Chapter 2 Water 2 1 Weak Interactions in Aqueous Systems Hydrogen Bonding Gives Water Its Unusual Properties The H O H bond angle is 104 5 less than a perfect tetrahedron Oxygen is more electronegative so there is a dipole and partial charges This allows for hydrogen bonds to form Extended networks of hydrogen bonded water molecules also form bridges between solutes that allow larger molecules to interact with each other without actually touching One molecule of oxygen is capable of making four hydrogen bonds Water Forms Hydrogen Bonds with Polar Solutes Hydrogen bonds are strongest when the bonded molecules are oriented to maximize electrostatic interaction which is when the hydrogen and the two atoms that share it are in a straight line Water Interacts Electrostatically with Charged Solutes Water dissolves salts such as NaCl by hydrating and stabilizing the Na and Cl ions weakening the electrostatic interactions between them Water is effective in screening the electrostatic interactions between dissolved ions because it has a high dielectric constant Entropy Increases as Crystalline Substances Dissolve This is why it is easy to dissolve salts in water There is a favorable free energy change Nonpolar Gases Are Poorly Soluble in Water The nonpolar nature of the gases and the decrease in entropy when they enter solution resulting from an increase in the constraint of the molecules motion makes them poorly soluble in water Nonpolar Compounds Force Energetically Unfavorable Changes in the Structure of Water crystalline compounds of nonpolar solutes and water The free energy change for dissolving a nonpolar solute in water is Clathrates unfavorable Amphipathic regions interactions compounds contain both polar or charged and nonpolar o When in water they form micelles held together by hydrophobic van der Waals Interactions Are Weak Interatomic Attractions Each atom has a van der Waals radius which is a measure of how close that atom will allow another to approach Weak Interactions Are Crucial to Macromolecular Structure and Function Covalent bonds are significantly stronger than other types of bonds o However the cumulative effect of other bonds can make a difference Solutes Affect the Colligative Properties of Aqueous Solutions Colligative properties o Vapor pressure o Boiling point o Melting point freezing point o Osmotic pressure Solutes affect the colligative properties of water based on the fact the water concentration is lower in solutions than in pure water o These affects depend only on the number of solutes not the chemical properties of the solute In a hypertonic solution the cell shrinks as water moves out In a hypotonic solution the cell swells as water enters o Cells generally contain higher concentrations of biomolecules and ions than their surrounds so osmotic pressure drives water into cells o Osmotic lysis is prevented in bacteria and plants because of the rigid cell wall Also some cells have contractile vacuoles that expel water from inside the cell Storing fuel as polysaccharides rather than simple molecules avoids an enormous increase in osmotic pressure 2 2 Ionization of Water Weak Acids and Weak Bases Pure Water Is Slightly Ionized H does not exist in solution they are immediately hydrated to hydronium ions Proton hopping causes water to carry electrical currents well Because of the high ionic mobility of H and OH acid base reactions occur The equilibrium constant is fixed for any chemical reaction at a specified quickly in aqueous solutions temperature The Ionization of Water Is Expressed by an Equilibrium Constant The pH Scale Designates the H and OH Concentrations The ionic product of water Kw is the basis for the pH scale o P denotes negative logarithm of Acidosis Alkalosis is when the pH of someone s blood is too low is when the pH of someone s blood is too high Weak Acids and Bases Have Characteristic Acid Dissociation Constants Strong acids and strong bases are completely ionized in dilute aqueous solutions Acids are proton donors and bases are proton acceptors The tendency of any acid to lose a proton and form its conjugate base is defined by the equilibrium constant Keq for the reversible reaction Equilibrium constants for ionization reactions are usually called ionization constants or acid dissociation constants Ka The stronger an acid the lower its pKa Titration Curves Reveal the pKa of Weak Acids In titration a measured volume of the acid is titrated with a solution of a strong base of known concentration A plot of pH against the amount of base added a titration curve reveals the pKa of the weak acid The pKa is the pH at the midpoint of the titration curve 2 3 Buffering against pH Changes in Biological Systems Buffers Are Mixtures of Weak Acids and Their Conjugate Bases Buffers of acid H or base OH are added are aqueous systems that resist changes in pH when small amounts o Consist of a weak acid and its conjugate base o Buffering region the addition of acid or base has much less effect on pH the relatively flat zone on a titration curve where pKa midpoint of a titration curve Buffering action results from 2 reversible reactions taking place at the same time and reaching equilibrium as determined by kw and ka pka 1 gives the buffer range of a conjugate acid base pair The Henderson Hasselbach Equation Relates pH pka and Buffer Concentration The Henderson Hasselbach Equation describes the shape of a titration curve Weak Acids or Bases Buffer Cells and Tissues against pH Changes proteins in the cytosol contain amino acids that are weak acids or bases Nucleotides as well as many small metabolites contain ionizable groups that can contribute buffering power to the cytoplasm A phosphate buffer system acts in the cytoplasm of all cells and resists changes to pH in the range between about 5 9 and 7 9 Blood plasma is buffered by the bicarbonate system o The pH of a bicarbonate buffer exposed to a gas phase is determined in the aqueous phase and by the partial by the concentration of HCO3 pressure of CO2 in the gas phase Untreated Diabetes Produces Life Threatening Acidosis Human blood plasma has a pH between 7 35 and 7 45 usually The catalytic activity of enzymes is especially sensitive and enzymes typically show maximal catalytic activity at a characteristic pH called the pH optimum 2 4 Water as a Reactant The formation of ATP from ADP and inorganic phosphate is a condensation reaction in which the elements of water are eliminated o The reverse of this is a