Protein purification essentially relies on differences in intermolecular forces between the target protein and impurities to achieve differential separation. The core intermolecular forces include electrostatic interactions, hydrophobic interactions, hydrogen bonds, van der Waals forces, and specific affinity interactions, each corresponding to a mainstream purification technique.

(i) Electrostatic interactions depend on charged amino acid residues on the protein surface and are regulated by buffer pH and ionic strength. They are the core principle of ion exchange chromatography. When the pH deviates from the protein’s isoelectric point (pI), the protein’s net charge increases, allowing reversible binding to charged media. This is suitable for impurity removal during initial recombinant protein purification.

(ii) Hydrophobic interactions arise from the hydrophobic aggregation effect of hydrophobic amino acid residues on the protein surface and are significantly enhanced under high‑salt conditions. They are the basis of hydrophobic interaction chromatography and are often used for fine purification of recombinant proteins.

(iii) Hydrogen bonds and van der Waals forces are weak molecular interactions that operate throughout all chromatography processes. They primarily assist protein‑media binding while maintaining protein spatial conformation. Specific affinity interactions represent precise molecular recognition, relying on the unique binding properties between ligands and tag proteins or between antigens and antibodies. They support affinity chromatography and are essential for producing recombinant tagged proteins and obtaining high‑purity proteins. The interplay of these forces constitutes a complete purification system for recombinant proteins, from crude extraction to polishing.

The stability of recombinant protein spatial conformation depends entirely on intramolecular hydrogen bonds, hydrophobic interactions, and electrostatic interactions. During purification, intermolecular forces directly alter protein conformation, thereby affecting final yield and product quality.

(i) Intramolecular hydrophobic interactions bury hydrophobic groups inside the protein, maintaining a compact and stable structure. If the salt concentration in the purification buffer is imbalanced, the hydrophobic balance is disrupted, exposing hydrophobic groups on the protein surface and leading to protein aggregation and precipitation, which significantly reduces purification yield.

(ii) Electrostatic interactions determine protein solubility. When the buffer pH is near the protein’s pI, the net protein charge approaches zero, electrostatic repulsion disappears, and the probability of intermolecular aggregation greatly increases, causing protein inactivation and loss.

(iii) Hydrogen bonds are easily affected by temperature and buffer additives. Hydrogen bond breakage leads to loosening of the protein secondary structure and denaturation. Precisely controlling affinity interactions and stabilizing weak molecular forces can reduce protein conformational damage. In practical recombinant protein production, optimizing the buffer system and regulating ionic strength and pH to balance the various intermolecular forces effectively enhances protein stability and maximizes purification yield.

Among recombinant protein production and purification processes, hydrophobic interactions are the core non‑specific forces influencing separation efficiency, protein stability, and product purity. After expression in prokaryotic or eukaryotic systems, recombinant proteins expose many hydrophobic amino acid residues on their surface. The strongly polar aqueous environment drives these hydrophobic groups to aggregate. This is both the primary reason why proteins tend to aggregate and precipitate and the key basis for protein separation by hydrophobic interaction chromatography.

Compared with electrostatic interactions, which are easily disturbed by pH and salt ions, and hydrogen bonds, which are weak and less stable, hydrophobic interactions are applicable to most soluble recombinant protein purification scenarios and have broader utility. In practice, high‑salt buffers compress the water layer, strengthening binding between the protein and the hydrophobic medium. Then, by gradually decreasing salt concentration, hydrophobic interactions are weakened, allowing stepwise elution of the target protein from impurities. Moreover, the conditions used for hydrophobic interactions are mild and do not disrupt protein secondary or tertiary structure, thus preserving recombinant protein biological activity to the greatest extent and avoiding denaturation, inactivation, and stability loss during purification. For these reasons, hydrophobic interactions are the key driving force for fine purification of recombinant proteins and production of active proteins.

Electrostatic interactions are the core principle of ion exchange chromatography and the most important molecular force in the initial (crude) purification of recombinant proteins. Their strength directly determines protein binding, elution efficiency, and separation purity. Acidic and basic amino acid residues on the protein surface change their charge state with buffer pH. When the buffer pH is below the protein’s pI, the protein becomes positively charged and can bind to cation exchange media; when the pH is above the pI, the protein is negatively charged and suitable for anion exchange media. This electrostatic adsorption enables protein immobilization.

During purification, the intensity of electrostatic interactions is precisely regulated by ionic strength. Low‑ionic‑strength buffer eliminates interference from free ions, enhancing electrostatic binding between the protein and the chromatography medium and maximizing target protein adsorption. As salt concentration is gradually increased, free ions in the buffer compete for binding sites on the medium, disrupting electrostatic interactions and achieving gradient elution. Because different proteins have different numbers and distributions of charged residues, their electrostatic binding strengths vary. Thus, by precisely adjusting pH and salt concentration, target recombinant proteins can be distinguished from impurities, and contaminants such as nucleic acids and other proteins can be efficiently removed, achieving preliminary crude purification and laying the foundation for subsequent fine purification.