Biomolecules in WaterBiomolecules in Water

• Water, the Biological Solvent

• Hydrogen Bonding and Solubility

• Cellular Reactions of Water

• Ionization, pH and pK

• The Henderson-Hasselbalch Equation

• Buffer Systems

• Water, the Biological Solvent

• Hydrogen Bonding and Solubility

• Cellular Reactions of Water

• Ionization, pH and pK

• The Henderson-Hasselbalch Equation

• Buffer Systems

WaterWater• The solvent of choice for biological systems.

• Medium for metabolism.

• Able to absorb large amounts of heat.

• Solvent for many materials.

• Used for transport - blood, cerebrospinal fluid, lymph, urine.

• Serves as a reactant or product of many biochemical reactions.

Structure of waterStructure of water

• Water is a polar moleculeWater is a polar molecule• Electronegativities of hydrogen (2.1) and

oxygen (3.5) result in a polar bond.

O

HH

104.5o

Water’s ‘bent’ shaperesults in the moleculehaving a + and - end.+

-

+

Structure of waterStructure of water

Hydrogen bondingHydrogen bonding• The small size of

hydrogen along with the shape and polarity of the water molecule all add up to a relatively strong attraction between water molecules.

• Hydrogen bonding is the strongest of the intermolecular forces.

Hydrogen bondingHydrogen bonding• This interaction

can occur between water and many biomolecules.

• It can also occur between two biomolecules.

O

CH2CH3H

O

HH

Example Example DNA relies on H bonding to hold thedouble helix together.

Hydrogen bonding and solubility Physical properties of water

Hydrogen bonding and solubility Physical properties of water

• Due to hydrogen bonding, water is unique for its molecular weight and size.

• Property H2O NH3 CH4 H2S

• Molecular weight 18 17 16 32• Boiling point (oC) 100 -33 -161 -60.7• Melting point(oC) 0 -78 -183 -85.5• Viscositya 1.01 0.25 0.10 0.15

• aUnits are centipoise.

Water as a solventWater as a solvent

• Water will dissolve biomolecules that are polar or ionic - hydrophilichydrophilic.

• It has a high ion solvating ability.

• Na+ 0.14 M• K+ 0.004M• Cl- 0.10 M

Concentrationsin blood.

Concentrationsin blood.

Dissolution of NaClDissolution of NaCl

Water as a solventWater as a solvent

• Nonpolar compounds like fats are not very soluble in water - hydrophobichydrophobic.

• Some materials have both polar and nonpolar ends - amphiphilicamphiphilic.

• One end tends to dissolve in polar solvents and the other in nonpolar ones.

CC

CC

CC

CC

CC

CC

CC

CC

O

OH

Example - saturated fatty acid

How detergents workHow detergents work

COO--OOC COO-

COO-

COO-

-OOC

-OOC

-OOC

Nonpolar taildissolves in oil.

Polar ‘heads’are attractedto the water.

MicellesMicelles

• Amphiphilic molecules tend to organize into micelle structures.

• The polar heads will point towards the aqueous environment and the nonpolar tails will be on the inside.

OH

Cellular reactions of waterCellular reactions of water

• Since water is the working solvent for biological systems, it is appropriate to review the acid/base chemistry of this solvent.

• AutoionizationAutoionization• pK, pKw, pKapK, pKw, pKa

• TitrationsTitrations• Buffers Buffers

Autoionization of waterAutoionization of water

• Water is an amphiproticamphiprotic substance that can act either as an acid or a base.

• HC2H3O2(aq) + H2O(l) H3O+ + C2H3O2-(aq)

• • acid base acid base

• H2O(l) + NH3(aq) NH4+

(aq) + OH-(aq)

• acid base acid base

Autoionization of waterAutoionization of water• AutoionizationAutoionization

– When water molecules react with one another to form ions.

• H2O(l) + H2O(l) H3O+(aq) + OH-

(aq)

– (10-7M) (10-7M)

• Kw = [ H3O+ ] [ OH- ]

• = 1.0 x 10-14 at 25oC

ion productof water

ion productof water

Acid dissociation constant, KaAcid dissociation constant, Ka• The strength of a weak acid can be expressed

as an equilibrium.

– HA (aq) + H2O(l) H3O+(aq) + A-

(aq)

• The strength of a weak acid is related to its equilibrium constant, Ka.

•

• Ka = [A-][H3O+]• [HA]

We omit water. It’salready includedin the constant.

We omit water. It’salready includedin the constant.

pH and pOHpH and pOH

• We need to measure and use acids and bases over a very large concentration range.

• pH and pOH are systems to keep track of these very large ranges.

– pH = -log[H3O+]

– pOH = -log[OH-]

– pH + pOH = 14

pH scalepH scale

• A log based scale used to keep track of the large change important to acids and bases.

When you add an acid, the pH gets smaller.

When you add a base, the pH gets larger.

14 7 0

10-14 M 10-7 M 1 M

Very basic Neutral Very acidic

H+ H+ H+