Water, ph and pka. Lecture 2: Margaret A. Daugherty. Fall Water: What makes it so good for life? Solvent properties.

Transcription

1 Lecture 2: Water, ph and pka Margaret A. Daugherty Fall 2004 Water: What makes it so good for life? Structure ice vs. water or more technically solid vs. liquid Solvent properties High heat capacity High dielectric constant Forms H-bonds with polar solutes Hydrophobic interaction Solvation of ions High heat of vaporization High boiling point High viscosity High density Colligative properties vapor pressure depression boiling point elevation freezing point depression osmotic pressure

2 The structure of water Key points: permanent dipole arises from difference in electronegativity between O & H from geometry Water Water is a unique solvent Lattice structure looser in frozen form (ice floats!) Hydrogen bond donor & acceptor with itself H 2 0 l :Hydrogen bonds are non-ideal; dynamic H 2 0 s :Hydrogen bonds ideal; rigid

3 Water is an excellent solvent Solvation of ions Water is highly polar, hence excellent solvent Dipole nature of water allows formation of electrostatic interactions with salts Hydration shells form around ions; dynamic, lifetimes ~10 nsec

4 Hydrophobic vs Hydrophilic Interactions hydrophilic (water loving) Ex: sugars, salts, amino acids, organic acids associate through hydrogen bonding hydrophobic (water fearing) Ex: alkanes, fats and oils, sterols lack functional groups that allow the formation of hydrogen bonds, thus they are excluded from the aqueous phase associate through hydrophobic interactions Hydrogen Bonds The polar nature and geometry of water allows water molecules to form hydrogen bonds with hydrophilic substances Water is a good solvent for polar molecules

5 Hydrogen Bonding The 3D structures of many biological molecules are determined by hydrogen bonding K E R S Hydrophobic Interactions tendency of nonpolar molecules to associate in H 2 O Water is ordered around hydrophobic molecules Entropy decreases Hyrophobes aggregate in H 2 0 Slice through IFABP Hydrophobic amino acids cluster inside the protein Water forms a clathrate cage about a nonpolar substance

6 Amphiphilic Molecules contain both hydrophobic and hydrophilic regions Ex: fatty acids, certain amino acids, detergents Amphiphilic molecules form micelles in H 2 0 COLLIGATIVE PROPERTIES OF WATER: influence of solute on the physical properties of water. A 1 molal solution: Freezing pt: o C Boiling pt: +.53 o C Vapor pressure depression: solute molecules present a barrier to escaping solvent molecules. Vapor pressure decreases. Boiling point elevation: a higher T is now required to raise the vapor pressure to get boiling. Freezing point depression: At Tm, the vapor pressure of liquid and solid water are equal. If the vapor pressure is lowered, then water must be cooled to below 0C to freeze it. Osmotic pressure: next page!

7 Osmotic pressure: a critical problem for cells More solute on side B Water moves to equalize concentration Osmotic pressure reached when rate of water movement balances in each direction isotonic hypotonic hypertonic Acid Base Properties: Definitions Many biological molecules (including AAs) are acids or bases; Acids are proton donors, bases are proton acceptors; Acids dissociate into protons and a conjugate base HA <--> H + + A - Bases associate with protons to become their conjugate acids: B + H + <--> BH + Strong electrolyte: substances that are almost completely dissociated to their respective ions. examples: strong acids & bases (HCl & NaOH) salts (NaCl, K 2 SO 4 ) Weak Electrolytes: substances that have a slight tendency to dissociate to their respective ions examples: acetic acid, lactic acid, phosphoric acid imidazole, histidine, carbonic acid/bicarbonate

8 Ionization of water H 2 O + H 2 0 <---> H OH - Simplify the reaction: H 2 0 <---> H + + OH - K EQ = [H + ][OH - ] / [H 2 O] or K EQ = 1.8 x At 25C: [H + ]=[OH - ]=1 x 10-7 M [H 2 0] = 55.5 M Since the concentration of H 2 0 is constant (negligible dissociation) Ion product of water: K w, 25C Kw = 55.5 * K EQ = [H + ][OH - ] = 1 x M At neutral ph: [H + ]=[OH - ]=1 x 10-7 M ph scale At neutral ph: [H + ]=[OH - ]=1 x 10-7 M (of course!) ph = -log [H + ] Physiological ph Insert table 2.2

9 Acid Base Properties -- pk a Definition: pk a for a functional group is the ph at which the acidic or basic group on 50% of the molecules are ionized; have equal amounts of HA and A- Ionization equilibrium of a weak acid: HA <=> H + + A - Ka = [H ] [A - ] [HA] pka = -log Ka = log Ka Henderson-Hasselbach Equation HA <-----> H + + A - Ka = [H+][A-]/HA rearrange H + = Ka[HA]/[A - ] log[h + ] = logka + log([ha]/[a - ]) ph = pka + log [A- ] (conj base) [HA] (conj acid)

10 Titration Curves: How does ph change with changin!g [!0!H -! ] H 3 PO 4 --> H 2 PO > HPO > PO 4 3- Buffers Mixtures of weak acid & their conjugate base [HA]= [A-] or ph = pka HA + OH - <==> A - + H 2 O Ammonium ion In general, weak acids are the most effective buffers Formic acid Beneficial thing about buffers: Resist changes in ph Titration curve

11 Proteins are optimized to function in their environments Digests dietary proteins; found in stomach Breaks down bacterial cell walls; found in tears Part of TCA cycle - converts malate to fumarate; found in mitochondria BUFFERS: the biological biggies Intracellular ph control ( ): phosphate system HPO 4 2- /H 2 PO 4 - pka = 7.2 Blood ph control (7.4): bicarbonate system H 2 CO 3 /HCO! 3 - with pka = 3.8 Role of proteins - help intracellularly: free histidine pka ~ 6.0 histidine in proteins pka ~7.0

12 Review 1). Water has many chemical & physical properties that make it ideal for life. 2). The dipole in H 2 0 arises from the difference in electronegativity between 0 & H and from the angle between the O-H bonds. 3). Liquid H 2 0 is fluid; frozen water has a lattice like structure. 4). H 2 O has a high dielectric constant, making it an excellent solvent. 5). Water forms clathrate cages around H 2 0. This is entropically unfavorable. 6). Colligative properties depend on the # of solute molecules; not their chemical composition. 7). Colligative properties are critical to life. 8). H 2 0 acts as a weak acid and weak base. 9). Many biological molecules are weak electrolytes. 10). ph is related to pka through the Henderson-Hasselbach equation 11). Titration curves permit us to determine pkas 12). Buffers are most effect at +/- 1 ph unit from pka 13). Buffers are important for cellular viability.