slides on biochemistry Water.pH.Buffers.pptx

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1. Water, pH and Buffers

2. By the end of this lecture you are expected to : Understand the properties of water which enabled it to act as an ideal solvent in biological systems. Define the hydrogen bond and give examples of major chemical groups involved in cellular hydrogen bonding. Apply the pH concept in examples of body fluids Define weak acid, pK and buffers State the Henderson- Hasselbalch equation and be able to use it for preparation of laboratory buffers Describe physiological buffering systems and discuss their role in health and disease Define respiratory alkalosis and respiratory acidosis Lecture Objectives:

3. Water is the fluid medium in which life exists. Approximately 60% of the adult human, by mass, is water. Water is not a passive medium in which substances exist and chemical reactions take place. It participates actively in a number of those chemical reactions, and determines the shapes of bio-molecules such as proteins & nucleic acids. Water

4. Water molecules form dipoles. Oxygen , being more electronegative, attracts electrons more strongly than the hydrogen does. This leaves the oxygen with a slightly negative nature “  - ”, and hydrogen with a slightly positive nature “  + ”. See structure Di-polarity of water is important for solvation of charged particles. Water approaches + ve ions by its ‘negative’ oxygen and surrounds – ve ions by its positive hydorgens . Dipoles

5. A hydrogen bond may be defined as the sharing of a hydrogen atom between two electro-negatives. The most important atoms in biochemical interactions involving hydrogen bonds are O, N and to a lesser extent S. Water solvates uncharged polar compounds by forming hydrogen bonds with them. Hydrogen bonding

6. A hydrogen bond is quite weak compared to covalent bonds. 4.5 kcal needed to break a H...O bond compared 110 kcal needed to break a to H ـــ O covalent bond. However , the value of hydrogen bonds in shaping the properties of H 2 O arises from the fact that they are numerous . In the liquid state each water molecule is surrounded by 3 or 4 other water molecules. This increases the cohesiveness, boiling point, melting point, heat of fusion, heat of vaporization and specific heat. H bonds and Properties of Water

7. Water has a very limited capacity to ionize . H 2 O = H + + OH - Protons usually combine with unionized water molecules producing hydronium ions, H 3 O + . H 2 O + H 2 O = H 3 O + + OH - This ionization contributes significantly to the properties of water. The above equation indicates that water can act as an acid (= a proton donor) and as a base (= a proton acceptor). Dissociation of Water

8. According to the Law of Mass Action, the equilibrium constant is obtained by: K eq = [H + ] X [OH - ]/[H 2 O] [H 2 O] = 1000/18 or ≈ 55.56 M; K eq was found to be = 1.8 X10 -16 M Solving in the above equation yields, [H + ] X [OH - ] = 1.0 X 10 -14 M 2 The reciprocal relationship between these ions indicates that when one is found at high concentration the 2 nd is low Dissociation of Water

9. At neutrality [H + ] = [OH - ] = 1.0 X 10 -7 M (see next slides) To avoid using many decimal places or negative exponents, the Danish scientist Sorensen 1909 introduced the pH concept: pH = -log [H + ] At neutrality pH = - (-7) = 7 At pH values lower than 7 the corresponding [H + ] is greater than [OH - ] ---------> acidic If [OH - ] = 1.0 X10 -5 M, what is the pH? Plasma pH = 7.4, Cell cytosol around 7, lysosome pH 4-5, saliva 6.8, gastric juice around 2 The pH Concept

10. Ion Concentration ( mol /l)

11. pH [H + ] Molar concentration [OH - ] M. C. 1 * 10 1 * 10 -14 1 1 * 10 -1 1 * 10 -13 2 1 * 10 -2 1 * 10 -12 3 1 * 10 -3 1 * 10 -11 4 1 * 10 -4 1 * 10 -10 5 1 * 10 -5 1 * 10 -9 6 1 * 10 -6 1 * 10 -8 7 1 * 10 -7 1 * 10 -7 8 1 * 10 -8 1 * 10 -6 9 1 * 10 -9 1 * 10 -5 10 1 * 10 -10 1 * 10 -4 11 1 * 10 -11 1 * 10 -3 12 1 * 10 -12 1 * 10 -2 13 1 * 10 -13 1 * 10 -1 14 1 * 10 -14 1 * 10

12. Weak acids dissociate only partially while strong acids dissociate almost completely. Most of the acids formed in our body are weak Ionization of a weak acid is represented as: HA = H + + A - from which , K eq = [H + ] X[A - ] /[HA] Solving for [H + ], taking –log of both sides and introducing the term pK yields the Henderson- Hasselbalch equation : pH = pK + log [A - ]/ [HA] Weak Acids

13. K eq = [H + ]*[A - ]/[HA] Solving for [H + ] [H + ] = K eq *[HA]/[A - ] Taking –log of both sides : -log[H + ] = -log(K*[HA]/[A - ]) or -log[H + ] = -logK –log ([HA]/[A - ]) Introduce the term pK for –logK, and inverting the fraction to eliminate the minus sign: pH = pK +log [A - ]/[HA] This is the Henderson-Hasselbalch equation Working it out

14. Analogous to pH, pK is the negative log of K. If [A - ] = [HA] then pH = pK pK is defined as the pH at which the acid is half dissociated. The value of pK is related inversely to acid strength: strong acids have lower values. Definition of pK

15. pK values of some acids

16. Solutions of weak acids and their conjugate bases (- or of weak bases and their conjugate acids) exhibit buffering: The tendency of a solution to resist more effectively a change in pH upon the addition of small amounts of acids or base than does an equal volume of water. Or simply, solutions that resist changes in pH upon addition of acid or base Buffering = change resistance Buffers

17. In the human body buffering is carried out by a number of systems, these are: The carbonic acid: bicarbonate buffer system CO 2 + H 2 O ⇆ H 2 CO 3 ⇆ H + +HCO - 3 This is the most important buffer system in plasma because of: The high concentration of HCO - 3 The readiness with which H 2 CO 3 may be increased through diminished lung activity or decreased through increased ventilation. Examples of Physiologic Buffers

18. Protein buffers: The most important buffer protein is hemoglobin. Proteins buffering capacity is due to the presence of charged amino acids.   The phosphate buffer, H 2 PO - 4 : H PO 2- 4 This plays a minor role in blood buffering, but it helps in the elimination of H + in the urine.   HPO 4 2- +H + ⇆ H 2 PO 4 - Weak acids buffer most effectively in the vicinity of their pK values (or pK ± 1pH unit) Examples of Physiologic Buffers

19. Hyperventilation , defined as a breathing rate more rapid than necessary for normal CO2 elimination from the body, can result in an inappropriately low [CO2(g)] in the blood. CNS disorders such as meningitis or cerebral hemorrhage, as well as a number of drug- or hormone-induced physiological changes, can lead to hyperventilation. As [CO2(g)] drops due to excessive exhalation, [H2CO3] in the blood plasma falls, followed by decline in [H + ] and [HCO3 - ] in the blood plasma. Blood pH rises from 7.4 to 7.74 in a few minutes, leading to respiratory alkalosis Respiratory Alkalosis

20. Hypoventilation is the opposite of hyperventilation and is characterized by an inability to excrete CO2 rapidly enough to meet physiological needs. Hypoventilation can be caused by narcotics, sedatives, anesthetics, and depressant drugs; diseases of the lung also lead to hypoventilation. Hypoventilation results in respiratory acidosis , as CO2(g) accumulates, giving rise to H2CO3, which dissociates to form H + and HCO3 - . Respiratory Acidosis

21. Structure of Water back