Biochem 3 Acid-Base Study Notes PDF

INTRODUCTION • Properties of water • Acids and Bases • pH • Weak acids and bases • Buffers UNUSUAL PROPERTIES OF WATER • • • • • • Melting point 0⁰C, Boiling point 100⁰C High surface tension Highest density at 4⁰C High heat capacity, High heat of vaporization Excellent solvent Ionizes POINTS TO...

INTRODUCTION • Properties of water • Acids and Bases • pH • Weak acids and bases • Buffers UNUSUAL PROPERTIES OF WATER • • • • • • Melting point 0⁰C, Boiling point 100⁰C High surface tension Highest density at 4⁰C High heat capacity, High heat of vaporization Excellent solvent Ionizes POINTS TO PONDER • Without these unusual properties of water, life could not exist as we know it! • https://www.youtube.com/watch?v=pjClihDmfeA WATER . . . . . . . . . HOH • H-O-H • Polarity of Molecules • Partial charges on regions of molecule • Soluble in polar solvents ( i. e. H2O) H-BONDS BETWEEN WATER MOLECULES Hydrogen bonds between water molecules. The oxygen atoms are shown in black. HYDROGEN BONDS - WATER MOLECULES HYDROGEN BONDS - WATER MOLECULES Each water molecule forms hydrogen bonds with 4 others HYDROGEN BOND VS COVALENT BOND What is the strength of a hydrogen bond compared to a covalent bond? Covalent 20-200 kcal/mol Noncovalent Ionic 10-100 kcal/mol Hydrogen 5 kcal/mol Van der Waals 0.5kcal/mol WATER IS THE MAIN SOLVENT IN BIOSYSTEMS Solutes dissolve in liquids Solvents dissolve solutes Solution: solute dissolves in solvent Solubility , ease of dissolving • Hydrophobic • Hydrophilic DISSOCIATION OF WATER H2O H+ + OH- can be used to see how much dissociation is in water: pH = -log[H+] DISSOCIATION OF WATER [H+][OH-] -16 Kd = [HOH] = 1.8X10 M [HOH] = 55 M Kw = [H+][OH-]=1X10-14 M2 log [H+]+ log[OH-]= -14 DISSOCIATION OF WATER log [H+]+ log[OH-]= -14 pH = - log [H+] pH + pOH = 14 At neutrality when pH=pOH the pH = 7 + PH AND H CONCENTRATION M M .1 10-1 .01 10-2 .001 10-3 .0001 10-4 .00001 10-5 .000001 10-6 .0000001 10-7 pH 1 2 3 4 5 6 7 CELL PH The pH range for cell survival is between 6.8 and 7.8. IONIZATION OF STRONG AND WEAK ACIDS ACIDS AND BASES STRONG ACIDS An acid is a molecule that can give up a H+ An example of a strong acid is HCl. When added to water it completely ionizes HCl H+ + Cl- If the concentration of HCl was .01M what would be the pH? If the concentration of HCl was .001M what would be the pH? ACIDS AND BASES STRONG BASES A base is a molecule that can give up a OHAn example of a strong base is NaOH. When added to water it completely ionizes NaOH Na+ + OH- If the concentration of NaOH was .01M what would be the pH? If the concentration of NaOH was .001M what would be the pH? + H CONCENTRATION IN BIOSYSTEMS ACIDS AND BASES WEAK ACIDS An example of a weak acid is CH3COOH. When added to water it will only partially ionize CH3COOH Acid CH3COO- + H+ Salt (Conjugate base) Acetic acid can act as a buffer and its properties can be described by the Henderson-Hasselbalch Equation ACIDS AND BASES HENDERSON-HASSELBALCH EQUATION CH3COOH Acid CH3COO- + H+ Salt (Conjugate base) pH = pkA + log [CH3COO-] [CH3COOH] pkA is the ionization constant TITRATION CURVE FOR ACETIC ACID ACIDS AND BASES STRONG BASES ACIDS AND BASES HENDERSON-HASSELBALCH EQUATION When the pH = pKa the amount of salt and acid are equal. The resulting pH is 4.74 Acetic acid will buffer best 1.0 pH units above and below this pKa. Weak Bases – A Brief Word QUESTION • At a pH of 3.74 for acetic acid what is the ratio of salt/acid? • At a pH of 6.74 for acetic acid what is the ratio of salt/acid? CELL PH • Due to metabolism, we produce enough acid per day to reduce the pH to less than 1 in body fluids. Yet the pH of the blood is maintained between 7.36 and 7.44 and intracellular pH is maintained between 6.9 and 7.4. • How is this accomplished? BUFFER SYSTEMS Extracellular fluid - bicarbonate-carbonic acid buffer system RBC – Hb buffer system Intracellular – Phosphate buffer system PHOSPHATE BUFFER H3PO4 + H pKa H2PO4- + H+ 2.1 7.2 HPO42- + H+ 12.0 PO43- + PHOSPHATE TITRATION CURVES You can assume that all hydrogens can be donated as you add more and more NaOH pH So what forms of H3PO4 are present at each pKa? NaOH added PHOSPHATE TITRATION CURVES QUESTION Phosphate is present inside cells and acts as a buffer. Show the ionization of phosphate and indicate the pH range that it buffers best. Physiological Buffers Buffer System [Intracellular] / [Extracellular] Effective Buffering Compartment IC - varies (lower than EC) Extracellular fluid – major buffer EC - 24 mmol/L about 80% of the buffering power Bicarbonate IC - 4-5 mmol/L Protein* Phosphate EC – 1 mmol/L IC – 0.7 mmol/L EC – 0.7 mmol/L Intracellular fluid – major buffer Extracellular fluid – minor buffer Intracellular fluid – major buffer Extracellular fluid – negligible buffering power *Note – hemoglobin has special buffering properties that we will consider later BICARBONATE – CARBONIC ACID BUFFER SYSTEM H2CO3 H+ + HCO3- This system is also in equilibrium with dissolved CO2 because of carbonic anhydrase action CARBONIC ANHYDRASE BICARBONATE-CARBONIC ACID BUFFER SYSTEM With these reactions working together the pKa is 6.1 and buffering will occur between 5.1 and 7.1 However, the bicarbonate buffer system is the most effective extracellular buffer… There’s more to this buffer than transitioning between the protonated and deprotonated forms Why Is The Bicarbonate Buffer So Effective? Carbonic acid Carbonic anhydrase H20 + CO2 H2CO3 Bicarbonate H+ + HCO3- • Carbonic anhydrase rapidly catalyzes the formation of carbonic acid from water and carbon dioxide (happens in the RBC then can become deprotonated) • Carbonic acid (protonated) dissociates into bicarbonate (deprotonated) and H+ ▪ pKa = 6.1 🡪 note: > 1 pH unit from a blood pH of 7.4 ▪ The blood has about 24 mmol of HCO3• Relatively high buffer concentration Why Is The Bicarbonate Buffer So Effective? • Bicarbonate buffer continued ▪ Determine the ratio of HCO3- to H2CO3 at pH = 7.4 pH = pKa + log [deprotonated] [protonated] 7.4 = 6.1 + log [HCO3-] [H2CO3] 1.3 = log [HCO3-] [H2CO3] 20 = [HCO3-] [H2CO3] Why Is The Bicarbonate Buffer So Effective? The lung can modify the rate of CO2 removal by increasing ventilation Carbonic acid Carbonic anhydrase H20 + CO2 H2CO3 Bicarbonate H+ + HCO3The kidney can modify its rate of bicarbonate excretion CARBON DIOXIDE FORMATION IN THE TISSUES When CO2 is produced as a result of fuel oxidation it will dissolve and be converted to H2CO3 by carbonic anhydrase in the red blood cell. H2CO3 will ionize and be exported in the plasma via the HCO3-/Cl- transporter BUFFERING SYSTEMS OF THE BODY CARBON DIOXIDE IN THE LUNGS When the red blood cell reaches the lungs HCO3- is transported back into the red blood cell. H2CO3 is converted to water and CO2 by carbonic anhydrase and the CO2 is released into the lungs CARBON DIOXIDE IN THE LUNGS • Why is carbonic anhydrase so important in this process? QUESTION • With hyperventilation the blood pH increases. Why? MAINTENANCE OF BODY PH Maintenance of body pH. The body produces approximately 13 to 22 mol/day of acid from normal metabolism. The body protects itself against this acidity by buffers that maintain a neutral pH and by the expiration of CO2 through the lungs and the excretion of NH4+ and other ions through the kidneys. TITRATION CURVE OF HISTIDINE CHARGES OF SIDE CHAINS OF AMINO ACIDS QUESTION • Why do cells die when the pH falls below 6.8?