Non-covalent binding encompasses molecular interactions that take place between molecules or within a single molecule without involving covalent bonds. Non-covalent binding serves an essential function within the chemical and biological sciences. Non-covalent binding depends on multiple weak forces such as hydrogen bonds and van der Waals forces along with ionic bonds and additional types. The hydrogen bond represents a weak association created between hydrogen atoms and electronegative atoms including nitrogen, oxygen, or fluorine. The function of biological macromolecules depends heavily on its ability to maintain their structure and function. The DNA double helix maintains its stability because of hydrogen bonds that connect its base pairs. Van der Waals interactions comprise orientation forces, induction forces, and dispersion forces. They are ubiquitous forces between molecules. The collective impact of numerous weak van der Waals forces results in significant changes to molecular aggregation state and physical properties.
Non-covalent bonds possess lower bond energy than covalent bonds which form strong interactions through electron pair sharing between atoms and exhibit greater stability. Non-covalent binding retains its dynamic nature because it permits reversible interactions.
Molecular recognition processes demonstrate the significant characteristics of non-covalent binding. The specific interaction between enzymes and substrates results from the cooperative action of multiple non-covalent interactions. Enzymes create enzyme-substrate complexes through hydrogen bonds and van der Waals forces as well as other interactions to bind substrate molecules and catalyze chemical reactions. The binding process displays both high specificity and selectivity through interactions that function similarly to how a key matches a lock. Through non-covalent interactions molecules spontaneously organize themselves into structured formations during the self-assembly process. Amphiphilic molecules present in water solutions self-organize into micelles and vesicles alongside various other structures by utilizing hydrophobic interactions together with hydrogen bonds. The utilization of these structures spans across multiple sectors including drug delivery systems and nanotechnology applications.
Non-covalent binding can be classified according to the type of force involved, mainly including hydrophobic interactions, electrostatic interactions, hydrogen bonding, van der Waals forces, and others. Hydrophobic interactions refer to the phenomenon where non-polar molecules aggregate in aqueous solutions to minimize contact with water molecules. This plays a significant role in processes such as membrane formation and protein folding. Electrostatic interactions refer to the interactions between charged molecules or ions, including ionic bonds and ion-dipole interactions. They are important in biological systems, such as ion channels and the charge distribution in proteins.
Dynamic reversibility is an important characteristic of non-covalent binding, which also affects its classification. Based on dynamic reversibility, non-covalent binding can be divided into reversible binding and quasi-reversible binding. Reversible binding means that the binding and dissociation processes between molecules can quickly reach equilibrium under certain conditions, while quasi-reversible binding may require external stimuli to convert between binding and dissociation.
Below is a comparison table of different interaction forces, their energy ranges, and typical application scenarios:
| Interaction Type | Energy Range (kJ/mol) | Typical Application Scenarios |
| Hydrophobic | 10 - 40 | Membrane formation, protein folding |
| Electrostatic | 20 - 100 | Ion channels, protein charge distribution |
| Hydrogen bonding | 10 - 40 | DNA structure stability, enzyme-substrate binding |
| Van der Waals | 0.4 - 40 | Molecular aggregation, physical property effects |
In molecular recognition processes, ligand-receptor binding often relies on the synergistic mechanisms of multiple non-covalent interactions. The binding between ligands and receptors does not occur through a single non-covalent force but involves various forces working together to achieve high specificity and affinity.
Taking antigen-antibody binding as an example, the specific structural domains on the antigen surface interact with the complementary determining regions (CDR) of the antibody. Among these, hydrogen bonds play a fine-tuning role in their binding. Multiple hydrogen bonds are formed between the amino acid residues of the antibody and specific atoms on the antigen, which contribute to the stability of the antigen-antibody complex. At the same time, van der Waals forces are widely present at the binding interface. Due to the presence of instantaneous dipoles between the atoms on the surfaces of the antigen and antibody molecules, van der Waals forces help bring them closer and facilitate their binding. Furthermore, electrostatic interactions cannot be overlooked. If there are regions with opposite charges on the surfaces of the antigen and antibody, electrostatic attraction will enhance the binding force between them. These three non-covalent interactions work synergistically, allowing the antibody to specifically recognize and bind the antigen, thereby ensuring the proper function of the immune system.
The formation of drug-protein complexes also depends on non-covalent synergistic effects. Once a drug molecule enters the body, it binds to specific protein targets. In this process, hydrophobic interactions may be the initial driving force for the drug-protein binding. The hydrophobic part of the drug molecule tends to interact with the hydrophobic pocket of the protein, thereby entering the binding site. Subsequently, hydrogen bonds and electrostatic interactions further stabilize the drug-protein complex. For example, polar groups in certain drug molecules can form hydrogen bonds with amino acid residues in the protein, while charged groups can attract the protein through electrostatic interactions.
In recent years, the impact of single-atom regulation on non-covalent forces has also received increasing attention. Studies have found that the introduction of single atoms in some systems can alter the electronic structure of molecules, thereby affecting the strength and direction of non-covalent interactions. For instance, in certain catalytic systems, single atoms can regulate non-covalent interactions between substrates and catalysts, improving the efficiency and selectivity of catalytic reactions.
(1) Nuclear Magnetic Resonance (NMR)
Principle: NMR technology utilizes the magnetic properties of atomic nuclei and their behavior in a magnetic field to provide molecular structural and dynamic information. For non-covalent complexes, NMR can determine the relative positions and interaction modes of molecules within the complex by detecting nuclear chemical shifts, coupling constants, and other parameters.
Limitations: NMR technology has relatively low sensitivity and requires high-concentration samples. Additionally, for large molecular complexes, spectral analysis may be complex and require specialized knowledge and experience.
(2) Mass Spectrometry (MS)
Principle: Mass spectrometry analyzes the molecular weight and structure of molecules by measuring the mass and relative abundance of sample ions. In non-covalent complex detection, MS can directly measure the molecular weight of the complex, thereby determining its composition and stoichiometry.
Limitations: Mass spectrometry typically requires sample ionization, which may cause dissociation of the non-covalent complex, potentially affecting the accuracy of detection. Moreover, for some unstable complexes, MS may not reliably detect their presence.
(3) Infrared Spectroscopy (IR)
Principle: Infrared spectroscopy analyzes molecular structure and chemical bonds by utilizing the absorption characteristics of molecules to infrared light. For non-covalent complexes, IR can detect changes in bond vibration frequencies caused by non-covalent interactions such as hydrogen bonds and van der Waals forces, providing information about the structure and interactions of the complex.
Limitations: The resolution of infrared spectroscopy is relatively low, and it may not accurately detect weaker non-covalent interactions. Additionally, spectral analysis requires certain experience and expertise.
(4) Surface Plasmon Resonance (SPR)
Principle: Surface plasmon resonance is an optical-based technique that can monitor molecular interactions in real-time. When biomolecules bind to a sensor surface, it causes changes in the surface plasmon resonance signal. By detecting this change, dynamic and thermodynamic information about molecular binding can be obtained.
Advantages: SPR technology offers real-time, label-free, and high-sensitivity monitoring, allowing direct observation of the formation and dissociation of non-covalent complexes.
Limitations: The detection range of SPR is relatively narrow, and it may not be effective for detecting small molecules or low-affinity interactions.
In the biomedical field, the construction of targeted drug delivery systems is an important research direction, with non-covalently functionalized nanocarriers showing significant potential. Carbon nanotubes and polymer nanoparticles, as common nanocarriers, each have distinct non-covalent functionalization design strategies.
For carbon nanotubes, their unique one-dimensional structure and excellent physicochemical properties make them ideal drug carriers. Drug molecules can be loaded onto the surface of carbon nanotubes through non-covalent interactions, such as π-π stacking and hydrophobic interactions. For example, drug molecules with aromatic structures can interact with the carbon nanotube walls via π-π stacking, facilitating effective drug loading. Additionally, to achieve targeted delivery, targeting ligands, such as antibodies or peptides, can be modified onto the surface of carbon nanotubes. These targeting ligands bind to the nanotubes through non-covalent interactions and can specifically recognize receptors on the surface of diseased cells, thereby delivering the drug accurately to the disease site.
Polymer nanoparticles are also commonly used drug carriers. By selecting suitable polymer materials and utilizing non-covalent interactions, such as electrostatic interactions and hydrogen bonding, drugs can be encapsulated and loaded. For example, positively charged polymer nanoparticles can interact with negatively charged drug molecules through electrostatic interactions, forming stable complexes. Additionally, polymer nanoparticles can self-assemble into core-shell structured nanocarriers, encapsulating the drug in the core to improve stability and bioavailability.
Non-covalently functionalized nanocarriers also exhibit good controlled release properties. The release of drugs can be regulated by adjusting the strength of non-covalent interactions and environmental factors. For example, in the tumor microenvironment, changes in pH, temperature, and other factors can weaken non-covalent interactions, causing the drug to be released from the nanocarrier. This controlled release property can improve drug efficacy and reduce toxic side effects on normal tissues.
Taking antibody-drug conjugates (ADCs) as an example, ADCs are drugs in which antibodies are conjugated to cytotoxic drugs through linkers. During the preparation of ADCs, non-covalent binding also plays an important role. Antibodies can specifically recognize antigens on tumor cell surfaces through non-covalent interactions, delivering cytotoxic drugs directly into tumor cells. At the same time, the linker design can utilize non-covalent interactions to control the drug's release. For example, some linkers can undergo hydrolysis or enzymatic cleavage in the tumor microenvironment, releasing the cytotoxic drug to exert therapeutic effects.
In biomolecular engineering, non-covalent binding plays a crucial role in interface regulation, especially in protein-small molecule complexes and cell surface modifications, which have broad applications in biosensors and cell therapy.
In biosensors, non-covalent binding of protein-small molecule complexes can be used for signal recognition and conversion. For example, in biosensors based on antibody-antigen interactions, antibodies act as recognition elements, binding to antigens through non-covalent interactions. When the antigen binds to the antibody, it causes a conformational change in the antibody, which can be converted into detectable electrical or optical signals via physical or chemical transformations. For example, research has used surface plasmon resonance (SPR) technology to detect antibody-antigen complex formation. Experimental data shows that when the antigen concentration is 10 nM, the SPR signal significantly changes, with a detection limit of 1 nM, demonstrating the high sensitivity and specificity of non-covalent binding in biosensor signal recognition.
In cell therapy, cell surface modification utilizes non-covalent binding to regulate the functions and behavior of cells. For example, small molecule ligands can be non-covalently attached to the surface of T cells in tumor immunotherapy. Experimental results show that T cells modified with small molecule ligands exhibit 30% increased cytotoxic activity against tumor cells. This is because the small molecule ligands bind non-covalently to receptors on the tumor cell surface, enhancing the interaction between T cells and tumor cells, thereby activating T cell immunity.
Additionally, protein-small molecule complex non-covalent binding can also regulate intracellular signaling pathways. Some small molecule drugs interact with intracellular protein targets through non-covalent binding, modulating the activity and function of proteins, thereby influencing processes such as cell growth, differentiation, and apoptosis. For example, a small molecule drug binding with a specific protein target has a binding affinity of Kd = 10^-8 M and inhibits the protein's activity through non-covalent binding, thus inhibiting the proliferation of tumor cells. These findings underscore the important value of non-covalent binding in biomolecular engineering interface regulation, providing new insights and methods for the development of biosensors and cell therapy.
In the field of materials science, the dynamic self-assembly of smart materials is an emerging and promising research direction. Among these materials, the non-covalent driving mechanisms in supramolecular polymers and liquid crystal materials are particularly noteworthy.
Supramolecular polymers are polymers formed by monomers connected via non-covalent interactions, such as hydrogen bonding, π-π stacking, and metal coordination. These non-covalent interactions are dynamic and reversible, allowing supramolecular polymers to undergo self-assembly and disassembly under different conditions. For example, in some supramolecular polymer systems, the formation and breakage of hydrogen bonds can be influenced by factors such as temperature and pH. When the temperature increases, hydrogen bonds break, causing the supramolecular polymer to disassemble; when the temperature decreases, hydrogen bonds reform, and the supramolecular polymer reassembles. This dynamic reversibility endows supramolecular polymers with unique properties, such as self-healing and stimulus responsiveness.
Liquid crystal materials are a class of materials with special optical and electrical properties, and their molecular arrangement is characterized by a certain degree of order. Non-covalent interactions play a key role in the self-assembly process of liquid crystal materials. For example, van der Waals forces and π-π stacking interactions can drive liquid crystal molecules to arrange in an ordered manner, forming a liquid crystal phase. Additionally, some liquid crystal materials also exhibit environmental responsiveness, enabling them to respond to external stimuli such as light, heat, and electric fields.
Taking light/thermo-sensitive materials as an example, certain light-sensitive liquid crystal materials undergo molecular structural changes when exposed to light, causing a transformation in the liquid crystal phase. This transformation can lead to changes in the material's optical properties, such as color and transparency. Thermo-sensitive materials, on the other hand, can adjust the state of the liquid crystal phase in response to temperature changes. For instance, a thermo-sensitive supramolecular polymer liquid crystal material shows a high degree of order in the liquid crystal phase at low temperatures. When the temperature increases, the non-covalent interactions weaken, and the material transitions into an unordered liquid state. This environmental responsiveness makes light/thermo-sensitive materials highly applicable in fields like smart displays and sensors.
In the field of energy materials, optimizing the performance of photoelectrode materials is critical for improving energy conversion efficiency. Single-atom coordination and non-covalent hole transport layers play essential roles in enhancing the charge separation efficiency of photoelectrode materials, and their role in water oxidation reactions provides a concrete example to better understand their mechanisms.
In water oxidation reactions, photoelectrode materials are required to convert light energy into chemical energy to split water and generate oxygen. However, the recombination of photo-generated charge carriers is a key issue affecting reaction efficiency. Single-atom coordination can effectively tune the electronic structure of photoelectrode materials, improving the charge separation efficiency of photo-generated carriers. Single atoms possess unique electronic properties and coordination environments, allowing them to form specific chemical bonds with atoms on the surface of photoelectrode materials, thus altering the material's band structure. For example, by introducing a single-atom catalyst into certain photoelectrode materials, the single atom forms coordination bonds with surface atoms, enabling photo-generated electrons to transfer more rapidly to the catalyst surface, participate in the water oxidation reaction, and reduce the recombination of electron-hole pairs.
Non-covalent hole transport layers also play a significant role in photoelectrode materials. The primary function of hole transport layers is to quickly transport photo-generated holes from the photoelectrode material to the reaction interface, promoting the water oxidation reaction. Non-covalent hole transport layers combine with photoelectrode materials via non-covalent interactions (such as π-π stacking and hydrogen bonding), providing good interface compatibility and charge transport properties. For example, conjugated organic small molecules can serve as non-covalent hole transport layer materials. They interact with the surface of photoelectrode materials through π-π stacking, forming continuous charge transport channels. This non-covalent binding not only facilitates efficient hole transport but also avoids damage to the photoelectrode material's structure, maintaining its stability.
In practical water oxidation reactions, the synergistic effect of single-atom coordination and non-covalent hole transport layers can significantly enhance the performance of photoelectrode materials. Studies have shown that after introducing a single-atom catalyst and a non-covalent hole transport layer into a particular photoelectrode material system, the onset potential for the water oxidation reaction decreased by 0.2 V, and the photocurrent density increased by three times. This indicates that single-atom coordination and non-covalent hole transport layers effectively promote the separation and transport of photo-generated carriers, improving the efficiency of photoelectrode materials in water oxidation reactions, and providing new pathways for the performance optimization of energy materials.
Non-covalent complexes face significant dissociation risks in both in vitro and in vivo environments, presenting major challenges to their stability and controllability. In vitro, factors such as temperature, pH, and ionic strength can weaken or disrupt non-covalent interactions, leading to dissociation of the complex. For instance, in some biosensor applications, an increase in environmental temperature can weaken non-covalent forces such as hydrogen bonds and van der Waals interactions, causing the dissociation of protein-small molecule complexes and impairing the sensor's detection performance.
In vivo environments are more complex. Biomolecules such as enzymes and proteins may interact with non-covalent complexes, leading to dissociation. Additionally, physiological conditions, including blood flow rates and osmotic pressure, also affect the stability of the complex. For example, in tumor-targeting therapy, non-covalently functionalized nanocarriers must remain stable in the bloodstream to deliver drugs accurately to tumor sites. However, proteins and cells in the blood may bind nonspecifically to the nanocarriers, shielding the targeted ligands on the carrier's surface and reducing targeting efficiency. The unique properties of the tumor microenvironment, such as low pH and high enzyme activity, can further weaken the non-covalent interactions between the nanocarrier and drug, leading to premature drug release and affecting therapeutic outcomes.
To address the challenge of balancing stability and controllability, molecular engineering modification strategies can be employed. For example, chemical modifications to the surface of nanocarriers can introduce functional groups that enhance non-covalent interactions between the nanocarrier and drug, improving the stability of the complex. Additionally, intelligent nanocarriers can be designed to allow controlled drug release in response to environmental changes. For instance, pH-sensitive nanocarriers can undergo structural changes in the acidic conditions of the tumor microenvironment to release the drug.
Artificial intelligence (AI) predictive models and high-throughput screening technologies have shown great potential in the study of non-covalent interactions. AI models can learn from vast amounts of experimental data to predict the strength and patterns of non-covalent interactions between molecules. For example, machine learning algorithms can model the structure and properties of molecules to predict the energy and direction of non-covalent interactions such as hydrogen bonds and van der Waals forces. This helps in quickly identifying compounds with potential activity in drug development, improving efficiency.
High-throughput screening technology enables the simultaneous testing of large numbers of molecules for non-covalent interactions. With automated experimental equipment and data analysis systems, interaction information can be quickly obtained, providing abundant data support for the study of non-covalent interactions. For example, in drug screening, high-throughput screening can test thousands of compounds to identify those with strong non-covalent interactions with specific targets.
There is increasing demand for innovation in green synthesis pathways. Non-covalent complexes can be synthesized using renewable resources and environmentally friendly methods. For instance, bio-based materials and aqueous-phase synthesis methods can be explored to reduce the use of organic solvents and minimize environmental impact.
The specific technical route is envisioned as follows: First, AI predictive models can be used for virtual screening of a molecular library to identify pairs of molecules with potential non-covalent interactions. Then, high-throughput screening technology can be employed to experimentally validate these pairs, determining the strength and patterns of their interactions. Finally, combining green synthesis pathways, non-covalent complexes with excellent performance can be prepared. Experimental data will be fed back into the AI models to continuously improve their accuracy and reliability.