CHMI-2227EL Lecture Notes - Lecture 3: Lithium Hydroxide, Sodium Hydroxide, Acid Dissociation Constant
27 Jun 2018
School
Course
Professor
Biochemistry - Day 3 2018.01.19
Net charge of amino acids as function of pH
-pI = iso electric point
-net charge is negative: if pH > pI more basic
-net charge is positive: if pH < pI more acidic
-pH=pI then this is zero
Acid-base properties of amino acids
-the isoelectric point or pI can be calculated from the pK values
-for monoamino monocarboxylic acid, the
expression is:
-pI = (pKa1 + pKa2) / 2
-where Ka1 and pKa2 are the pKs of the
carboxyl and amino groups, respectively
-Most amino acids have a pKa1 around 2 and
pKa2 ranging from 8 to 10
-The isoelectric point or pI can be determined
by subjecting the amino acid to electrophoresis
in buffers of varying pH
Electrophoresis
-Electrophoresis: the process of separating
compounds on the basis of their electric charge
-Electrophoresis of amino acids can be carried out
using paper, starch, agar, certain plastics, and
cellulose acetate as solid supports
-In paper electrophoresis, a paper strip is
saturated with an aqueous buffer of
predetermined pH, which serves as a bridge
between two electrode vessels
-A sample of amino acids is applied as a spot on
the paper strip
-An electric potential is applied to the electrode vessels and amino acids migrate
towards the electrode with charge opposite their own
-Molecules with a high charge density move faster then those with low charge density
-Molecules at their isoelectric point remain at the origin (where the charge on the atom
is neutral)
-After separation is complete, the strip is dried and developed to make the separate
amino acids visible
find more resources at oneclass.com
find more resources at oneclass.com
Acids and Bases
-Strong acid: completely ionized in aqueous solution
-HCl, HBr, HI, HNO3, HClO4, H2SO4
-Strong base: completely ionized in aqueous solution
-LiOH, NaOH, KOH, Ca(OH)2, Ba(OH)2
Weak Acids and Bases
-An acid or base that is not completely ionized in aqueous solution
-Carboxylic acids, such as acetic acid, are weak acids
Ionization of H2O and pH
-55.5 M is obtained by dividing the number of grams of water in one litre, 1000 grams,
by the molecular weight of water, which is 18 grams/mole
-Calculations
-[H+] = 10-7 M = [OH-]
-H2O -> OH- + H+
-Ka= [H+][OH-]/[H20] = [H+][OH-]/55.55
-Kw = Ka x 55.5 = [H=][OH-]
-Kw= 10-14
-The equation for the ionization of a weak acid, HA, in water and the acid ionization
constant, Ka, for this equilibrium are:
-HA + H2O -> A- + H3O+
-Ka= Keq [H2O] = [H3O+][A-]/[HA]
-pKa = -logKa
Henderson - Hasselbalch
-We can calculate the ration of weak acid, HA, to its
conjugate base A-, in the following way
-The acid dissociation constant for the weak acid is
-Ka= [conjugate base][H+] / [weak acid]
--log [H+]=-logKa + log [conjugate base]/[weal acid]
-pH = pKa + log [conjugate base]/[weak acid]
-from this equation we see that when the concentration
find more resources at oneclass.com
find more resources at oneclass.com
