der Waals attractive forces. The assembly of casein, either in monomer or micellar
states, with a cationic polymer of dodecyltrimethylammonium has been described
[123]. Repeating units of the cationic polymer at low concentrations bind to acidic
amino acids of the casein molecules, making the casein micelles more compact due
to the reduction of the overall net surface charge. Increasing the concentration of
cationic polymer induced aggregation and formation of insoluble casein–surfactant
complexes. A re-solubilisation of these complexes was reached for a large excess of
polymer with respect to the casein because of the repulsion caused by an excess of
positive charges at the surface of assembled particles. The re-solubilised complexes
exhibited new physicochemical properties compared to the initial complexes,
which may, according to the authors, broaden the application range of casein
micelles in food science, as well as in the cosmetic and medical domains. This
study illustrates the key effect of the relative proportions of mixed oppositely
charged molecules on the final properties of formed supramolecular structures.
The use of neutron scattering provides access to the internal structure of objects
resulting from the mixture of a protein and a polyelectrolyte with opposite charge.
Studies of mixtures of an anionic polyelectrolyte with LYS showed that the
assembly leads to the formation of several types of supramolecular structures
depending on the concentration, pH and the size of the polyelectrolyte chain
[120]. The study clearly shows that the initial charge ratio and the polyelectrolyte
concentration are the main factors governing the assembly. Under certain
conditions, the two molecules form dense primary complexes with an internal
positive/negative charge ratio close to 1. According to the initial charge ratio, either
free protein in solution or polyelectrolyte forming a crown around the assembled
structure were detected. The variation of physicochemical parameters allowed the
authors to draw the following three conclusions: (a) the inner charge ratio of formed
complex remains close to 1 whatever the initial charge ratio; (b) the assembled
complexes exhibit a very high density when the charge density of mixed molecules
is maximum; and (c) the finite size of complexes (up to several microns) is limited
by the electrostatic repulsions and can therefore be modulated by the ionic strength
of the medium. These laws of interaction have been also reported in the case of two
synthetic oppositely charged polyelectrolytes [127]. A strong or a weak assembly
was reached according to the charge and concentration ratio of the constituents. The
stoichiometry between the two partners found in the assembled supramolecular
structures depends on the system studied. The difference in size between mixed
molecules was assumed to be important for the stoichiometry in the supramolecular
structures. This is well illustrated during cross-assembly of LYS with different
polyelectrolytes [118]. One LYS molecule self-assembles with either 20 mol of
poly(vinyl sulfonic acid) or 500 mol of poly(acrylic acid). The apparent energy
associated with the formation of the complexes is very high but once normalised by
the number of involved molecules, it accounts for about 10 kcal/mol, corresponding
to the energy of a normal Coulombic interaction between two electrical charges in
solution. This explains the low salt concentration typically required for total
dissociation of the formed supramolecular structures.
84
S. Bouhallab and T. Croguennec
states, with a cationic polymer of dodecyltrimethylammonium has been described
[123]. Repeating units of the cationic polymer at low concentrations bind to acidic
amino acids of the casein molecules, making the casein micelles more compact due
to the reduction of the overall net surface charge. Increasing the concentration of
cationic polymer induced aggregation and formation of insoluble casein–surfactant
complexes. A re-solubilisation of these complexes was reached for a large excess of
polymer with respect to the casein because of the repulsion caused by an excess of
positive charges at the surface of assembled particles. The re-solubilised complexes
exhibited new physicochemical properties compared to the initial complexes,
which may, according to the authors, broaden the application range of casein
micelles in food science, as well as in the cosmetic and medical domains. This
study illustrates the key effect of the relative proportions of mixed oppositely
charged molecules on the final properties of formed supramolecular structures.
The use of neutron scattering provides access to the internal structure of objects
resulting from the mixture of a protein and a polyelectrolyte with opposite charge.
Studies of mixtures of an anionic polyelectrolyte with LYS showed that the
assembly leads to the formation of several types of supramolecular structures
depending on the concentration, pH and the size of the polyelectrolyte chain
[120]. The study clearly shows that the initial charge ratio and the polyelectrolyte
concentration are the main factors governing the assembly. Under certain
conditions, the two molecules form dense primary complexes with an internal
positive/negative charge ratio close to 1. According to the initial charge ratio, either
free protein in solution or polyelectrolyte forming a crown around the assembled
structure were detected. The variation of physicochemical parameters allowed the
authors to draw the following three conclusions: (a) the inner charge ratio of formed
complex remains close to 1 whatever the initial charge ratio; (b) the assembled
complexes exhibit a very high density when the charge density of mixed molecules
is maximum; and (c) the finite size of complexes (up to several microns) is limited
by the electrostatic repulsions and can therefore be modulated by the ionic strength
of the medium. These laws of interaction have been also reported in the case of two
synthetic oppositely charged polyelectrolytes [127]. A strong or a weak assembly
was reached according to the charge and concentration ratio of the constituents. The
stoichiometry between the two partners found in the assembled supramolecular
structures depends on the system studied. The difference in size between mixed
molecules was assumed to be important for the stoichiometry in the supramolecular
structures. This is well illustrated during cross-assembly of LYS with different
polyelectrolytes [118]. One LYS molecule self-assembles with either 20 mol of
poly(vinyl sulfonic acid) or 500 mol of poly(acrylic acid). The apparent energy
associated with the formation of the complexes is very high but once normalised by
the number of involved molecules, it accounts for about 10 kcal/mol, corresponding
to the energy of a normal Coulombic interaction between two electrical charges in
solution. This explains the low salt concentration typically required for total
dissociation of the formed supramolecular structures.
84
S. Bouhallab and T. Croguennec