere with the hydrogen bonding among water molecules

Increasing the enthalpy and decreasing entropy

Using the Gibbs free energy equation the dissolving of nonpolar compounds in water end up requiring energy

Ordering of Water Molecules Around Nonpolar Solutes

Water molecules form a highly ordered cage like shell around each solute molecule

Maximizing solvent solvent hydrogen bonding

This brings back order

Amphipathic Compounds in Aqueous Solutions

Polar, hydrophilic region interacts favorably with H2O and tends to dissolve

Nonpolar, hydrophobic region tends to avoid contact with H2O and cluster together

Highly ordered water molecules form cages around the hydrophobic alkyl chains

The Hydrophobic Effect

Nonpolar regions cluster together

Polar regions arrange to maximize interaction with each other and with the solvent

Micelles

Thermodynamically stable structures of amphipathic compounds of water

Lipids in Water

Dispersion of lipids: Each lipid molecule forces surrounding H2O molecules to become highly ordered

Cluster of lipids: Only lipid portions at the edge of the cluster force the ordering of water, fewer water molecules are ordered, and entropy is increased

Micelles: All hydrophobic groups are sequestered from water, ordered shells of H2O molecules is minimized and entropy is further increased

Release of Ordered Water

Favors formation of an enzyme substrate complex

Van der Waals Interactions

Weak interatomic attractions

Distance dependent weak attractions and repulsions between transient dipoles

Van der Waals radius: measure of how close an atom will allow another to approach

Why are weak interaction important

Weak interactions are crucial to macromolecular structure and function

Noncovalent interactions are much weaker than covalent bonds so they continuously form and break

The Cumulative Effect of Weak Interactions

For macromolecules the most stable structure usually maximizes weak interactions

H2O molecules are often found to be bound so tightly to biomolecules that they are part of the crystal structure

Producing Osmotic Pressure

Concentration of solutes

The movement of water from higher to lower concentrations

Solutes alter the colligative properties of the solvent (vapor pressure, boiling point, melting point/freezing point, osmotic pressure)

Effect depends on the number of solute particles (molecules or ions) in a given amount of water

Osmotic Pressure

The force necessary to resist water movement

It is approximated by the van’t Hoff equation pi=icRT

Calculating Osmotic Pressure

The van’t Hoff factor, a measure of the extent to which the solution dissociates into 2+ ionic species (i)

For a nonionizing solute i=1 and a solute that dissociate into two ions i=2

The solute’s molar concentrations (c)

R is the gas constant and T is the absolute temperature

Osmolarity and Osmosis

Osmolarity is the measure of the solute concentration and dictates how many solute osmoles are existing in 1 liter of a standard solution

This equals the product of the van’t Hoff factor i and the solute molar concentration c

Osmosis is water movement across a semipermeable membrane driven by differences in osmotic pressure

Osmotic environments and RBC

Hypotonic: cell swells, more solute on inside than out so water rushes in

Hypertonic: cell shrinks, more solute outside than in so water leaves

Isotonic: no net gain or loss