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Summary chapter 1-3 from Lehninger (biochemistry)
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Summary of the properties of water and the basic properties of amino acids.
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TEST BANK FOR LEHNINGER PRINCIPLES OF BIOCHEMISTRY NELSON 6TH EDITION
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Hoofdstuk 2 pagina 58-65Ionization of Water, Weak Acids, and Weak Bases /The Ionization of Water is expressed by an
Equilibrium Constant
There is a small degree of Ionization of water to H+ and OH- and can be described by an equilibrium
[𝐻 + ][𝑂𝐻 − ]
constant([H2O]=55.5M). 𝐾𝑒𝑞 = & 1.0𝑥10−14 𝑀2 = [𝐻 + ][𝑂𝐻 − ]
[55.5𝑀]
Weak acids ionize and weak bases protonate when dissolved. [H+] pH.
The net movement of H+ (and OH- in the opposite direction) is caused by proton hopping over H-
bonds, because of this high ionic mobility reactions in aqueous solutions are exceptionally fast.
Reversible ionization is crucial to the role of water in cellular function.
[𝐶]𝑔 [𝐷]ℎ
Standard expression equilibrium constant for eA+fB ↔ gC+hD is 𝐾𝑒𝑞 = [𝐴]𝑒 [𝐵]𝑓
Keq is dimensionless, fixed and characteristic for any chemical reaction at a specified temperature.
∆G◦ is directly related to Keq.
The pH Scale Designates the H+ and OH- Concentrations
Convenient means of designation concentration of H+ in an aqueous solution from 1.0M H+ to 1.0M
OH-. 𝑝𝐻 = − log[𝐻 + ] pH water at 25◦C is 7.0. pH is logarithmic, 1 unit is 10x more/less.
pH > 7 alkaline or basic ( 7.4 alkalosis in body)
pH < 7 acidic ( 7.4 acidosis in body)
Some indicators: litmus, phenolphthalein and phenol red. (change colour as proton dissociates from
dye)
Accurate measurements: glass electrodes
Weak Acids and Bases have Characteristic Acid Dissociation Constants
Strong acids/bases completely ionized in dilute aqueous solution
Proton donor and corresponding acceptor conjugate acid-base pair with reversible reaction (K-
[𝐻 + ][𝐴− ]
eq=Ka) 𝐾𝑎 = (HA ↔ H+ + A-) Ka= Ionization/acid dissociation constant
[𝐻𝐴]
𝑝𝐾𝑎 = − log 𝐾𝑎
Weak acids have a small Ionization constant with a high pKa.
Titration Curves Revail the pKa of Weak Acids
A titration curve (the pH against the amount of OH- added) reveals the pKa. When base is added it
gets protonated, which causes the weak acid to further ionize because it will want to satisfy its own
equilibrium. Halfway through the titration HA(donor) equals A-(acceptor), here pH = pKa . The
endpoint of such titration is about pH 7.0 and the 2 equilibria coexist.
Buffering against pH Changes in Biological Systems
The protonated amino and carboxyl groups of amino acids and the phosphate groups of nucleotides
function as weak acids are ionizable groups with their characteristic pKa, their ionic state determent
by their surroundings’ pH. (Away from water their pKa can be significantly different)
Cells and organisms maintain a specific, constant cytosolic (also extracellular )pH (usually around 7)
for optimal ionic state of biomolecules. This is primarily done by biological buffers.
Buffers Are Mixtures of Weak Acids and Their Conjugate Bases
Buffers are aqueous systems that resist pH changes when small amounts of an acid or a base is
added. The flat midpoint of the titration curve(= buffering region) is the pH at which I conjugate pair
acts maximal as buffer, which again is equal to pKa . There is a change in pH but this is very small
compared to the change in pH in water or the salt of a strong acid/base. (NaCl). Result of 2 reversible
reaction equilibria with nearly equal concentration a small change in ratio relative concentrations
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