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pH and buffers
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Summary of pH, amino acids and buffers. How to calculate pH using pKa value?
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pH and buffersStrong acid – fully dissociated in solution into proton and a conjugate base – HCl
Weak acid – partially dissociated in solution into proton and a conjugate base – H2CO3
Conjugate base – when an acid dissociates into its ions in water, it loses a hydrogen ion. The species
that is formed is the acid’s conjugate base
Strong base – always fully dissociated in solution; good acceptor of H+ ions
Weak base – can be partially dissociated in solution; poor acceptor of H+ ions
Blood pH:
Normally, a very narrow range (7.35 to 7.45);
Lower – acidosis (7.0-7.35)
Higher – alkalosis (7.45-7.8)
Living range: pH 7.0-7.8 outside these limits it may be fatal;
Most acids are generated by:
1. breakdown of proteins
2. incomplete oxidation of fats or glucose
3. loading and transport of carbon dioxide in the blood.
Acid-base balance is regulated in the body by:
1. the lungs
2. the kidneys
3. systems in the blood known as chemical buffers
Acids- proton donors
Bases – proton acceptors
Buffers – mixture of both
Ka – dissociation constant
Kw –ionic product of water; Kw= [H+]. [OH-]=10-14M2
Keq = 1.8 x 10-14; At neutrality, [H+] = [OH-] = 10-7M; Pure water is a 55.6M solution;
pH – measure of H+ concentration, pH=-log10[H+]
pKa – pH at which the acid is half dissociated, pKa=-log10Ka
buffer solution – mixture of weak acid and its conjugate base; resists changes in pH upon addition
of H+ and OH- ions
α-carboxyl proton- the least strongly bound to amid acid molecule
, ‘basic’ amino acids – the α amino proton will be removed before that from the side chain
In some cases, it is the proton associated with the amino acid side chain (eg. aspartic and glutamic
acids, histidine and cysteine). In the case of other (‘basic’) amino acids (eg. arginine and lysine), the
-amino group’s proton will be removed before that from the side chain.
Acidic ionic form of molecule – fully protonated
Acidic amino acids – Aspartate, Glutamate
Basic amino acids – Lysine, Arginine, Histidine
All solutions are made up in 2M NaCl titration of arginine, aspartate and histidine
A buffer is a dilute solution of weak acid and its conjugate base such that the addition of small
amount of acid or base does not change the pH ( or the pH does not change that much)
The buffer is at its most effective when there is 50% weak acid and 50% weak base, the pH of this
solution is the pKa. If a small amount of strong base is added to this buffer solution some more of the
weak acid dissociates and donates a proton to the hydroxide ion making water.
If a small amount of strong acid is added to this buffer solution some of the weak base accepts a
proton, as the pH is a measure of the free proton concentration the pH is unchanged.
The α amino and α carboxyl groups cannot contribute to the buffering of the amino acid because
they form peptide bond – each protein will have a single α amino group and a single α carboxyl but
there are not significant. It is only the R side chain which could potentially contribute to buffering.
Buffering at two pKa and No buffering at the zwitterion
pKa values:
α COOH ~ pH2 Glycine – -HOOC.CH2.NH3+
α amino ~pH9.5 Diprotic
-
OOC.CH2.NH3 – zwitterion
R group aspartate ~pH4.0 Positively charged in pH1
R group arginine ~pH12.5 Negatively charged in pH12
R group histidine ~pH6
Physiologically important buffers: H2CO3 (H2CO3 is proportional to the pCO2)
Haemoglobin is an important buffer in the blood.
The presence of a large number of histidine residues .
Oxyhaemoglobin pKa = 6.8 • Deoxyhaemoglobin pKa = 7.8
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