The intermolecular forces of Nisin

Nisin, a natural antibacterial peptide produced by Streptococcus lactis, consists of 34 amino acid residues in its molecular structure, with 5 unique lanthionine (Lan) and β-methyllanthionine (MeLan) cross-linking structures. These structures endow nisin with a specific spatial conformation and determine the types of its intermolecular forces as well as their functional relevance. The intermolecular forces of nisin not only affect its own aggregation state, solubility, and stability but also directly participate in its interaction with the cell membranes of target microorganisms (e.g., Gram-positive bacteria), thereby exerting antibacterial activity. Its main intermolecular forces can be analyzed from two dimensions: intermolecular interactions among Nisin molecules themselves and intermolecular interactions between nisin and target molecules.

I. Intermolecular Forces Among Nisin Molecules Themselves: Influencing Aggregation State and Stability

Nisin molecules form aggregates through non-covalent intermolecular interactions. This aggregation behavior is closely related to nisin’s solubility, stability, and antibacterial activity, with the core forces including hydrophobic interactions, hydrogen bonds, and electrostatic interactions:

1. Hydrophobic Interactions: The Core Driving Force for Molecular Aggregation

The molecular structure of nisin contains a relatively large number of hydrophobic amino acid residues (e.g., alanine, valine, isoleucine). The hydrophobic side chains of these residues tend to avoid contact with water, thereby driving multiple nisin molecules in aqueous solutions to form aggregates by bringing their hydrophobic regions close to each other. The strength of hydrophobic interactions is affected by environmental conditions (e.g., pH value, ionic strength):

Under neutral or weakly acidic conditions (nisin has an isoelectric point of approximately 8.0 and carries a positive charge in acidic environments), the exposure of hydrophobic residues is relatively low, resulting in weak molecular aggregation and good solubility.

In environments near or above the isoelectric point, the surface charge of molecules decreases, hydrophobic interactions are enhanced, and large aggregates are easily formed, which may lead to reduced solubility.

This aggregation state is not completely meaningless—moderate molecular aggregation can enhance nisin’s affinity for cell membranes, while excessive aggregation may reduce the efficiency of its binding to targets. Therefore, hydrophobic interactions play an important regulatory role in balancing Nisin’s activity.

2. Hydrogen Bonds: Maintaining Molecular Conformation and Aggregate Stability

The amino acid residues of Nisin contain groups capable of forming hydrogen bonds, such as hydroxyl groups (e.g., in serine, threonine), amino groups (e.g., in lysine), and carbonyl groups (C=O in peptide bonds):

Intramolecularly, hydrogen bonds participate in maintaining Nisin’s unique "hairpin-like" secondary structure (fixed by lanthionine cross-links).

Intermolecularly, polar groups of different Nisin molecules can be connected to each other through hydrogen bonds (e.g., the peptide bond carbonyl of one molecule forms an O-H…O hydrogen bond with the amino acid side-chain hydroxyl of another molecule), further stabilizing the structure of molecular aggregates and reducing the dissociation of aggregates.

In addition, hydrogen bonds can enhance the interaction between Nisin and water molecules, alleviating excessive aggregation caused by hydrophobic interactions to a certain extent and maintaining Nisin’s dispersibility in aqueous solutions.

3. Electrostatic Interactions: Regulating Molecular Solubility and Aggregation Tendency

Nisin molecules contain 2 lysine residues (with positive charges) and 1 aspartic acid residue (with a negative charge), and generally carry a net positive charge under physiological pH (approximately 7.4) or acidic conditions. This surface charge results in weak electrostatic repulsion between Nisin molecules: the positively charged molecular surfaces repel each other, preventing excessive aggregation caused by hydrophobic interactions and thus maintaining the dispersibility of molecules in solutions.

When the environmental pH rises to near or above the isoelectric point, the net charge of molecules decreases, electrostatic repulsion weakens, hydrophobic interactions dominate, and molecules tend to aggregate. In contrast, at low pH values (e.g., in acidic food systems), the positive charge of molecules is enhanced, electrostatic repulsion is significant, and solubility is improved—this is one of the important reasons why nisin exhibits good stability when applied in acidic foods (e.g., yogurt, pickles).

II. Intermolecular Forces Between Nisin and Target Molecules: The Key to Determining Antibacterial Activity

The core of nisin’s antibacterial mechanism lies in its binding to target molecules (mainly lipid Ⅱ, a precursor substance for cell wall synthesis) on the cell membranes of Gram-positive bacteria, thereby destroying the structure and function of cell membranes. This process relies on specific intermolecular forces between nisin and target molecules:

1. Hydrophobic Interactions with Lipid Ⅱ: The Basis for Achieving Targeted Binding

Lipid Ⅱ is an amphiphilic molecule composed of isoprenoid alcohol phosphate (hydrophobic tail) and muramic acid-pentapeptide (hydrophilic head), with its hydrophobic tail embedded in the lipid bilayer of bacterial cell membranes. The hydrophobic regions of nisin molecules (e.g., the segments of amino acid residues 1–12 and 23–34) can tightly bind to the hydrophobic tail of lipid Ⅱ through hydrophobic interactions. This binding has a certain degree of specificity—the spatial conformation of nisin (a rigid structure formed by lanthionine cross-links) enables its hydrophobic regions to accurately match the structure of the hydrophobic tail of lipid Ⅱ, thereby achieving "molecular recognition." This ensures that nisin preferentially binds to lipid Ⅱ on bacterial cell membranes rather than to host cells (host cell membranes do not contain lipid Ⅱ).

2. Hydrogen Bonds and Electrostatic Interactions with Lipid Ⅱ: Enhancing Binding Affinity

In addition to hydrophobic interactions, hydrogen bonds and electrostatic interactions also exist between nisin and lipid Ⅱ, further enhancing the binding strength:

On one hand, polar groups in nisin molecules (e.g., carbonyl groups in peptide bonds, hydroxyl groups in serine) can form hydrogen bonds with groups such as amino groups and hydroxyl groups in the hydrophilic head of lipid Ⅱ (e.g., the hydroxyl group of Thr28 in nisin forms an O-H…O hydrogen bond with the hydroxyl group of muramic acid in lipid Ⅱ), making the binding between the two more stable.

On the other hand, the positively charged lysine residues (Lys12, Lys22) in Nisin molecules can form electrostatic interactions (positive-negative charge attraction) with the negatively charged phosphate groups in the head of lipid Ⅱ. These electrostatic interactions not only enhance the binding affinity between nisin and lipid Ⅱ but also guide nisin molecules to move directionally to regions enriched with lipid Ⅱ (the surface of bacterial cell membranes), improving binding efficiency.

3. Hydrophobic Interactions with Bacterial Cell Membrane Lipids: Promoting Pore Formation

After nisin binds to lipid Ⅱ, the formed complex further inserts into the lipid bilayer of bacterial cell membranes through hydrophobic interactions. The hydrophobic segments of nisin molecules interact with the hydrophobic tails of cell membrane lipids, disrupting the original ordered structure of the cell membrane. At the same time, multiple nisin-lipid Ⅱ complexes aggregate through intermolecular hydrophobic interactions and hydrogen bonds to form transmembrane pores. These pores increase the permeability of the cell membrane, causing small molecules inside the cell (e.g., potassium ions, amino acids) to leak out and harmful substances from the outside to enter the cell, ultimately leading to bacterial lysis and death.

The intermolecular forces of nisin run through the entire process of its own existence state and antibacterial function: hydrophobic interactions, hydrogen bonds, and electrostatic interactions among nisin molecules themselves jointly regulate its aggregation state, solubility, and stability, ensuring that nisin maintains activity in different application environments (e.g., food systems, biological agents). Meanwhile, hydrophobic interactions, hydrogen bonds, and electrostatic interactions between nisin and target molecules (lipid Ⅱ, bacterial cell membrane lipids) are the core mechanisms for nisin to achieve targeted binding, form transmembrane pores, and exert antibacterial activity. In-depth understanding of these intermolecular forces can not only provide a theoretical basis for optimizing nisin’s production process (e.g., improving solubility, reducing aggregation) but also guide its efficient application in fields such as food preservation and pharmaceutical antibacterial (e.g., designing more stable Nisin derivatives).