of Ova was essential to initiate cross-assembly when mixed with LYS at pH 7.6 and
low ionic strength. A LYS:Ova molar ratio of 1.5 was found in the formed supramolecular structures. No turbidity (aggregation) was detected by these authors when
LYS was mixed with native (unheated) Ova. The aggregation between LYS and
denatured Ova was also inhibited through the decrease of the positive net charge of
LYS by chemical acetylation. Similarly, an increase in ionic strength beyond 50 mM
strongly reduces the aggregation between the two proteins. The authors conclude that
electrostatic interactions are important during cross-assembly of LYS with the
unfolded chain of Ova. Electrostatic cross-assembly between oppositely charged
molecules was also reported for binary systems involving unstructured proteins.
This is the case when mixing gelatin-A (pI ¼ 9) and gelatin-B (pI ¼ 5) at molar
ratio gelatin-A:gelatin-B of 1.5 [133]. This pH-controlled electrostatic crossassembly was explored by these authors for encapsulation and release of a hydrophilic drug.
Electrostatic interactions are also described to be the main driving force in the
self-assembly of structures between Lf (a globular protein) and α-casein, β-casein
or κ-casein (unstructured proteins) at neutral pH [132]. As expected for
electrostatic-driven complexes, the size of the formed complexes is affected by
pH and ionic strength. The size of the complex decreases with increasing salt or
when the pH shifts away from the pH for charge equilibration. For this latter case,
it is assumed that the growth of the complexes is limited by the accumulation of
charge on the surface of the self-assembled structures.
Recent and fundamental studies have been done on the physical chemistry of
aggregation and assembly between proteins with opposite charge. Biesheuvel et al.
[130] have shown the importance of the surface properties of proteins for their
interaction and subsequent assembly and phase separation in the case of a binary
protein mixture. For this, they studied the assembly between native LYS, with an
overall charge at neutral pH of þ7, and succinylated LYS, with an overall opposite
charge at neutral pH of À7. The study was conducted under conditions whereby the
overall conformation of proteins are not affected. By combining experimental and
modelling approaches, these authors showed that the assembly between these two
proteins was modulated by different physicochemical factors such as pH, ionic
strength, the molar ratio of the two proteins and the temperature [130, 131]. The
assembly process between these two protein forms decreases with increasing ionic
strength and is inhibited for pH values where the overall charges of the two proteins
has the same sign. A complete phase diagram, depending on the ionic strength and
protein concentration, was described and commented on by the authors. In this
protein mixture, electrostatic interactions and temperature are shown to be the main
driving forces for protein interaction, for subsequent aggregation and then for phase
separation. The theoretical model used to describe the assembly of oppositely
charged proteins takes into account electrostatic interactions, steric effects and
temperature. Optimal conditions for liquid–liquid phase separation in this system
include: low ionic strength, low temperature, a symmetrical charge ratio, high
protein concentration and a pH value at which the protein charge densities are
highest, i.e. pH 7.5. In this work, protein aggregation and phase separation were
assessed by turbidimetry measurements but no structural characterisation of the
Spontaneous Assembly and Induced Aggregation of Food Proteins
87
low ionic strength. A LYS:Ova molar ratio of 1.5 was found in the formed supramolecular structures. No turbidity (aggregation) was detected by these authors when
LYS was mixed with native (unheated) Ova. The aggregation between LYS and
denatured Ova was also inhibited through the decrease of the positive net charge of
LYS by chemical acetylation. Similarly, an increase in ionic strength beyond 50 mM
strongly reduces the aggregation between the two proteins. The authors conclude that
electrostatic interactions are important during cross-assembly of LYS with the
unfolded chain of Ova. Electrostatic cross-assembly between oppositely charged
molecules was also reported for binary systems involving unstructured proteins.
This is the case when mixing gelatin-A (pI ¼ 9) and gelatin-B (pI ¼ 5) at molar
ratio gelatin-A:gelatin-B of 1.5 [133]. This pH-controlled electrostatic crossassembly was explored by these authors for encapsulation and release of a hydrophilic drug.
Electrostatic interactions are also described to be the main driving force in the
self-assembly of structures between Lf (a globular protein) and α-casein, β-casein
or κ-casein (unstructured proteins) at neutral pH [132]. As expected for
electrostatic-driven complexes, the size of the formed complexes is affected by
pH and ionic strength. The size of the complex decreases with increasing salt or
when the pH shifts away from the pH for charge equilibration. For this latter case,
it is assumed that the growth of the complexes is limited by the accumulation of
charge on the surface of the self-assembled structures.
Recent and fundamental studies have been done on the physical chemistry of
aggregation and assembly between proteins with opposite charge. Biesheuvel et al.
[130] have shown the importance of the surface properties of proteins for their
interaction and subsequent assembly and phase separation in the case of a binary
protein mixture. For this, they studied the assembly between native LYS, with an
overall charge at neutral pH of þ7, and succinylated LYS, with an overall opposite
charge at neutral pH of À7. The study was conducted under conditions whereby the
overall conformation of proteins are not affected. By combining experimental and
modelling approaches, these authors showed that the assembly between these two
proteins was modulated by different physicochemical factors such as pH, ionic
strength, the molar ratio of the two proteins and the temperature [130, 131]. The
assembly process between these two protein forms decreases with increasing ionic
strength and is inhibited for pH values where the overall charges of the two proteins
has the same sign. A complete phase diagram, depending on the ionic strength and
protein concentration, was described and commented on by the authors. In this
protein mixture, electrostatic interactions and temperature are shown to be the main
driving forces for protein interaction, for subsequent aggregation and then for phase
separation. The theoretical model used to describe the assembly of oppositely
charged proteins takes into account electrostatic interactions, steric effects and
temperature. Optimal conditions for liquid–liquid phase separation in this system
include: low ionic strength, low temperature, a symmetrical charge ratio, high
protein concentration and a pH value at which the protein charge densities are
highest, i.e. pH 7.5. In this work, protein aggregation and phase separation were
assessed by turbidimetry measurements but no structural characterisation of the
Spontaneous Assembly and Induced Aggregation of Food Proteins
87