Biophysics & Biomolecules

1. Welcome
to
The House of BIOCHEMISTRY
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2. Biophysics
Dr. Mohammad Shiblee Zaman
MBBS, MD (Biochemistry)
Lecturer
Dhaka Medical College

3. 3
Topics
1 5
3 4
2 6
Introduction to
Biochemistry
Law of mass
action
pH, pK and pH
scale
Buffer
Acid, Base
Handerson-
Hasselbach
equation

4. Acid and Base
❑ Thomas Lowry (England)
or JN Bronsted (Denmark)
(1923)
❑ Arrhenius (traditional)
❑ GN Lewis (1923)
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5. Acid
❑ Acid…..proton donor in aqueous solution
❑ Conjugate base of acid…..remaining anionic part
after proton donation
❑ Strong acid…..quick & complete dissociation
❑ Weak acid…..slow & incomplete dissociation
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6. Base
❑ Base…..proton acceptor in aqueous solution
❑ Conjugate acid of a base…..formed acid after
accepting proton by that base
❑ Strong base…..quick & complete dissociation
❑ Weak base…..slow & incomplete or no
dissociation
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7. Acid, Base contd.
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Fig: Acid & its conjugate base, Base & its conjugate acid

8. Acid, Base contd.
❑ Sources of acids:
1. Protein metabolism specially
sulpher containing AAs
2. Lipid metabolism -
phospholipids
3. Nucleic acids metabolism
4. Glucose metabolism
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❑ Sources of bases:
1. Metabolism of citrus
foods and vegetables
Net production of
acid is more than
base in human body

9. Ampholyte
Metallic hydroxides of alkaline metals like Na and K
which in solution ionize to OH- ions that can bind H+
ion to form H2O
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Alkali
Can act both as an acid and a base
eg. H2O → H+ + OH–
H2O + H+ → H3O+

10. pH
❑ Term introduced by Sorenson in 1909
❑ Negative logarithm of hydrogen ion
concentration of a solution when H+
conc. is expressed in terms of mol/L
pH = – log [H+]
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11. pH contd.
❑ Relationship between pH and [H+].....inverse
❑ ↓ pH of 1 unit.....10 fold ↑ [H+].....vice versa
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12. pH contd.
❑ Normal H+ conc. of blood: 35 – 45 nmol/L
❑ Normal pH of blood: 7.35 - 7.45 (Ave. 7.4)
❑ This pH is maintained by:
 Body fluid buffer system
 Respiratory buffer system
 Renal buffer system
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13. Measurement of pH
❑ Non specific methods:
1. Body fluid buffers
2. Using indicators
3. pH paper
4. Litmus paper
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❑ Specific methods:
1. PH meter
2. pH gas electrode
3. Hydrogen electrode
4. Calomel electrode

14. Importance of pH
❑ Maintenance of optimum pH is essential for life as
most enzymatic reactions occurs at that pH
❑ Maintenance of optimum pH is essential for body
homeostasis
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Optimum pH
Level of pH at which maximum rate of enzymatic
reaction takes place

15. pH scale
❑ Range of pH that covers practical range
of acidity and alkalinity of commonly
used solution expressed as scale
❑ Range: 0 -14
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Fig: pH scale

16. pH scale contd.
❑ Shows relationship between H+, OH- and pH of
aqueous solution at 25 0C when Kw = 10-14
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Fig: Conceptual diagram of pH scale

17. pK
 Negative logarithm of ionization or dissociation
constant (K)
 pH at which an acid is half dissociated, existing as
equal proportions of acid and conjugate base
pK = – log K
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18. Buffer
 Mixture of weak acid & its conjugate base usually
in form of salt
 Has ability to resist change of pH of a solution when
moderate amount of acid or base is added
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Buffer =
Conjugate base
or,
Salt
Weak acid Weak acid

19. Buffer system Conjugate base/ salt Weak acid pK
Bicarbonate** HCO3
– or NaHCO3 H2CO3 6.1
Phosphate HPO4
2– or Na2HPO4 H2PO4- or NaH2PO4 6.8
Plasma protein Pr- HPr 7.3
Hemoglobin Hb– HHb 7.3
Oxyhemoglobin HbO2
– HHbO2 7.3
Ammonia NH3 NH4
+ 9.0
Common buffer systems
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20. Compartment Buffers (in order of importance)
ECF/ Plasma Bicarbonate, Phosphate, Protein
ICF Protein, Bicarbonate, Phosphate
RBC Hemoglobin, Phosphate, Bicarbonate
Blood Bicarbonate, Hemoglobin, Phosphate,
Protein
Urine Ammonia, Phosphate, Bicarbonate
Distribution of body buffers
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21. Mechanism of buffer action
❑ Buffer acts by converting
 strong acid into weak acid
 strong base into weak base or neutral salt
 Thereby minimizes effect of strong acid or strong
base on pH of a solution
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22. Mechanism of buffer action contd.
22
HCl
H2CO3 NaCl
NaHCO3
CO2 H2O

23. Mechanism of buffer action contd.
23
NaOH
NaHCO3
H20
H2CO3

24. Alternatively,
❑ Buffer acts by
 donating proton in proton deficit (alkalosis)
 accepting proton in proton excess (acidosis)
 Thereby keep acid/base tied up to minimize pH change 24
HB
H+
B-
HB
H+
B-

25. Bone Buffer: Hydroxyapatite crystal (HAC)
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Ca10(PO4)6(OH)2 Ca9(PO4)6 + Ca++ + 2H2O
2H+
In persistent acidosis, when body fluid buffer fails

26. Buffering capacity
❑ Amount of acid or base required to produce one unit
change of pH in solution
❑ Smaller the pH change, greater the buffering capacity
❑ Depends on
 pK of buffer – maximum capacity when pH = pK
 Base/acid ratio – ratio closer to 1, more is the capacity
 Total amount – more effective at higher concentration
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27. Bicarbonate Buffer: most effective
❑ High buffering capacity (60% of total)
❑ Wide field of buffering activity (both in ECF & ICF)
❑ Conjugate base (HCO3
-) conc. (24 mmol/L) is 20
times more than its acid (H2CO3) (1.2 mmol/L)
❑ Works synergistically with Hb buffer system
❑ Open end buffer system
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29. Importance of buffer systems
❑ Maintain normal pH of body as optimum pH is
essential for normal enzymatic activity
❑ Regulate narrow limit of body pH, compatible for life
of most cells
❑ Determine pH with indicator
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30. Law of mass action
Rate of any reversible chemical reaction at any
instant, at a given temperature is directly
proportional to the product of molar concentration of
the reactants
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A + B
V1
→
C + D
V = velocity
←
V2
[ ] = molar conc.

31. Explanation
According to Law of mass action:
V1 α [A][B], So, V1= K1 [A][B]
V2 α [C][D], So, V2= K2 [C][D]
At equilibrium, V1= V2
So, K1 [A][B] = K2 [C][D]
K1/ K2= [C][D] / [A][B]
K = [C][D] / [A][B]
K= Equilibrium constant or dissociation constant
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32. Henderson-Hasselbalch Equation (HHE)
❑ Define relationship between pH, pK and
concentration of an acid and its conjugate base
❑ Explain relationship between pH of a weak acid in
solution and its dissociation constant
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pH = pK + log
Conjugate base or salt
Weak acid

33. Explanation
Dissociation of a weak acid can be represented as
HA (weak acid)↔H+ + A- (Conjugate Base)
According to Law of mass action,
K = [H+][A-] / [HA] (K=dissociation constant of acid)
[H+][A-] = K[HA] (by cross multiply)
[H] = K [HA] / [A-] (divide both side by [A-])
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34. Explanation contd.
log [H+] = log {K[HA] / [A-]} (taking log on both side)
log [H+] = log K + log [HA] / [A-]
- log [H+] = - log K - log [HA] / [A-] (multiply by -1)
pH = pK - log [HA] / [A-]
pH = pK + log [A-] / [HA]
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So, pH = pK + log
Conjugate base
Weak acid

35. Importance of HHE: used to
❑ determine the pH of weak acid
❑ determine the pH of buffer solution
❑ quantify the buffer components needed to prepare a
buffer solution of definite pH
❑ determine blood pH & to evaluate the acid base
status of patients
❑ spell out the concept of acidosis & alkalosis
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