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Biomolecules in Water

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 solvent of choice for biological systems. Medium for metabolism.

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Biomolecules in Water

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  1. 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

  2. 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.

  3. O H H 104.5o Structure of water • Water is a polar molecule • Electronegativities of hydrogen (2.1) and oxygen (3.5) result in a polar bond. - Water’s ‘bent’ shape results in the molecule having a + and - end. + +

  4. Structure of water

  5. 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.

  6. H CH2CH3 O O H H Hydrogen bonding • This interaction can occur between water and many biomolecules. • It can also occur between two biomolecules. Example DNA relies on H bonding to hold the double helix together.

  7. 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.

  8. Water as a solvent • Water will dissolve biomolecules that are polar or ionic - hydrophilic. • It has a high ion solvating ability. • Na+ 0.14 M • K+ 0.004M • Cl- 0.10 M Concentrations in blood.

  9. Dissolution of NaCl

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

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

  12. 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.

  13. 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. • Autoionization • pK, pKw, pKa • Titrations • Buffers

  14. Autoionization of water • Water is an amphiprotic 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

  15. Autoionization of water • Autoionization • 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 product of water

  16. Acid 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’s already included in the constant.

  17. 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[H3O+] • pOH = -log[OH-] • pH + pOH = 14

  18. 14 7 0 10-14 M 10-7 M 1 M Very basic Neutral Very acidic H+ H+ H+ 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.

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