Biomolecules 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

Water 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 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 o Water’s ‘bent’ shape results in the molecule having a  + and  - end. ++ -- ++

Structure of water

Hydrogen 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 bonding This interaction can occur between water and many biomolecules. It can also occur between two biomolecules. O CH 2 CH 3 H O HH Example Example DNA relies on H bonding to hold the double helix together.

Hydrogen bonding and solubility Physical properties of water Due to hydrogen bonding, water is unique for its molecular weight and size. PropertyH 2 ONH 3 CH 4 H 2 S Molecular weight Boiling point ( o C) Melting point( o C) Viscosity a a Units are centipoise.

Water as a solvent hydrophilicWater will dissolve biomolecules that are polar or ionic - hydrophilic. It has a high ion solvating ability. Na M K M Cl M Concentrations in blood. Concentrations in blood.

Dissolution of NaCl

Water as a solvent hydrophobicNonpolar compounds like fats are not very soluble in water - hydrophobic. amphiphilicSome materials have both polar and nonpolar ends - amphiphilic. One end tends to dissolve in polar solvents and the other in nonpolar ones. C C C C C C C C C C C C C C C C O OH Example - saturated fatty acid

How detergents work COO - - OOC COO - - OOC Nonpolar tail dissolves in oil. Polar ‘heads’ are attracted to the water.

Micelles 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.

Cellular 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 BuffersBuffers

Autoionization of water amphiproticWater is an amphiprotic substance that can act either as an acid or a base. HC 2 H 3 O 2(aq) + H 2 O (l) H 3 O + + C 2 H 3 O 2 - (aq) acid base acid base H 2 O (l) + NH 3(aq) NH 4 + (aq) + OH - (aq) acid base acid base

Autoionization of water AutoionizationAutoionization –When water molecules react with one another to form ions. H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) – (10 -7 M) (10 -7 M) K w = [ H 3 O + ] [ OH - ] = 1.0 x at 25 o C ion product of water ion product of water

Acid dissociation constant, K a The strength of a weak acid can be expressed as an equilibrium. – HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) The strength of a weak acid is related to its equilibrium constant, K a. K a = [A - ][H 3 O + ] [HA] We omit water. It’s already included in the constant. We omit water. It’s already included in the constant.

pH 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[H 3 O + ] –pOH = -log[OH - ] –pH + pOH = 14

pH 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 M M 1 M Very basic Neutral Very acidic H+H+ H+H+ H+H+