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3.1. POLYELECTROLYTE FUNDAMENTALS
3.1.1. Definition and
introduction.
A majority of the biopolymers are polyelectrolytes. Polyelectrolytes can be
defined as a polymer carrying fixed charges. Polymers carrying only negative
charged substituents (typically due to carboxylate, sulphate or phosphate) are
called polyanions, whereas polymers carrying only positively charged
substituents (typically various forms of amines) are called polycations. DNA
and RNA are polyanions due to the phosphate groups.
Examples of polyanionic polysaccharides (Charged group in parentheses):
•
Alginates (-COO
-
)
•
Pectin (-COO
-
)
•
CMC (carboxymethyl cellulose, (-COO
-
)
•
Carrageenans (-SO
4
-
)
•
Heparin (-COO
-
and –SO
4
-
)
•
Hyaluronan (-COO
-
)
Examples of polycationic polysaccharides:
•
Chitosans (-NH
3
+
)
•
Cationic starches (-N(CH
3
)
3
+
)
Polymers carrying both positive and negative charges are called ampholytes
or polyampholytes. Obviously, most proteins are ampholytes because they
generally contain both negatively charged amino acids (R-groups of Asp, Glu)
and the C-terminal carboxylate, and negatively charged groups (R-groups of
Lys, Arg, His) and the N-terminal amine.
3.1.2. Counterions: Essential
components with major
influence
Charged substituents are always accompanied by oppositely charged
counterions to maintain macroscopic electroneutrality. Their influence and
importance are large and polyelectrolytes cannot be considered independently
from them, as will become apparent throughout the course.
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-COO
-
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+
-NH
3
+