Several theoretical models describing the interactions and assembly between
two charged polymers are available. However, the application of these models is
often difficult when proteins are involved because of their structural complexity.
Chain flexibility and electric charge density are the main structural parameters of a
given synthetic polyelectrolyte that influence its interacting properties. The importance of molecular flexibility for self-assembly and complex formation is well
illustrated when mixing synthetic polyelectrolyte poly(vinylsulfate) or poly
(methacrylic acid) with oppositely charged poly(L-lysine), whose conformation
can be modulated by pH or salt concentration change [126]. Spherical particles
were obtained with random coil conformation of the polypeptide chain whereas αhelical conformation resulted in needle-like particles. Concerning proteins, the
situation is much more complex due to the presence of charges of opposite sign
distributed all over the surface of the molecule, with specificity in local charge
density. In general, proteins carry patches of negative and positive charges on their
surface. For acidic proteins, the number of negative residues on the surface is larger
than the number of positive residues. Hence, the overall charge of these proteins
under neutral pH condition is negative (see [114]). The inverse is true for basic
proteins that carry positive charge under the same conditions. The presence of
positive (negative) patches on the surface of acidic (basic) proteins regulates the
surface concentration of counterions of opposite charges that affect subsequent
interaction and assembly processes between oppositely charged partners.
4.2 Oppositely Charged Proteins
The physicochemical laws that govern protein assembly in a system containing more
than one protein are presumably different from those described for the single protein
systems. In binary protein systems, there is a range of pH for which the two
proteins carry opposite net charges. This is especially true when mixing a basic
protein and an acidic protein. Under these conditions, electrostatic interactions
between the two proteins of opposite charge are attractive and can lead to supramolecular assembly. For a visual illustration, Fig. 5 summarizes how ionic strength
affects the assembly of proteins in system containing one type of protein (all proteins
carry the same net charge at a given pH) or a mixture of proteins with opposite net
charge at a given pH.
The interaction and spontaneous assembly between food proteins with opposite
charge under mild conditions are poorly described. Some published examples are
summarised in Table 3. In contrast to protein assemblies induced by drastic conditions
(see above), the advantage of mild conditions is the possibility of fabricating reversible
supramolecular assemblies because only weak non covalent interactions are involved.
This offers a real opportunity for better control of both the assembly process of proteins
into nano- and microstructures and the disassembly process.
Kobayashi’s group and Lewis’s group were the first to report the formation
of turbid solutions by mixing two oppositely charged globular proteins from
Spontaneous Assembly and Induced Aggregation of Food Proteins
85
two charged polymers are available. However, the application of these models is
often difficult when proteins are involved because of their structural complexity.
Chain flexibility and electric charge density are the main structural parameters of a
given synthetic polyelectrolyte that influence its interacting properties. The importance of molecular flexibility for self-assembly and complex formation is well
illustrated when mixing synthetic polyelectrolyte poly(vinylsulfate) or poly
(methacrylic acid) with oppositely charged poly(L-lysine), whose conformation
can be modulated by pH or salt concentration change [126]. Spherical particles
were obtained with random coil conformation of the polypeptide chain whereas αhelical conformation resulted in needle-like particles. Concerning proteins, the
situation is much more complex due to the presence of charges of opposite sign
distributed all over the surface of the molecule, with specificity in local charge
density. In general, proteins carry patches of negative and positive charges on their
surface. For acidic proteins, the number of negative residues on the surface is larger
than the number of positive residues. Hence, the overall charge of these proteins
under neutral pH condition is negative (see [114]). The inverse is true for basic
proteins that carry positive charge under the same conditions. The presence of
positive (negative) patches on the surface of acidic (basic) proteins regulates the
surface concentration of counterions of opposite charges that affect subsequent
interaction and assembly processes between oppositely charged partners.
4.2 Oppositely Charged Proteins
The physicochemical laws that govern protein assembly in a system containing more
than one protein are presumably different from those described for the single protein
systems. In binary protein systems, there is a range of pH for which the two
proteins carry opposite net charges. This is especially true when mixing a basic
protein and an acidic protein. Under these conditions, electrostatic interactions
between the two proteins of opposite charge are attractive and can lead to supramolecular assembly. For a visual illustration, Fig. 5 summarizes how ionic strength
affects the assembly of proteins in system containing one type of protein (all proteins
carry the same net charge at a given pH) or a mixture of proteins with opposite net
charge at a given pH.
The interaction and spontaneous assembly between food proteins with opposite
charge under mild conditions are poorly described. Some published examples are
summarised in Table 3. In contrast to protein assemblies induced by drastic conditions
(see above), the advantage of mild conditions is the possibility of fabricating reversible
supramolecular assemblies because only weak non covalent interactions are involved.
This offers a real opportunity for better control of both the assembly process of proteins
into nano- and microstructures and the disassembly process.
Kobayashi’s group and Lewis’s group were the first to report the formation
of turbid solutions by mixing two oppositely charged globular proteins from
Spontaneous Assembly and Induced Aggregation of Food Proteins
85