Acid-Base Chemistry 2025 BMS 531 PDF

Acid-Base Chemistry and Henderson-Hasselbalch Equation Emily Masser, PhD BMS 531.04 2025 1 Objectives 1. Describe the relationship between pH and hydrogen ion concentration. 2. Relate pKa to the ratio of associated and dissociated compounds. 3. If given the pKa and pH, be abl...

Acid-Base Chemistry and Henderson-Hasselbalch Equation Emily Masser, PhD BMS 531.04 2025 1 Objectives 1. Describe the relationship between pH and hydrogen ion concentration. 2. Relate pKa to the ratio of associated and dissociated compounds. 3. If given the pKa and pH, be able to estimate the percentage of a compound that will be dissociated and/or associated. 4. Describe the function of a buffer in a biological system. 5. Explain how the bicarbonate system is an effective physiological buffer. 6. Describe why a buffer is more effective near its pKa(s). 7. Compare the ability of a compound to cross a biological membrane in the charged vs. uncharged state (assume no transport mechanism). 2 What is pH? Hydrogen ion + (H ) concentration in an aqueous solution Symbol p denotes “negative logarithm of” pH = negative logarithm of the hydrogen ion concentration of an aqueous solution ↓ pH, ↑ H concentration + Logarithmic scale Change in one pH → 10-fold + change in H concentration 3  Why is pH important? pH affects structure and activity of biological macromolecules Small changes in pH can cause a large change in structure and function pH of blood and urine commonly used in medical diagnoses Blood pH < 7.4 – acidosis Capacity of the body to buffer H+ is diminished Blood pH > 7.4 – alkalosis 4 Lehninger Principles of Biochemistry, 8e: Figure 2-14 Conjugate Acid-Base Pair pH determined by H+ H depends on solutes that + function as: Acids (proton donors) Bases (proton acceptors) Biological systems contain weak acids and their conjugate bases Strength of the tendency to lose or gain a proton in H2O Proton donor and corresponding proton acceptor 5 Acid Dissociation Constant (Ka) We can calculate a value that describes the affinity of an acid for the dissociable H + Equilibrium equation Keq for the reaction is called the acid dissociation constant, Ka Weak acids have a low Ka High HA concentration Strong acids have a high Ka Most in dissociated form (A ) - 6 Acid Dissociation Constant (Ka) Ka value is usually very small Ka expressed as pKa Weak acids have a high pKa Low Ka Strong acids have a low pKa High Ka For example, Ammonia pKa of 9.25 Acetic acid pKa of 4.76 Loses its H+ more easily 7  Conjugate Acid-Base Pair Monoprotic acids give up only one proton Acetic acid (CH3COOH) When a proton donor loses a proton → proton acceptor acetate (CH3COO )- Ammonium ion Diprotic acids Carbonic acid and glycine Triprotic acid Phosphoric acid 8 Lehninger Principles of Biochemistry, 8e: Figure 2-15  Titration Curve Titration can be used to determine amount of acid Acid is titrated with a solution of a strong base (NaOH) Amounts of acid and base expressed in equivalents Amount of OH required to - deprotonate the acid Plot of pH against amount of NaOH added Reveals pKa of the weak acid 9 Lehninger Principles of Biochemistry, 8e: Figure 2-16  Titration Curve As NaOH is added, OH- combines with to form H2OH+ + As H is removed, CH3COOH dissociates further Net result, more acetic acid ionizes → acetate At midpoint, concentration of proton donor = concentration proton acceptor (pH = pKa) Endpoint, acetic acid has lost its protons → water and acetate 10 Lehninger Principles of Biochemistry, 8e: Figure 2-16  Titration Curve Three weak acids with different dissociation constants Acetic acid (pKa 4.76) Highest Ka (lowest pKa) Strongest of the three Loses its H most readily + Dihydrogen phosphate, H2PO4 - (pKa 6.86) + Ammonium ion, NH4 (pKa 9.25) Conjugate acid-base pair can act as a buffer 11 Lehninger Principles of Biochemistry, 8e: Figure 2-17  Buffers Resist changes in pH when small amounts of acid (H+) and base (OH-) are added Buffers do not prevent pH changes, minimize changes Buffer system consists of a weak acid (proton donor) and its conjugate base (proton acceptor) Buffering region ~1 pH unit above and below its midpoint 12 Lehninger Principles of Biochemistry, 8e: Figure 2-17  Acetic Acid-Acetate as a Buffer System Nearly equal concentrations of proton donor and its conjugate proton acceptor Small change in the ratio of weak acid and its anion Small change in pH Decrease of one component in the system is balanced by an increase in the other Characteristic pH zone in which it is an effective buffer 13 Lehninger Principles of Biochemistry, 8e: Figure 2-18 Henderson-Hasselbalch Equation Shape of the titration curve of any weak acid is described by the Henderson-Hasselbalch equation Relates pH, pKa and ratio of - proton acceptor (A ) and proton donor (HA) Shows why the pKa of a weak acid is equal to the pH of the solution at the midpoint of its titration 14 Henderson-Hasselbalch Equation Knowing the pKa of an acid and pH of the solution, can predict mostly protonated or mostly deprotonated ↑ pH ↑ OH (OH > H ) - - + Base pulls H → A + - ↓ pH ↑ H (H > OH ) + + - Acid in the protonated form, HA 15  Amino Acids Buffer Cells and Tissues Intracellular and extracellular fluids have a characteristic and nearly constant pH Defense against changes in internal pH – buffer systems Cytoplasm of most cells contains high concentrations of proteins Functional groups that are weak acids or bases Histidine buffers effectively near neutral pH 16 Lehninger Principles of Biochemistry, 8e: Figure 2-19  Ionization of Histidine in the Blood The pKa value of the imidazole side chain of histidine = 6.0, what is the ratio of protonated: deprotonated in the blood (pH = 7.3)? We can estimate the ratio should be between 1: __ (pH = 7.0) and 1: __ (pH = 8.0) protonated: deprotonated. 7.0 = 6.0 + log [A ]/[HA] - 1 = log [A ]/[HA] - 101 = [A-]/[HA] 10 = [A ]/[HA] - 17 Lehninger Principles of Biochemistry, 8e: Figure 2-19  Ionization of Histidine in the Blood The pKa value of the imidazole side chain of histidine = 6.0, what is the ratio of protonated: deprotonated in the blood (pH = 7.3)? We can estimate the ratio should be between 1: 10 (pH = 7.0) and 1: __ (pH = 8.0) protonated: deprotonated. 7.0 = 6.0 + log [A ]/[HA] - 1 = log [A ]/[HA] - 101 = [A-]/[HA] 10 = [A ]/[HA] - 18 Lehninger Principles of Biochemistry, 8e: Figure 2-19  Ionization of Histidine in the Blood The pKa value of the imidazole side chain of histidine = 6.0, what is the ratio of protonated: deprotonated in the blood (pH = 7.3)? We can estimate the ratio should be between 1: 10 (pH = 7.0) and 1: 100 (pH = 8.0) protonated: deprotonated. 8.0 = 6.0 + log [A ]/[HA] - 2 = log [A ]/[HA] - 102 = [A-]/[HA] 100 = [A ]/[HA] - 19 Lehninger Principles of Biochemistry, 8e: Figure 2-19  Ionization of Histidine in the Blood The pKa value of the imidazole side chain of histidine = 6.0, what is the ratio of protonated: deprotonated in the blood (pH = 7.3)? We can estimate the ratio should be between 1: 10 (pH = 7.0) and 1: 100 (pH = 8.0) protonated: deprotonated. Therefore, less than 10% will be protonated histidine and greater than 90% will be deprotonated histidine. 20 Lehninger Principles of Biochemistry, 8e: Figure 2-19 Phosphate Buffer System Cytoplasm of all cells Dihydrogen phosphate (H2PO4 ) - as the proton donor Hydrogen phosphate (HPO4 ) as 2- the proton acceptor Buffer system at neutral pH Maximally effective at a pH close to its pKa of 6.86 Effective buffer in biological fluids Extracellular and intracellular pH of 6.9 – 7.4 21 Bicarbonate Buffer System Carbonic acid (H2CO3) as the proton donor - Bicarbonate (HCO3 ) as the proton acceptor Carbonic acid formed from dissolved CO2 and H2O CO2(aq) is in equilibrium with CO2(g) pH is ultimately determined by HCO3- in the aqueous phase and CO2(g) 22  Bicarbonate Buffer System Effective near pH 7.4 Three reversible reactions CO2 in the lungs and bicarbonate in the blood ↑ H+ → H2CO3 → CO2(aq) → CO2(g) Rate of respiration can quickly adjust the equilibrium of these reactions to maintain blood pH Controlled by the brain stem Detect CO2 and pH changes 23 Lehninger Principles of Biochemistry, 8e: Figure 2-20  Bicarbonate Buffer System Hyperventilation tips the balance of O2 in and CO2 out in favor of too much CO2 out Raising blood pH Alkalosis Dizziness, headache, weakness and fainting Home remedy for mild alkalosis is to breathe into a paper bag Air in the bag becomes enriched in CO2 24 Lehninger Principles of Biochemistry, 8e: Figure 2-20  Bicarbonate Buffer System ↓ pH ↑ H2CO3 ↓ CO2(aq) through hyperventilation (exhaling CO2) ↓ excretion of HCO3- ↑ pH H and HCO3 formation + - Reduced breathing rate to increase CO2(aq) ↑ excretion of HCO3- 25 Biochemistry, 2e: Figure 2.39  Influence of pH on Ionizable Drugs Many drugs are weak acids that are present in solution as both: Lipid-soluble, diffusible nonionized form Lipid-insoluble, poorly diffusible ionized form Distribution of a weak acid is influenced by its pKa and the pH gradient across the membrane The pKa is the pH at which half the drug is in its ionized form 26 Adapted from Goodman & Gilman’s: The Pharmacological Basis of Therapeutics, 13e: Chapter 2  Influence of pH on Ionizable Drugs Henderson-Hasselbalch explains how pH influences the dissociation of a weak acid: Weak acid (pKa = 4.4) in pH = 7.4 (blood) 7.4 = 4.4 + log [A ]/[HA] - 1000 = [A ]/[HA] - Increased dissociation at a higher pH 27 Adapted from Goodman & Gilman’s: The Pharmacological Basis of Therapeutics, 13e: Chapter 2  Influence of pH on Ionizable Drugs Henderson-Hasselbalch explains how pH influences the dissociation of a weak acid: Weak acid (pKa = 4.4) in pH = 1.4 (stomach) 1.4 = 4.4 + log [A ]/[HA] - 0.001 = [A ]/[HA] - Decreased dissociation at a lower pH 28 Adapted from Goodman & Gilman’s: The Pharmacological Basis of Therapeutics, 13e: Chapter 2  Aspirin (Acetylsalicylic Acid) Dissociation of Aspirin Aspirin pKa = 3.5 At pH = 3.5, aspirin and acetylsalicylate are present at equal concentrations. Acetylsalicylate (conjugate base) Aspirin is 1:1 associated: dissociated at this pH. What is the associated: dissociated in the stomach (pH = 1.5)? 29 https://www.sigmaaldrich.com/US/en  Aspirin (Acetylsalicylic Acid) Dissociation of Aspirin The pH in the stomach is lower, and therefore, higher H + concentration. When the hydrogen ion Acetylsalicylate (conjugate base) concentration increases, the protonated form (associated) will predominate. 30 https://www.sigmaaldrich.com/US/en  Aspirin (Acetylsalicylic Acid) Dissociation of Aspirin What is the association: dissociation in the duodenum (pH = 6.5)? The higher the pH, the lower the Acetylsalicylate (conjugate base) hydrogen ion concentration. The deprotonated form (dissociated) will predominate. 31 https://www.sigmaaldrich.com/US/en  Aspirin (Acetylsalicylic Acid) Dissociation of Aspirin We know at pH = 3.5 aspirin is 1:1 associated: dissociated What is the % dissociated in the stomach? Acetylsalicylate (conjugate base) What is the pH? H concentration higher or + lower compared to 3.5? 32 https://www.sigmaaldrich.com/US/en  Aspirin (Acetylsalicylic Acid) Dissociation of Aspirin There is 10-fold change in H+ concentration with a change in one pH unit. 100-fold with two pH units. Acetylsalicylate (conjugate base) 1000-fold with three pH units. The % dissociated will follow the same ratios. 33 https://www.sigmaaldrich.com/US/en  Estimating % Dissociated pH = 3.5 What is the associated: dissociated at pH = 3.5? 34 https://www.sigmaaldrich.com/US/en  Estimating % Dissociated pH = 3.5 What is the associated: dissociated at pH = 3.5? What is the associated: dissociated at pH = 2.5? What is the associated: pH = 2.5 dissociated at pH = 1.5? pH = 1.5 35 https://www.sigmaaldrich.com/US/en  Estimating % Dissociated pH = 3.5 What is the percentage of aspirin associated in the stomach? 99% associated What is the percentage of aspirin dissociated in the stomach? pH = 2.5 1% dissociated pH = 1.5 36 https://www.sigmaaldrich.com/US/en Aspirin in the Duodenum What percentage of aspirin (pKa = 3.5) is associated in the duodenum (pH = 6.5)? First, is the predominate species associated or dissociated? pH = 6.5 is a higher or lower H concentration? + Therefore, most of the aspirin will be ___. Second, the difference in the pKa and pH is three pH units (pKa = 3.5 and pH = 6.5) 1000 = [A ]/[HA] = __% associated. - 37 Aspirin in the Duodenum What percentage of aspirin (pKa = 3.5) is associated in the duodenum (pH = 6.5)? First, is the predominate species associated or dissociated? pH = 6.5 is a higher or lower H concentration? + Therefore, most of the aspirin will be dissociated. Second, the difference in the pKa and pH is three pH units (pKa = 3.5 and pH = 6.5) 1000 = [A ]/[HA] = __% associated. - 38 Aspirin in the Duodenum What percentage of aspirin (pKa = 3.5) is associated in the duodenum (pH = 6.5)? First, is the predominate species associated or dissociated? pH = 6.5 is a higher or lower H concentration? + Therefore, most of the aspirin will be dissociated. Second, the difference in the pKa and pH is three pH units (pKa = 3.5 and pH = 6.5) 1000 = [A ]/[HA] = 0.1% associated. - 39