The physicochemical properties of Nisin

The physicochemical properties of Nisin (nisin) are closely linked to its unique molecular structure, and the interplay of pH, temperature, and solubility is a core factor determining its application scenarios. As a lantibiotic consisting of 34 amino acids, its molecule contains multiple unusual amino acids linked by thioether bonds (e.g., lanthionine, β-methyllanthionine). These structures not only endow it with antimicrobial activity but also profoundly influence its stability and dissolution behavior in different environments.

I. Impact of pH on Nisin’s Physicochemical Properties

Nisin’s stability and solubility are highly sensitive to pH changes, exhibiting a significant acid-dependent characteristic.

Under acidic conditions (pH 2.0–6.0), the Nisin molecule carries a positive charge, with a compact and stable structure. This is because its amino acid sequence contains abundant basic amino acids (e.g., lysine), which are resistant to hydrolysis in acidic environments. Additionally, strong intermolecular repulsion prevents aggregation. For example, in a pH 3.0 citric acid buffer, Nisin retains over 90% of its activity; even after autoclaving at 121°C, it still preserves approximately 70% of its antimicrobial capacity. This property makes it particularly suitable for preserving acidic foods (e.g., yogurt, pickled products).

In neutral or alkaline conditions (pH >7.0), the situation differs drastically. As pH increases, basic amino acids in Nisin gradually deprotonate, causing the molecular structure to loosen. Thioether bonds are prone to hydrolytic cleavage, leading to rapid loss of activity. For instance, in a pH 8.0 environment, its activity decreases by over 50% even after 24 hours of storage at room temperature; in strongly alkaline conditions (pH >9.0), it becomes completely inactive within hours. Furthermore, Nisin’s solubility drops significantly under alkaline conditions, with a tendency to precipitate, further limiting its efficacy.

II. Relationship Between Temperature and Nisin Stability

Nisin’s thermal stability is also regulated by pH, following the rule of "stable in acid, labile in neutral/alkaline environments."

In acidic environments, Nisin exhibits exceptional heat resistance. For example, at pH 2.0–4.0, it retains 60%–80% of its antimicrobial activity after boiling at 100°C for 30 minutes or autoclaving at 121°C for 15 minutes. This heat resistance stems from the rigidity of its molecular structure under acidic conditions: thioether bonds and peptide bonds are resistant to heat-induced cleavage, and the compact conformation reduces the likelihood of thermal denaturation. This trait allows it to withstand pasteurization or high-temperature sterilization in food processing, making it one of the few natural antimicrobials suitable for heat-processed foods.

In neutral or alkaline conditions, elevated temperatures accelerate Nisin inactivation. For example, at pH 7.0, treatment at 60°C for 30 minutes reduces its activity by 40%; at 100°C, activity is completely lost within 10 minutes. This phenomenon is associated with the loose molecular structure in neutral environments: high temperatures exacerbate hydrolysis of peptide and thioether bonds, destroying key structures for antimicrobial activity (e.g., hydrophobic regions responsible for inserting into bacterial cell membranes).

III. Interaction Between Solubility and Environmental Factors

Nisin’s solubility primarily depends on pH and solvent properties. Its solubility in pure water is low (approximately 400 μg/mL) but increases significantly in acidic solutions—for example, reaching over 10 mg/mL in 0.1 M hydrochloric acid or 1% citric acid. This is because positive charges under acidic conditions enhance hydrogen bonding with water molecules, reducing aggregation.

Additionally, ionic strength in solvents affects solubility: low-concentration salt solutions (e.g., 0.1 M NaCl) promote dissolution through charge shielding; however, high-concentration salts (e.g., 1 M NaCl) may cause precipitation via salting-out effects. Organic solvents (e.g., ethanol, methanol) have minimal impact on solubility, but excessive organic phases can disrupt hydrophobic interactions, leading to structural denaturation and activity loss.

Nisin’s physicochemical properties exhibit distinct environmental dependence: acidic conditions are critical for maintaining its stability, high solubility, and heat resistance, while neutral/alkaline environments and high temperatures significantly impair its activity. These traits both limit its main application scenarios (e.g., acidic foods, low-pH pharmaceutical formulations) and provide clear optimization directions for practical use—for example, adjusting system pH or adding acidic stabilizers can maximize its natural antimicrobial efficacy.