Biomedical Engineering Reference
In-Depth Information
α
=
dilution
1). The definition of ionic strength I with respect to
incomplete dissociation is
i
n
c
i
·
α
z
i
1
2
·
=
I
·
i
=
i
1
Thedegreeofdissociationiscalculatedby
10
−pK
10
−pK
+10
−pH
pK: dissociation constant; pH: value of pH for the respective buffer.
Since proteins contain a lot of acidic and basic side chains
acting as weak acids and bases, respectively, proteins are buffering
substances, too. If you mix buffer solutions with protein solutions,
pH may be altered because the concentration of protein's buffering
residues may exceed the capacity of the (chemical) buffer. For
instance, bovine serum albumin contains 59 basic (Lys) and 99
acidic (59 Asp plus 40 Glu) residues per mole; a solution of 10 mg
α
=
i
/
ml
(1%) BSA contains 9 mM basic and 14.5 mM acidic residues, and
phosphate-buffered saline (PBS) contains only 10 mM phosphate.
As a consequence of this example (a) the concentration of the
chemical buffer should be high enough to act as a buffer, (b) choose
a chemical buffer the pK of which is nearby the pH to be stabilized,
and (c) adjust the pH after all components are mixed.
β
Fig. 7.3.
Buffer capacity
depending on distance of pH to pK
References
Stoll VS, Blanchard JS (1990) Meth Enzymol 182:24
Beynon RJ, Easterby JS (1996) Buffer solutions. IRL Press, Oxford
