Water Biochem 4511 Figures Essentials of Biochemistry 3rd Ed OSU Custom Edition Principles of Biochemistry 5th Ed Moran et al Lehninger Principles of Biochemistry 5th Ed Nelson Cox Fundamentals of Biochemistry 2nd Ed Voet Voet Pratt Water is abundant in living organisms About 60 of humans is water 2014 John Wiley Sons Inc All rights reserved Molecule of Water is polar Water has tetrahedral geometry because of its electronic structure Water is polar Oxygen has 2 lone pair electrons 2014 John Wiley Sons Inc All rights reserved An electron from hydrogen is shared with one from oxygen Molecule of water is polar 1 Electronegativity difference between atoms 2 Orientation of polar bonds Hydrogen Bonding Hydrogen Acceptor Hydrogen Donor Hydrogen bond distance 1 8 Sum of van der Waals 2 8 1 nm 10 Donors and Acceptors A molecule can be both a hydrogen bond donor and acceptor Acceptor provides lone pair Donor donates proton Acceptor provides lone pair Donor donates proton Atomic distances Covalent bond Hydrogen bond No bond van der Waals radii 2 7 1 nm 10 Hydrogen Bonding 1 Due to tetrahedral geometry a single water molecule may form up to 4 hydrogen bonds with other water molecules or with other molecules 4 2 3 Ice Crystalline Water H bond formation is maximized Crystal lattice Tetrahedral arrangement Low density H bonds Define Properties of Water of hydrogen bonds Ice floats on top of warmer water H bonds Define Many Physical Properties of Water Irregular but significant hydrogen bonding Surface tension is high Heat capacity is very high effects of oceans on climate Ice has lower density and thus floats High boiling point 100 C Hydrogen Bonding in Macromolecules Not limited to water water interactions Hydrogen bonding capability is based on electronegativity Carbon does not form H bonds Electronegativity is a periodic function The farther the element is to the right of the Periodic Table the more electronegative it is there are exceptions Which of the above elements will not form a hydrogen bond 2014 John Wiley Sons Inc All rights reserved Hydrogen bonds are weak 20 kJ mol 1 4 8 kcal mol 1 1cal 4 2J 1J 0 239 cal DNA But plentiful Proteins and therefore play critical roles in biomolecules Water as a solvent Ionic solutes Ionic solutes NaCl each ion is separately surrounded by network of ordered water Interactions between the ions are weakened Water as a solvent Common Biological Functional groups Alcohol Carbonyl Carboxylate Amine Hydrophilicity Hydrophilic Water loving Molecule forms favorable interactions with water and dissolves well Hydrophobic Water fearing Molecule does not make favorable interactions with water and does not dissolve well Thermodynamics G H T S G Gibbs free energy change H Enthalpy change S Entropy change Reaction is spontaneous favorable when G 0 i e H T S Water as solvent Nonpolar Solutes No hydrogen bonds available with solute Water maximizes H bonds to itself forming a highly ordered water cage Water cage has a huge entropic cost and is extremely important for proper protein folding and the formation of cellular membranes Amphipathic Molecules Part hydrophilic and part hydrophobic Amphipathic Molecules Part hydrophilic and part hydrophobic Water Soluble Hydrophilic Hydrophobic or Amphipathic Glucose Tryptophan Testosterone Guanine Sodium dodecyl sulfate pH and Buffers Biochem 4511 Figures Essentials of Biochemistry 3rd Ed OSU Custom Edition Lehninger Principles of Biochemistry 5th Ed Nelson Cox Fundamentals of Biochemistry 2nd Ed Voet Voet Pratt Acids and Bases HA B HB A Br nsted acid HA can donate a proton to the conjugate base ABr nsted base B can accept a proton to form conjugate acid HB Water Serves as Both an Acid and Base H 2O H H does not exist as a separate species in water H3O 2 H 2O Hydroxide ion Hydronium ion HA H 3O B HB A Hydronium ions H2O H2O OH H3O H does not exist as a separate species in water H3O Delocalization Proton Jumping Rapid H mobility Water as an Acid and Base Keq H2O H2O OH H3O Equilibrium constant for acid base reactions is defined as Keq Keq products reactants products reactants H3O A HA H2O Keq may be experimentally determined for H2O Keq 1 8 x 10 16 Water as an Acid and Base Kw H2O H2O OH H With water we consider a special equilibrium constant Kw the ion product constant for water Kw is defined as the general equilibrium constant multiplied by H2O which is a constant of 55 M Kw Keq H2O H OH 10 14 Kw 1 0 x 10 14 Water as an Acid and Base Kw Kw H OH 1 0 x 10 14 Kw may be used to calculate H and OH for a given water sample In neutral water H OH and therefore Kw H OH 10 14 H 1 0 x 10 7 M OH pH scale H 1 0 x 10 7 M OH These numbers are very small compared to bulk water 55 5 M However the variations are significant A new scale makes H easier to report and makes calculations simpler pH log H log 1 H For neutral water described above pH log 1 0 x 10 7 M 7 0 pH log H Less H3O more basic More OH more basic pH is higher More H3O more acidic Less OH less acidic pH is lower pH and pOH pH log H pOH log OH As pH and pOH are in equilibrium as defined by Kw Examples pH pOH 14 If H 0 1 M then pH log 1 x 10 1 M pH 1 If H 1 M then pH log 1 x 10 6 M pH 6 If OH 1 x 10 1 M then pOH 1 and pH 14 1 13 or H Kw OH 1x10 14 1x10 1 1x10 13 Temperature H2O and pressure Affects pH H OH Reaction is endothermic absorbs heat By Le Chatelier s Principle if you increase the temperature the forward reaction is favored and H is increased 25 C pH of pure H2O 7 0 0 C pH 7 47 40 C pH 6 77 The pH of a solution changes in an incubator or the cold room Acids and Bases Equilibrium constant for acid base reactions HA H2O Keq products reactants A H3O H3O A HA H2O Acids and Bases Ka For acids and bases in an aqueous environment it is convenient to consider a simplified equilibrium constant termed the acid dissociation constant Ka Ka Keq H2O H3O A HA H2O H2O H3O A HA When water is the solvent H2O 55 5 M and this value essentially does not change Definition Strong Acid or Base Ka H3O A HA For a Strong Acid Ka 1 Almost completely dissociated in water Hydrochloric acid Sulfuric acid Nitric acid For a Weak Acid Only partially ionized in water Ka 1 Acetic acid Phosphoric acid Carbonic acid Can find Ka in tables if needed in 4511 we will provide Determining pH Strong Acids What is the pH of a 1 L solution of water after the addition of 10 L 1M HCl 1 Is the acid weak or strong HCl is a strong acid 2 A …