Isoelectric pH Flashcards
Defintion pH
power of hydrogen
Definition of isoelectric pH
represents the pH at which the acid dissociation equals with the basic dissociation, the overall net electrical charge of the amino acid or protein is = 0.
Calculation of the isoelectric pH
The order of dissociation is:
- dissociates the COOH groups (first those attached to the carbon atom situated in alpha α position, then beta β, gamma γ, delta Δ, epsilon ε, etc)
- dissociation of NH3+ groups follows (first those attached to the carbon atom situated in alpha α position, then beta β, gamma γ, delta Δ, epsilon ε, etc)
CAVE: You can take away protons only linked to oxygen or nitrogen.
In the case of amino acids/ proteins, in acidic environment, the dissociation starts from the fully protonated form.
Examples phi
Isoelectric ph: calculation example
glutamic acid
Isoelectric ph: calculation example
lysine
Isoelectric ph: calculation example
alanine
Depending on the pH of the medium we can have for an amnio acid ..
a cationic form (positive charge)
a anionic form (negative charge)
zwitterion (isoelectric form or neutral form) (charge = O)
The pH at which a molecule exists as a zwitterion is called isoelectric pH (pHi).
At pHi …
- the charge of the molecule is O
- there is no mobility in an electrical field
the solubility or buffering capacity is minimum
pKa1
when 50% of the molecules are in isoelectric form and 50% of the molecules are in cationic form +
= PKa1 (with regard to COOH)
pKa2
when 50% of the molecules are in isoelectric form and 50% of the moleules are in anionic form -
= pKa 2 (with regard to NH2)
Monoamino monocarboxylic
amino acids
pHi = 0,5 (pka1 + pKa2)
Glycine, Alanin, Valine, Leucine, Isoleucin, Serine, Theorine, Cystein, Methionine, Glutamine, Asparagin
Monoamino dicarboyxlic
amino acids
pHi = 0,5 (pKa1 + pKa2)
Aspartic acid, Glutamic acid
Diamino monocarboxylic
amino acids
pHi = 0,5 (pKa2 + pKa3)
Lysine, Arginine, Histidine
pKa/b
for strong acids/ strong base
pK<strong>a/b</strong> for strong base/ strong acid is
between -10 and 2.
pKa/b
for waek acid or base
pKa/b for waek acid or base is
higher than 2
Calaculate pKa1, pKa2 and pHi of glycine knowing that for titration of this amino acid with NaOH the values from the following table were obtained:
Equation of pHi
