The interaction between L-Arginine and food proteins

L-Arginine (abbreviated as L-Arg) is an essential basic amino acid in the human body. It not only participates in protein synthesis but is also widely used in food processing as a nutritional fortifier, flavor modifier, or functional ingredient. Food proteins (such as whey protein, soy protein, casein, ovalbumin, etc.) are core components in food systems, undertaking key roles such as nutrient supply and structural support (e.g., gelation, emulsification). The interaction between the two runs through food processing (e.g., heating, fermentation, enzymatic hydrolysis), storage, and digestion processes. Its mechanism is closely related to the nutritional properties, texture quality, and functional stability of food, and is mainly achieved through three pathways: intermolecular forces, chemical modification reactions, and enzymatic reactions.

I. Non-Covalent Interactions Based on Intermolecular Forces

Non-covalent interactions between l-arginine and food proteins do not involve the breaking or formation of chemical bonds; they are mainly maintained by weak intermolecular interactions, regulated by factors such as pH, temperature, and ionic strength of the food system. These interactions significantly affect the solubility, conformational stability, and functional properties (e.g., emulsification, foaming) of proteins.

1. Electrostatic Interactions

The side chain of the l-arginine molecule contains a guanidino group (-C(NH₂)₂⁺), which exhibits strong positive charge under physiological pH (6.0–7.5) and the pH range of most food processing (4.0–8.0). Food proteins (e.g., β-lactoglobulin in whey protein, 11S globulin in soy protein) have negatively charged groups such as carboxyl (-COO⁻) and hydroxyl (-OH) on their surfaces. The two can form electrostatic complexes through "positive-negative charge" attraction. This interaction can significantly change protein solubility: for example, adding l-arginine near the isoelectric point (pI) of a protein (where the protein has a net charge of 0 and tends to aggregate and precipitate), its positive charge can neutralize the negative charge on the protein surface or reduce electrostatic repulsion between protein molecules through "charge shielding," breaking the aggregated state and redispersing the protein, thereby improving solubility. This property makes l-arginine often used as a "protein solubilizer." For instance, in the processing of plant protein beverages (e.g., soy protein beverages), it solves the problem of protein precipitation and stratification during low-temperature storage.

2. Hydrogen Bonding and Hydrophobic Interactions

The hydrogen atoms in the amino group (-NH₂) and guanidino group of l-arginine can form hydrogen bonds with the oxygen atoms (C=O) in protein peptide bonds and hydroxyl groups (-OH) in side chains. This interaction can stabilize the secondary structure of proteins (e.g., α-helix, β-sheet) and reduce conformational denaturation caused by heating or pH fluctuations. At the same time, the hydrophobic groups (e.g., methylene chains) in the l-arginine molecule can bind to the exposed hydrophobic regions inside the protein, and through hydrophobic interactions, inhibit excessive aggregation of protein molecules. For example, in the preparation of whey protein gels, adding an appropriate amount of l-arginine can synergistically regulate the cross-linking degree of proteins through hydrogen bonding and hydrophobic interactions, making the gel network more uniform and improving the elasticity and water-holding capacity of the gel; however, in excess, its highly polar guanidino group may competitively bind water molecules on the protein surface, weakening the interaction between protein and water, and instead deteriorating the gel texture.

II. Covalent Interactions Based on Chemical Modification

Under high-temperature, oxidative, or Maillard reaction conditions in food processing, l-arginine and food proteins undergo covalent bonding. By changing the molecular structure, charge distribution, and functional group properties of proteins, this affects the color, flavor, nutritional safety, and functional activity of food. The most typical reactions are the Maillard reaction and oxidative cross-linking reaction.

1. Covalent Bonding in the Maillard Reaction

The Maillard reaction is a non-enzymatic browning reaction between amino acids and the amino/carbonyl groups of proteins (or carbohydrates) in food processing, widely present in processes such as baking (e.g., bread, biscuits), frying (e.g., meat products), and fermentation. As an amino acid with free amino groups,l-arginine can undergo condensation, rearrangement, and other reactions between its α-amino group, side-chain guanidino group, and the aldehyde groups at the ends of protein peptide chains (or aldehydes produced by carbohydrate decomposition in food), ultimately forming products such as melanoidins, aldehydes, and ketones. This covalent modification has two effects: on one hand, the melanoidins produced by the reaction endow food with a roasted flavor (e.g., the aroma of baked bread) and a brown color, improving sensory quality; on the other hand, excessive reaction (e.g., high-temperature and long-term processing) can destroy essential amino acids in proteins (including l-arginine itself), reducing the protein nutritional value of food, and may even produce potential harmful substances such as acrylamide (the risk is relatively higher when l-arginine reacts with reducing sugars at high temperatures than with other amino acids). In addition, the Maillard reaction can also change the functional properties of proteins—for example, reducing emulsifying activity (due to modification of surface active groups) but possibly enhancing gelation ability (due to increased intermolecular cross-linking).

2. Oxidation-Induced Covalent Cross-Linking

During food storage or processing, if there are reactive oxygen species (e.g., oxygen, hydrogen peroxide) or metal ions (e.g., iron, copper ions) in the system, protein oxidation reactions will be triggered, causing side-chain amino acid residues (e.g., cysteine, tyrosine) to be oxidized into free radicals or active groups. The guanidino group of l-arginine is easily attacked under oxidative conditions, generating imine intermediates, which then combine with free radicals produced by protein oxidation to form intermolecular covalent cross-links. This cross-linking significantly increases the molecular weight of proteins, reduces solubility, and even forms insoluble polymers, affecting food texture: for example, during the storage of meat products (e.g., sausages, cured meat), oxidative cross-linking between l-arginine and myofibrillar proteins leads to hardening of meat and reduced water-holding capacity, affecting taste; however, in the preparation of certain functional foods (e.g., edible films), this cross-linking can enhance the mechanical strength and barrier properties of the film, improving its preservation effect.

III. Biotransformation Interactions Based on Enzymatic Reactions

Natural enzymes (e.g., proteases, nitric oxide synthase) present in food systems or enzyme preparations added during processing mediate the biotransformation of l-arginine and food proteins. This interaction not only affects the digestion and absorption efficiency of proteins but also produces physiologically active functional substances, endowing food with additional health value.

1. Protease-Mediated Co-Hydrolysis and Absorption Synergy

L-arginine can act as a "substrate activator" for proteases (e.g., trypsin, pepsin), promoting the hydrolysis of food proteins. On one hand, its structure is similar to arginine residues in proteins, so it can competitively bind to the active sites of proteases, improving enzymatic catalytic efficiency; on the other hand, its positive charge can stretch protein molecules through electrostatic interactions, exposing more peptide bonds that can be cleaved by enzymes (e.g., peptide bonds on both sides of arginine residues), accelerating the hydrolysis of proteins into small-molecule peptides and free amino acids. This co-hydrolysis can improve the digestion and absorption rate of proteins. For example, in infant formula, co-enzymatic hydrolysis of l-arginine and whey protein can generate easily absorbable small-molecule peptides while increasing the content of free l-arginine, meeting the essential amino acid needs of infants. In addition, the "L-Arg-protein peptide complex" produced by hydrolysis may have synergistic physiological activities, such as enhancing immunity and regulating intestinal flora.

2. Nitric Oxide Synthase (NOS)-Mediated Bioactive Substance Production

In certain fermented foods (e.g., fermented milk, natto) or foods containing viable cells (e.g., probiotics), nitric oxide synthase (NOS) in the system can catalyze the conversion of l-arginine into nitric oxide (NO) and citrulline. Some amino acid residues in food proteins (e.g., cysteine) can act as "carriers" for NO, combining with NO to form S-nitrosoproteins (SNOs). NO is an important signaling molecule with effects such as vasodilation and immune regulation, while S-nitrosoproteins can stabilize NO, extending its retention time in food and action time in the body. For example, in fermented milk, the NOS of lactic acid bacteria can use added l-arginine to generate NO, which combines with whey protein to form S-nitrosowhey protein, endowing fermented milk with the potential function of auxiliary blood pressure regulation; at the same time, the presence of protein can also protect l-arginine from degradation by other enzymes, improving its stability during food processing and storage.

IV. Practical Significance and Regulation of Interactions for Food Applications

The interaction between l-arginine and food proteins is a "double-edged sword." Rational regulation can improve food quality and functionality, while improper regulation may lead to nutrient loss or quality deterioration. Two key aspects should be focused on in food processing:

1. Optimizing Food Quality and Functionality

Using the non-covalent interactions between the two, adding l-arginine can improve the processing properties of proteins such as solubility and emulsibility—for example, solving stratification problems in plant protein beverages and enhancing the foaming stability of meringue in baked foods; using enzymatic co-hydrolysis, high-digestibility protein nutritional preparations (e.g., protein powder, enteral nutrition agents) can be prepared, or functional foods with NO-regulating functions can be developed.

2. Avoiding Negative Effects

Processing conditions (e.g., temperature, time, pH) should be controlled to reduce problems caused by excessive covalent interactions. For example, in high-temperature processing (e.g., frying, baking), the Maillard reaction between l-arginine and proteins is reduced by controlling temperature (avoiding exceeding 180℃) and shortening time, thereby decreasing essential amino acid loss and harmful substance production; in meat product storage, oxidative cross-linking is inhibited by adding antioxidants (e.g., vitamin C, tea polyphenols) to prevent meat hardening.

The interaction between l-arginine and food proteins is essentially a synergy of "forces and reactions" at the molecular level. Its mechanism changes dynamically with food processing conditions and directly determines the nutritional value, texture, flavor, and functional properties of food. In-depth understanding of this interaction can provide key theoretical support for food formula optimization (e.g., functional amino acid fortification), processing technology improvement (e.g., reducing nutrient loss), and the development of new functional foods (e.g., high-activity protein-based foods).