agronom.me
Menu

Agrochemistry

The effect of acidity and salts on protein solubility in agricultural chemistry

For students

6 min read

The effect of acidity and salts on protein solubility in agricultural chemistry

Proteins play an important role in the formation of the structure of working solutions and cellular metabolism in plants. Their solubility, ability to retain moisture, and stabilize suspensions depend directly on the acidity of the medium and the concentration of mineral salts. Understanding these mechanisms helps agronomists control the stability of multicomponent tank mixtures and avoid the precipitation of active ingredients.

How medium acidity determines protein solubility

Most proteins are hydrophilic, meaning they are capable of binding water due to polar and charged groups on the molecule's surface. However, their solubility in water is unstable and depends on the pH level of the solution. Proteins have minimal solubility at their isoelectric point. In this state, the net electrical charge of the molecule is zero, electrostatic repulsion disappears, and proteins quickly clump together into large flakes.

The mechanism of protein precipitation when pH changes is clearly demonstrated by the process of milk curdling:

  1. In fresh milk, the protein casein is completely dissolved because the initial pH of the medium is significantly higher than its isoelectric point.
  2. As the medium is acidified with lactic acid, the pH level of the solution gradually decreases.
  3. Upon reaching the isoelectric point of casein (pH 4.7), the protein loses its charge, sheds its hydration shell, and precipitates.

In living plant cells, this balance is strictly controlled. The isoelectric points of most cytoplasmic proteins lie just above pH 6. Since the pH of healthy cytoplasm approaches 7, cellular proteins are always in a slightly alkaline environment relative to their isoelectric point. Thanks to this, they carry a negative charge, repel each other, and remain stable in a dissolved state.

At a pH below the isoelectric point, a protein carries a net positive charge, and at a pH above it — a negative one. The resulting electrostatic repulsion between like-charged molecules prevents them from clumping and keeps the proteins in solution.

The influence of mineral salts and colloidal properties of proteins

Introducing mineral salts into a solution can drastically alter protein solubility. In low concentrations, neutral salts such as ammonium sulfate, sodium sulfate, or magnesium sulfate increase solubility. They increase the dissociation of ionized protein groups, reducing protein-protein interaction. This effect is called salting-in.

High salt concentrations in a tank mixture lead to the opposite effect — salting-out. The salt removes water from protein molecules for its own hydration, causing the protein to lose solubility and precipitate. This can inactivate biological products and clog sprayer nozzles.

Besides solubility, proteins possess distinct hydrophilic (water-binding) and hydrophobic (fat-binding) properties. Due to this, they can act as emulsifiers, holding together mutually insoluble liquids like water and oil. In working solutions, proteins exhibit typical colloidal properties:

  • diffuse slowly in media;
  • are unable to pass through semipermeable membranes;
  • scatter light;
  • are characterized by high viscosity.

To control the physical properties of solutions and assess the state of plant tissues, the following basic indicators of protein systems are used:

  • Water and fat binding at the surface — 0.2–0.4 g per 1 g of protein
  • Isoelectric point of casein — pH 4.7
  • Isoelectric point of cytoplasmic proteins — above pH 6
  • Normal cytoplasm pH of cells — about 7

Plant proteins and components of amino acid biostimulants possess amphoteric properties — they can behave both as acids and as alkalis. The carboxyl groups of aspartic and glutamic amino acids are responsible for acidic properties, while the radicals of arginine, lysine, and histidine are responsible for alkaline ones. Thanks to this, proteins act as a natural buffer, smoothing out pH fluctuations in plant cells. Furthermore, the active groups of amino acids easily bind nutrients, forming strong peptide, hydrogen, and disulfide bonds within the molecule.

The amphoteric nature of proteins helps plants maintain an internal pH balance during sudden changes in the acidity of the soil or working solution.

Denaturation and protein solubility conditions

The spatial structure of a protein molecule is very labile and easily destroyed under the influence of external stresses. When the bonds holding the secondary, tertiary, or quaternary structure break, the protein denatures — it folds into a random coil. In doing so, it loses biological activity and changes its viscosity and solubility. If the damage has not affected the primary structure (the amino acid chain), the protein can recover after the threat is removed — this process is called renaturation.

The denaturation of protein compounds is caused by the following factors:

  • physical: critical temperatures, pressure, mechanical stress, ultrasonic and ionizing radiation;
  • chemical: heavy metal salts, acids, alkalis, organic solvents, and alkaloids.

The application of protein preparations and biostimulants together with copper-based fungicides or in extreme heat is ineffective. Heavy metals and high temperatures cause irreversible protein denaturation, completely stripping the preparation of its functional properties.

The solubility of proteins in water depends directly on their hydrophilicity. A charged protein molecule attracts water dipoles, creating a hydration shell around itself. This shell prevents the molecules from sticking together and precipitating. The ability to retain water depends on the protein structure:

  • albumins possess high hydrophilicity and form a large water shell;
  • globulins bind to water significantly more weakly, so their shell is thinner.

The stability of any protein solution relies on two factors: the electrical charge of the molecule and the presence of a hydration shell. If the charge is neutralized or the water shell is destroyed, the protein will precipitate. Depending on the conditions, this process can be reversible or irreversible.

Read next