Presentation on theme: "CHAPTER 2 Water and Aqueous Solutions"— Presentation transcript:
1CHAPTER 2 Water and Aqueous Solutions Learning ObjectivesTypes of non-covalent interactions between moleculesProperties of water – THE medium for lifeHydrophobic -- nonpolar -- moieties aggregate in waterSolute effects on bulk properties of waterWeak acids and basesBuffers theory and practiceWater as participant in biochemical reactions
2Physics of Non-covalent Interactions Non-covalent interactions do not involve sharing a pair of electrons. Based on their physical origin, one can distinguish betweenIonic (Coulombic) InteractionsElectrostatic interactions between permanently charged species,or between the ion and a permanent dipoleDipole InteractionsElectrostatic interactions between uncharged, but polar moleculesVan der Waals InteractionsWeak interactions between all atoms, regardless of polarityAttractive (dispersion) and repulsive (steric) componentHydrophobic EffectComplex phenomenon associated with the ordering of water molecules around non-polar substances
3Noncovalent Forces and Interactions Hydrogen bondsIon-Ion 1/rIon-dipole 1/r2Dipole-dipole 1/r3Dipole - Induced dipole - 1/r5ID – ID (Van der Waals) - 1/r6Hydrophobic
4Hydrogen BondsStrong dipole-dipole or charge-dipole interaction that arises between an acid (proton donor) and a base (proton acceptor)Typically 4-6 kJ/mol for bonds with neutral atoms,and 6-10 kJ/mol for bonds with one charged atomTypically involves two electronegative atoms (frequently nitrogen and oxygen)Hydrogen bonds are strongest whenthe bonded molecules are oriented tomaximize electrostatic interaction.Ideally the three atoms involved are in a line
6Importance of Hydrogen Bonds Source of unique properties of waterStructure and function of proteinsStructure and function of DNAStructure and function of polysaccharidesBinding of a substrates to enzymesBinding of hormones to receptorsMatching of mRNA and tRNA
8Van der Waals Interactions Van der Waals interactions have two components:Attractive force (London dispersion) Depends on the polarizabilityRepulsive force (Steric repulsion) Depends on the size of atomsAttraction dominates at longer distances (typically nm)Repulsion dominates at very short distancesThere is a minimum energy distance (van der Waals contact distance)
9Biochemical Significance of Van der Waals Interactions Weak individuallyEasily broken, reversibleUniversal:Occur between any two atoms that are near each otherImportancedetermines steric complementaritystabilizes biological macromolecules (stacking in DNA)facilitates binding of polarizable ligands
10Water is the Medium for Life Life evolved in water (UV protection)Organisms typically contain 70-90% waterChemical reactions occur in aqueous milieuWater is a critical determinant of the structure and function of proteins, nucleic acids, and membranes
11Structure of the Water Molecule Four electron pairs on foursp3 orbitals (distorted tetrahedron)Two pairs covalently link hydrogen atoms to a central oxygen atom.Two remaining pairs remain nonbonding (lone pairs)The electronegativity of the oxygen atom induces a net dipole momentWater can serve as both a hydrogen bond donor and acceptor.
12Hydrogen Bonding in Water Up to four H-bonds H2Ohigh boiling pointhigh melting pointlarge surface tensionHydrogen bonding in water is cooperative.Hydrogen bonds between neighboring molecules are weak (20 kJ/mole) relative to the H–O covalent bonds (420 kJ/mol)
13Water as a SolventWater is a good solvent for charged and polar substancesamino acids and peptidessmall alcoholscarbohydratesWater is a poor solvent for nonpolar substancesnonpolar gasesaromatic moietiesaliphatic chains
15Water Dissolves Many Salts High dielectric constant of water (ε = 80 ) shields oppositely charged ions;Almost no attraction > 40 nmElectrostatics of solvation lowers the energy of the systemEntropy increases as ordered crystal lattice is dissolvedNaCl(s) <=>Na+ + Cl-
16Ice: H2O(s)Water has many different crystal forms; the hexagonal ice is the most commonHexagonal ice forms a regular lattice, and thus has a low entropyHexagonal ice has lower density than liquid water; ice floats
17The Hydrophobic Effect Refers to the association or folding of non-polar molecules in the aqueous solutionIs one of the main factors behind:Protein foldingProtein-protein associationFormation of lipid micellesBinding of steroid hormones to their receptorsDoes not arise because of some attractive direct force between two non-polar molecules
18Solubility of Polar and Non-polar Solutes Why are non-polar molecules poorly soluble in water?
19Low Solubility of Hydrophobic Solutes Disruption of H-bonded H2O networks“Ordered” Water near a hydrophobic soluteCavity formation in a medium with high surface tension
20Hydrophobic EffectLipid molecules disperse in the solution; nonpolar tail of each lipid molecule is surrounded by ordered water moleculesLipid aggregates – Water released, surface area reduced
21FIGURE 2-7b (part 3) Amphipathic compounds in aqueous solution FIGURE 2-7b (part 3) Amphipathic compounds in aqueous solution. (b) By clustering together in micelles, the fatty acid molecules expose the smallest possible hydrophobic surface area to the water, and fewer water molecules are required in the shell of ordered water. The energy gained by freeing immobilized water molecules stabilizes the micelle.
22Hydrophobic Effect Favors Ligand Binding Binding sites in enzymes and receptors are often hydrophobicSuch sites can bind hydrophobic substrates and ligands such as steroid hormonesMany drugs are designed to take advantage of the hydrophobic effect
23Colligative Properties Some properties of solution — boiling point, melting point, and osmolarity — do not depend strongly on the nature of the dissolved substance. These are called colligative propertiesOther properties — viscosity, surface tension, taste, and color, among other — depend strongly on the chemical nature of the solute. These are non-colligative properties.Cytoplasm of cells are highly concentrated solutions and have high osmotic pressure
24FIGURE 2-11 Osmosis and the measurement of osmotic pressure FIGURE 2-11 Osmosis and the measurement of osmotic pressure. (a) The initial state. The tube contains an aqueous solution, the beaker contains pure water, and the semipermeable membrane allows the passage of water but not solute. Water flows from the beaker into the tube to equalize its concentration across the membrane. (b) The final state. Water has moved into the solution of the nonpermeant compound, diluting it and raising the column of water within the tube. At equilibrium, the force of gravity operating on the solution in the tube exactly balances the tendency of water to move into the tube, where its concentration is lower. (c) Osmotic pressure (Π) is measured as the force that must be applied to return the solution in the tube to the level of that in the beaker. This force is proportional to the height, h, of the column in (b).
25Effect of Extracellular Osmolarity Osmotic PressureFor a single soluteΠ = RT (ic)Where i is extent of dissociation and c is concentration.For mixturesΠ = RT Σ (ic)
27FIGURE 2-10 Water chain in cytochrome f FIGURE 2-10 Water chain in cytochrome f. Water is bound in a proton channel of the membrane protein cytochrome f, which is part of the energy-trapping machinery of photosynthesis in chloroplasts (see Figure 19-64). Five water molecules are hydrogen-bonded to each other and to functional groups of the protein: the peptide backbone atoms of valine, proline, arginine, and alanine residues, and the side chains of three asparagine and two glutamine residues. The protein has a bound heme (see Figure 5-1), its iron ion facilitating electron flow during photosynthesis. Electron flow is coupled to the movement of protons across the membrane, which probably involves “proton hopping” (see Figure 2-13) through this chain of bound water molecules.
28Ionization of Water H2O H+ + OH- O-H bonds are polar and can dissociate heterolyticallyProducts are a proton (H+) and a hydroxide ion (OH-)Dissociation of water is a rapid reversible processMost water molecules remain un-ionized, thus pure waterhas very low electrical conductivity (resistance: 18 M•cm)The equilibrium H2O H+ + OH- is strongly to the leftExtent of dissociation depends on the temperature
29Proton Hydration Protons do not exist free in solution. They are immediately hydrated to form hydronium (oxonium) ionsA hydronium ion is a water molecule with a proton associated with one of the non-bonding electron pairsHydronium ions are solvated by nearby water moleculesThe covalent and hydrogen bonds are interchangeable. This allows for an extremely fast mobility of protons in water via “proton hopping”
30Proton HoppingHydrogen bonded networks form natural chains for rapid Proton transfer
31Ionization of Water: Quantitative Treatment Concentrations of participating species in an equilibrium processare not independent but are related via the equilibrium constant[H+]•[OH-]H2O H+ + OH-Keq = ————[H2O]Keq can be determined experimentally, it is 1.8•10-16 M at 25 °C[H2O] can be determined from water density, it is 55.5 MIonic product of water:In pure water [H+] = [OH-] = 10-7 M
32What is pH?pH is defined as the negative logarithm of the hydrogen ion concentration.Simplifies equationsThe pH and pOH must always add to 14pH can be negative ([H+] = 6 M)In neutral solution, [H+] = [OH-] and the pH is 7pH = -log[H+]
34Dissociation of Weak Electrolytes: Principle Weak electrolytes dissociate only partially in waterExtent of dissociation is determined by the acid dissociation constant KaWe can calculate the pH if the Ka is known. But some algebra is needed!
35Dissociation of Weak Electrolytes: Example What is the final pH of a solution when 0.1 moles of acetic acid is adjusted to 1 L of water?We assume that the only source of H+ is the weak acidTo find the [H+], a quadratic equation must be solved.0.1 – x x xx = , pH = 2.883Ax2 + bx + c = 0
36Dissociation of Weak Electrolytes: Simplification The equation can be simplified if the amount of dissociated species is much less than the amount of undissociated acidApproximation works for sufficiently weak acids and basesCheck that x << [Total Acid]0.1 – x x xx xx = , pH = 2.880
37pKa = -log Ka (strong acid large Ka small pKa) pKa measures aciditypKa = -log Ka (strong acid large Ka small pKa)
38Buffers are mixtures of weak acids and their anions Buffers resist change in pHAt pH = pKa, there is a 50:50 mixture of acid and anion forms of the compoundBuffering capacity of acid/anion system is greatestat pH = pKaBuffering capacity is lost when the pH differs from pKaby more than 1 pH unit
39FIGURE 2-16 The titration curve of acetic acid FIGURE 2-16 The titration curve of acetic acid. After addition of each increment of NaOH to the acetic acid solution, the pH of the mixture is measured. This value is plotted against the amount of NaOH added, expressed as a fraction of the total NaOH required to convert all the acetic acid (CH3COOH) to its deprotonated form, acetate (CH3COO–). The points so obtained yield the titration curve. Shown in the boxes are the predominant ionic forms at the points designated. At the midpoint of the titration, the concentrations of the proton donor and proton acceptor are equal, and the pH is numerically equal to the pKa. The shaded zone is the useful region of buffering power, generally between 10% and 90% titration of the weak acid.
41Biological Buffer Systems Maintenance of intracellular pH is vital to all cellsEnzyme-catalyzed reactions have optimal pHSolubility of polar molecules depends on H-bond donors and acceptorsEquilibrium between CO2 gas and dissolved HCO3- depends on pHBuffer systems in vivo are mainly based onphosphate, concentration in millimolar rangebicarbonate, important for blood plasmahistidine, efficient buffer at neutral pHBuffer systems in vitro are often based on sulfonic acids of cyclic aminesHEPESPIPESCHES
43Chapter 2: SummaryThe goal of this chapter was to help you to better understand:The nature of intermolecular forcesThe properties and structure of liquid waterThe behavior of weak acids and bases in waterThe way water can participate in biochemical reactions