Simulation method of chitosan oligosaccharide nanofiltration process

Abstract

The invention relates to a method for simulating a chitosan oligosaccharide nanofiltration process. The method comprises the following steps: S1, constructing an initial structure model of chitosan oligosaccharide, water and hydroxyl; s2, constructing a three-dimensional structure initial model of the nanofiltration membrane; s3, constructing a chitosan oligosaccharide-nanofiltration membrane-water ternary model according to the initial structure model of chitosan oligosaccharide, water and hydroxyl obtained in the step S1 and the three-dimensional structure initial model of the nanofiltration membrane obtained in the step S2; s4, carrying out molecular dynamics simulation on the chitosan oligosaccharide-nanofiltration membrane-water ternary model obtained in S3; and S5, performing result analysis on the model subjected to molecular dynamics simulation in the step S4 from the aspects of diffusion trajectory, diffusion coefficient, interaction energy, RDF and hydrogen bonds. According to the method, the nanofiltration process of the chitosan oligosaccharide is simulated through a computer, the interaction condition among the chitosan oligosaccharide, a membrane material, water and other components can be explored from the microscopic level, the influence factors of the nanofiltration process of the chitosan oligosaccharide are determined, the diffusion mechanism of the nanofiltration process of the chitosan oligosaccharide is revealed, and the method has theoretical value.

Description

Technical Field

[0001] This invention relates to the field of computer simulation technology, and more specifically to a simulation method for a chitosan oligosaccharide nanofiltration process. Background Technology

[0002] Chitosan oligosaccharides are oligosaccharides with a degree of polymerization of 2-10, composed of glucosamine or a small amount of N-acetylglucosamine linked by β-1,4-glycosidic bonds, and their salts. They possess various physiological activities, including antioxidant, antibacterial, antitumor, and immunomodulatory effects, and are widely used in food, medicine, and agriculture. Chitosan oligosaccharides with different degrees of polymerization exhibit different biological activities. Therefore, the precise separation of chitosan oligosaccharides with different degrees of polymerization is particularly important for their application research.

[0003] Membrane separation and chromatographic separation are currently the main methods for separating and purifying chitosan oligosaccharides. Compared with chromatographic separation, membrane separation technology has advantages such as low energy consumption, convenient operation, good continuity, and scalability for production, making it the most promising method for industrial-scale separation and purification of chitosan oligosaccharides. Currently, nanofiltration is the main membrane separation technology used for chitosan oligosaccharide separation. Previous studies have shown that the chitosan oligosaccharides obtained by nanofiltration are still mixtures within a certain degree of polymerization range, and cannot yield chitosan oligosaccharide products with a single degree of polymerization. This is mainly because the molecular size and physicochemical properties of chitosan oligosaccharides with different degrees of polymerization are relatively similar, making it difficult for existing nanofiltration technology to achieve precise separation of chitosan oligosaccharides with a single degree of polymerization.

[0004] Membrane materials are the core of nanofiltration technology. The development of novel membrane materials is crucial for achieving precise separation of chitosan oligosaccharides with a single degree of polymerization. Currently, various membrane materials are available for the preparation of novel nanofiltration membranes and can be further modified through experimental methods such as surface grafting and matrix modification.

[0005] However, experimental studies have limitations; crucial parameters affecting the nanofiltration process of chitosan oligosaccharides, such as the interaction between the membrane material and the oligosaccharide, are difficult to obtain experimentally. Molecular simulations using computational software can overcome these limitations. While numerous studies have applied molecular simulations to nanofiltration, there are currently no reports on using molecular simulations to study the nanofiltration separation process of chitosan oligosaccharides. Therefore, it is essential to develop a simulation method for the chitosan oligosaccharide nanofiltration process using computational software. Summary of the Invention

[0006] To address the aforementioned challenges, this invention provides a simulation method for the chitosan oligosaccharide nanofiltration process based on Materials Studio. This method explores the interaction mechanism between chitosan oligosaccharides, membrane materials, and water molecules at the molecular level, elucidates the diffusion mechanism of chitosan oligosaccharides in the nanofiltration process, and provides theoretical guidance for the precise separation of chitosan oligosaccharides.

[0007] The objective of this invention can be achieved through the following technical solutions: This invention provides a simulation method for the chitosan oligosaccharide nanofiltration process based on Materials Studio, comprising the following steps: S1. Construct initial structural models of chitosan oligosaccharide, water, and hydroxide ions; S2. Construct the initial three-dimensional structural model of the nanofiltration membrane; S3. Based on the initial structural models of chitosan oligosaccharide, water, and hydroxide ions obtained in S1 and the initial three-dimensional structural model of nanofiltration membrane obtained in S2, construct a ternary model of chitosan oligosaccharide-nanofiltration membrane-water. S4. Molecular dynamics simulations were performed on the chitosan oligosaccharide-nanofiltration membrane-water ternary model obtained in S3. S5. The results of the molecular dynamics simulations following S4 are analyzed from the aspects of diffusion trajectory, diffusion coefficient, interaction energy, RDF, and hydrogen bonds.

[0008] Furthermore, S1 includes the following steps: S11. In Materials Studio, construct the initial structural model of chitosan oligosaccharide, water, and hydroxide ions; S12. In Materials Studio, use the Forcite module to optimize the initial structural models of chitosan oligosaccharide, water, and hydroxide obtained in S11.

[0009] Furthermore, in S11, the initial structural model of chitosan oligosaccharide, water, and hydroxide ions is constructed to give COMPASSII a force field and corresponding charge; Furthermore, in S12, the charge distribution during structural optimization is selected using current, and both the electrostatic and van der Waals terms are obtained using an atom-based summation method.

[0010] Furthermore, S2 includes the following steps: S21. In Materials Studio, construct the monomer and polymer chain model of the nanofiltration membrane; S22. In Materials Studio, build the monomer and polymer chain model of the nanofiltration membrane based on S21, and use the Amorphous Cell module to build the initial three-dimensional structural model of the nanofiltration membrane. S23. In Materials Studio, use the Forcite module to perform geometric optimization on the three-dimensional structural model of the nanofiltration membrane obtained in S22. S24. In Materials Studio, perform annealing and molecular dynamics simulations on the geometry-optimized model from S23 to obtain a more stable conformation. Furthermore, in S21, the constructed nanofiltration membrane monomer model is given a force field by COMPASSII, while the charge is set to 0; Furthermore, in S21, the head and tail atoms of the monomer are selected when constructing the polymer; Furthermore, in S23, the COMPASSII force field and Smart algorithm are used, and the maximum number of iterations is set to 5000.

[0011] Furthermore, in S24, the annealing process is set to 5 annealing cycles, the kinetic time for each temperature gradient is set to 100 ps, ​​the ensemble is set to NPT, the pressure is set to 0.0001 GPa, and geometry optimization is checked.

[0012] Furthermore, in S24, the molecular dynamics simulation was performed at a temperature of 298 K, with an ensemble of NVT and a total duration of 2000 ps.

[0013] Furthermore, S3 includes the following steps: S31. In Materials Studio, use the Amorphous Cell module to construct a ternary model of "chitosan oligosaccharide-nanofiltration membrane-water".

[0014] Furthermore, in S31, the number of output configurations is set to 10, and the configuration with the lowest energy is selected for subsequent analysis.

[0015] Furthermore, S4 includes the following steps: S41. In Materials Studio, fix the chitosan oligosaccharide and nanofiltration membrane, and use the software's Forcite module to optimize the structure of the chitosan oligosaccharide-nanofiltration membrane-water ternary model. S42. In Materials Studio, use the software's Forcite module to perform molecular dynamics simulations on the optimized configuration obtained in S41. S43. In Materials Studio, remove the fixation of chitosan oligosaccharide and re-optimize the geometry of the final structure obtained in S42. S44. In Materials Studio, perform molecular dynamics simulations on the geometry-optimized structure from S43.

[0016] Furthermore, in S41, both chitosan oligosaccharide and nanofiltration membrane in the chitosan oligosaccharide-nanofiltration membrane-water ternary model are selected, and the "Fix Cartesian position" is checked. Furthermore, in S42, the ensemble for the molecular dynamics simulation is NVT, with a total time of 20 ps; Furthermore, in S43, select chitosan oligosaccharide under the chitosan-nanofiltration membrane-water ternary model, and uncheck FixCartesian position; Furthermore, in S44, the ensemble for the molecular dynamics simulation is NVT, with a total time of 1500 ps.

[0017] Furthermore, in S5, the diffusion trajectory to be investigated requires the selection of the centroid of chitosan oligosaccharide, and then the three-dimensional coordinates of the centroid of chitosan oligosaccharide during the diffusion process in the nanofiltration membrane are extracted using a Perl script to obtain the diffusion trajectory. Furthermore, in S5, the analysis function of the Forcite module is used to obtain the MSD data of chitosan oligosaccharide in the ternary system. The diffusion coefficient can be obtained by combining the EINSTEIN formula. When analyzing, it is necessary to set the object to be analyzed as chitosan oligosaccharide with the corresponding degree of polymerization and select the trajectory file of the last 1000 ps for analysis. Furthermore, in S5, Perl scripts are used to calculate the interaction energy, which includes the total interaction energy, electrostatic interaction energy, and van der Waals interaction energy. Furthermore, in S5, the analysis function of the Forcite module can be used to obtain the RDF diagrams of chitosan oligosaccharide-water and chitosan oligosaccharide-membrane material in the ternary system, and the trajectory file of the last 1000 ps can be selected for analysis. Furthermore, in S5, a Perl script is used to calculate hydrogen bonds between substances. The calculated hydrogen bond data includes the number of hydrogen bonds, the length of hydrogen bonds, and the angle of hydrogen bonds. During the calculation, the angle of hydrogen bonds is specified to be 120°-180°, and the length of hydrogen bonds does not exceed the cutoff radius given in the RDF plot.

[0018] Compared with existing technologies, this invention provides a simulation method for the nanofiltration process of chitosan oligosaccharides based on Materials Studio. By simulating the nanofiltration process of chitosan oligosaccharides using computer simulation, the interaction between chitosan oligosaccharides and membrane materials, water and other components can be explored at the microscopic level, the influencing factors of the chitosan oligosaccharide nanofiltration process can be identified, and the diffusion mechanism of the chitosan oligosaccharide nanofiltration process can be revealed, which has theoretical value. Attached Figure Description

[0019] Figure 1 This is a flowchart of the chitosan oligosaccharide nanofiltration process simulation method based on Materials Studio according to the present invention.

[0020] Figure 2 This is a 3D model diagram of PPTA after geometric optimization according to an embodiment of the present invention.

[0021] Figure 3 This is a diagram of the geometrically optimized "chitosan oligosaccharide-PPTA-water" ternary model according to an embodiment of the present invention.

[0022] Figure 4 This is a diffusion trajectory diagram of chitosan oligosaccharide (chitosan disaccharide as an example) in a ternary system according to an embodiment of the present invention.

[0023] Figure 5This is an energy diagram of the interaction between chitosan oligosaccharide and water in the ternary system of this invention.

[0024] Figure 6 This is an interaction energy diagram of chitosan oligosaccharide and PPTA in the ternary system of this invention.

[0025] Figure 7 This is an RDF diagram of the corresponding atoms of chitosan oligosaccharide (chitobiose as an example) and water in the ternary system of this invention.

[0026] Figure 8 This is an RDF diagram of the corresponding atoms of chitosan oligosaccharide (chitobiose as an example) and PPTA in the ternary system of this invention. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0028] Example refer to Figure 1 This embodiment takes chitosan oligosaccharides and PPTA with a degree of polymerization of 2-10 as the research objects, and provides a method for simulating the nanofiltration process of chitosan oligosaccharides based on Materials Studio, which specifically includes the following steps: (1) Model construction of chitosan oligosaccharide, water molecules and hydroxide ions: Based on the structures of the corresponding degree of polymerization chitosan oligosaccharide, water molecules, and hydroxide ions, draw a basic molecular model. Click the Modify button on the toolbar, select Charges, and change the total charge to the corresponding charge to assign it a charge. Click the energy option in the Forcite module task card, set the force field to COMPASSII, click the More button on the right, and then click the Calculate option to the right of the force field and charge to distribute the force field and charge.

[0029] (2) Perform geometric optimization on the completed model: In the Forcite module, geometric optimization is performed on chitosan oligosaccharide, water molecule and hydroxide ion models at various degrees of polymerization. The algorithm is Smart, the maximum number of iterations is changed to 5000, the charge distribution is selected as Use current, and the electrostatic and van der Waals terms are both Atom-based summation methods. Click Run to obtain the corresponding optimized configuration.

[0030] (3) Construct monomer and polymer chain models for PPTA; Based on the polymer monomer structure, draw the PPTA monomer model. Then use the Homopolymer tool in the Build tab to construct the polymer chain, selecting the head and tail atoms and setting the number of repeating units to 35.

[0031] (4) Construct the initial three-dimensional structural model of PPTA: The PPTA spatial structure was constructed using the Amorphous Cell module. Since PPTA contains benzene rings, its initial density was set to 0.1 g / cm³ to prevent coupling or puncture of the benzene rings. 3 The target density was set at 1.33 g / cm³. 3 To ensure it is a nanofiltration membrane structure, 10 configurations with different energies were constructed at 298 K, and the configuration with the lowest energy was selected for subsequent structural optimization.

[0032] (5) Perform geometric optimization on the 3D structural model of PPTA: The Forcite module was used to perform geometric optimization on the 3D structure to achieve energy convergence. The algorithm was set to Smart, the maximum number of iterations was changed to 5000, the charge distribution was set to Use current, the electrostatic term was performed using the Ewald summation method, and the van der Waals term was performed using the Atom-based summation method.

[0033] (6) Annealing and molecular dynamics simulations were performed on the obtained PPTA structural model: Annealing is performed in the Forcite module, with 5 annealing cycles, 100 ps for each temperature gradient kinetic time, NPT ensemble, and 0.0001 GPa pressure. Geometric optimization is also enabled. Finally, a kinetic simulation of 2000 ps is performed in the Forcite module at 298 K with the NVT ensemble to stabilize the model's energy, thus completing the establishment of the PPTA 3D model. Figure 2 As shown.

[0034] (7) Construct a ternary model of "chitosan oligosaccharide-PPTA-water": In the Amorphous Cell module, select the Packing option. Add one chitosan oligosaccharide, 900 water molecules, and an appropriate amount of hydroxide ions to balance the charge in the constructed PPTA model (two hydroxide ions for chitobiose, and so on for other sugars with different degrees of polymerization). Set the system density according to Table 1. Then, output 10 frames of configurations under a force field of 298 K and COMPASS II, and select the configuration with the lowest energy for further processing.

[0035] Table 1 Density settings of the "chitosan oligosaccharide-PPTA-water" ternary system (8) Fix chitosan oligosaccharide and PPTA, and optimize the structure of the ternary model: Select chitosan oligosaccharide and PPTA, and check "Fix Cartesian position" in the Constraints option. Then, perform structure optimization in the Forcite module with a maximum of 5000 iterations. The optimization algorithm is Smart, the charge assignment is Forcefield assigned, the electrostatic term is the Ewald summation method, and the van der Waals term is the Atom-based summation method. The optimized model is as follows. Figure 3 As shown.

[0036] (9) Perform molecular dynamics simulations on the obtained optimized configuration: A kinetic simulation of 20 ps was performed under the NVT ensemble to adjust water molecules and hydroxide ions to the potential field of the system.

[0037] (10) Remove the fixation of chitosan oligosaccharide and perform geometric optimization on the final structure obtained in the previous step; Select chitosan oligosaccharide, and uncheck "Fix Cartesian position" in the Constraints option. Then, in the Forcite module, perform geometric optimization using the Smart algorithm with a maximum of 5000 iterations. Select Forcefield assigned for charge assignment, Ewald summation method for electrostatics, and Atom-based summation method for van der Waals terms.

[0038] (11) Perform molecular dynamics simulations on the geometry-optimized structure from the previous step: A kinetic simulation of 1500 ps was performed under the NVT ensemble, with the first 500 ps used for the equilibrium system and the last 1000 ps used for analyzing the system properties.

[0039] (12) Analyze the results of the structural files obtained from the dynamic simulation: To investigate the diffusion trajectory of chitosan oligosaccharide, it is necessary to select the centroid of chitosan oligosaccharide, and then use a Perl script to extract the three-dimensional coordinates of the centroid of chitosan oligosaccharide during the diffusion process in the nanofiltration membrane, so as to obtain the diffusion trajectory. Figure 4 This is a diffusion trajectory diagram of chitosan oligosaccharides, using chitobiose as an example.

[0040] Depend on Figure 4 It can be seen that the centroid trajectory points of chitosan oligosaccharides are highly concentrated in a narrow space, exhibiting a spherical aggregate state, which shows extremely strong confinement. This is because the PPTA structure is highly rigid and the polymer chains are tightly packed.

[0041] Using the analysis function of the Forcite module, the analysis object was selected as chitosan oligosaccharide, and the analysis trajectory range was the last 1000 ps. The MSD data of chitosan oligosaccharide in the ternary system was then obtained. The diffusion coefficient can be obtained by combining the EINSTEIN formula, as shown in Table 2.

[0042] Table 2. Diffusion coefficients of chitosan oligosaccharides in PPTA under the ternary system. It can be observed that as the degree of polymerization of chitosan oligosaccharides increases, its diffusion coefficient gradually decreases, and the rate of decrease gradually diminishes, indicating that the diffusion coefficient is affected by the molecular size of chitosan oligosaccharides and the interactions between substances.

[0043] The interaction energy between matter can be calculated using Perl scripts. The calculated interaction energy includes the total interaction energy, electrostatic interaction energy, and van der Waals interaction energy. Figure 5 This is a diagram showing the interaction energy between chitosan oligosaccharides and water in a ternary system. Figure 6 This is a diagram showing the interaction energy between chitosan oligosaccharide and PPTA in a ternary system.

[0044] It can be seen that in the ternary system, the absolute values ​​of the total interaction energy, electrostatic interaction energy, and van der Waals interaction energy between chitosan oligosaccharide and water, and between chitosan oligosaccharide and PPTA, increase with the increase of the degree of polymerization, indicating that the molecular size of chitosan oligosaccharide is an important factor affecting the interaction between chitosan oligosaccharide and water, and between chitosan oligosaccharide and PPTA. Further analysis of the contributions of electrostatic interaction energy and van der Waals interaction energy reveals that the absolute value of electrostatic interaction energy is much larger than that of van der Waals interaction energy, indicating that electrostatic interaction is the main source of interaction between chitosan oligosaccharide and water, and between chitosan oligosaccharide and PPTA.

[0045] By using the analysis function of the Forcite module to select the corresponding atom pairs and specifying the analysis range as the last 1000 ps, ​​the RDF diagrams of chitosan oligosaccharide-water and chitosan oligosaccharide-membrane materials in the ternary system can be obtained. Figure 7 This is an RDF diagram of chitosan oligosaccharide and water atoms in a ternary system, using chitobiose as an example. Figure 8 This is the RDF diagram of the corresponding atoms of chitosan oligosaccharide and PPTA in the ternary system.

[0046] like Figure 7As shown, the RDF curve of the interaction between the hydroxyl hydrogen atom of chitosan oligosaccharide and the oxygen atom in water exhibits a significant main peak at 1.69–1.73 Å, indicating that this is the most important hydrogen bonding site between chitosan oligosaccharide and water molecules. The main peak of the interaction between the amino hydrogen atom of chitosan oligosaccharide and the oxygen atom in water appears between 1.81–1.85 Å, with a relatively high peak value, indicating a similarly strong hydrogen bonding interaction at this location. In contrast, the main peak of the RDF curve of the interaction between the hydroxyl oxygen atom of chitosan oligosaccharide and the hydrogen atom in water appears around 1.77–1.95 Å, with a relatively low peak value and a broader peak shape, indicating a weaker interaction and limited hydrogen bond formation ability in this combination. Therefore, the hydrogen bonds between the ternary system chitosan oligosaccharide and water are mainly formed between the hydroxyl hydrogen atoms and amino hydrogen atoms of chitosan oligosaccharide and the oxygen atoms in water. Figure 8 It can be observed that the main peak of the hydroxyl hydrogen atom of chitosan oligosaccharide and the oxygen atom of PPTA appears at 1.77 Å, while the peak position of the amino hydrogen atom of chitosan oligosaccharide and the oxygen atom of PPTA is 1.95 Å. Both are within the typical hydrogen bonding range, indicating a strong electrostatic attraction between the oxygen atom of PPTA and the hydrogen-donating atom of chitosan oligosaccharide. This is the main site for hydrogen bonding between chitosan oligosaccharide and the PPTA membrane. The cutoff radius can then be determined based on the peak and valley positions of the atomic pairs within the hydrogen bonding range, and the hydrogen bonds between substances in the ternary system can then be calculated.

[0047] Hydrogen bonds between substances can be calculated using Perl scripts. The calculated hydrogen bond data includes the number of hydrogen bonds, the bond length, and the bond angle. During the calculation, the hydrogen bond angle is specified to be 120°-180°, and the hydrogen bond length does not exceed the cutoff radius given in the RDF plot. The calculation results are shown in Tables 3 and 4.

[0048] Table 3. Hydrogen bond data between chitosan oligosaccharides and water in the PPTA ternary system. As shown in Table 3, the number of hydrogen bonds increases significantly with the increase of the degree of polymerization of chitosan oligosaccharides, indicating that the hydrogen bonds between chitosan oligosaccharides and water molecules in the ternary system are mainly related to the molecular size of chitosan oligosaccharides. The hydrogen bond length and hydrogen bond angle hardly change with the increase of the degree of polymerization of chitosan oligosaccharides, indicating that the formed hydrogen bonds are relatively stable.

[0049] Table 4. Hydrogen bond data between chitosan oligosaccharide and PPTA in the ternary system. As shown in Table 4, the number of hydrogen bonds between chitosan oligosaccharide and PPTA in the ternary system tends to increase with the degree of polymerization of chitosan oligosaccharide, while the trends of hydrogen bond length and hydrogen bond angle vary with the degree of polymerization. Furthermore, it can be observed that the number of hydrogen bonds formed between chitosan oligosaccharide and PPTA is greater than 2, indicating that the electrostatic interaction between chitosan oligosaccharide and PPTA is further enhanced by the formation of hydrogen bonds.

[0050] Based on the above results, it can be concluded that the main influencing factor on the diffusion of chitosan oligosaccharides in PPTA under the ternary system is the molecular size of the chitosan oligosaccharides. Simultaneously, the interactions between chitosan oligosaccharides and water, and between chitosan oligosaccharides and PPTA, also affect the diffusion of chitosan oligosaccharides. The main interactions between chitosan oligosaccharides and water, and between chitosan oligosaccharides and PPTA, are electrostatic interactions, and the formed hydrogen bonds further enhance these electrostatic interactions. This conclusion also demonstrates the feasibility of this invention in exploring the field of chitosan oligosaccharide nanofiltration at the microscopic level.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for simulating the chitosan oligosaccharide nanofiltration process, characterized in that, Includes the following steps: S1. Construct initial structural models of chitosan oligosaccharide, water, and hydroxide ions; S2. Construct the initial three-dimensional structural model of the nanofiltration membrane; S3. Based on the initial structural models of chitosan oligosaccharide, water, and hydroxide ions obtained in S1 and the initial three-dimensional structural model of nanofiltration membrane obtained in S2, construct a ternary model of chitosan oligosaccharide-nanofiltration membrane-water. S4. Molecular dynamics simulations were performed on the chitosan oligosaccharide-nanofiltration membrane-water ternary model obtained in S3. S5. The results of the molecular dynamics simulations following S4 are analyzed from the aspects of diffusion trajectory, diffusion coefficient, interaction energy, RDF, and hydrogen bonds.

2. The method for simulating the chitosan oligosaccharide nanofiltration process according to claim 1, characterized in that, S1 includes the following steps: S11. In Materials Studio, construct the initial structural model of chitosan oligosaccharide, water, and hydroxide ions; S12. In Materials Studio, use the Forcite module to optimize the initial structural models of chitosan oligosaccharide, water, and hydroxide obtained in S11.

3. The method for simulating the chitosan oligosaccharide nanofiltration process according to claim 2, characterized in that, In S11, the initial structural model of chitosan oligosaccharide, water, and hydroxide ions is constructed to give COMPASSII a force field and corresponding charge. In S12, the charge distribution is selected using current during structural optimization, and both the electrostatic and van der Waals terms are obtained using an atom-based summation method.

4. The method for simulating the chitosan oligosaccharide nanofiltration process according to claim 1, characterized in that, S2 includes the following steps: S21. In Materials Studio, construct the monomer and polymer chain model of the nanofiltration membrane; S22. In Materials Studio, build the monomer and polymer chain model of the nanofiltration membrane based on S21, and use the Amorphous Cell module to build the initial three-dimensional structural model of the nanofiltration membrane. S23. In Materials Studio, use the Forcite module to perform geometric optimization on the three-dimensional structural model of the nanofiltration membrane obtained in S22. S24. In Materials Studio, perform annealing and molecular dynamics simulations on the geometry-optimized model from S23 to obtain a more stable conformation.

5. The method for simulating the chitosan oligosaccharide nanofiltration process according to claim 4, characterized in that, In S21, the constructed nanofiltration membrane monomer model is given a force field by COMPASSII, while the charge is set to 0. In S21, the head and tail atoms of the monomer are selected when constructing the polymer; In S23, COMPASSII force field and Smart algorithm are used, and the maximum number of iterations is set to 5000. In S24, the annealing process is set to 5 annealing cycles, the kinetic time for each temperature gradient is set to 100 ps, ​​the ensemble is set to NPT, the pressure is set to 0.0001 GPa, and geometry optimization is checked. In S24, the molecular dynamics simulation was performed at a temperature of 298 K, with an ensemble of NVT and a total duration of 2000 ps.

6. The method for simulating the chitosan oligosaccharide nanofiltration process according to claim 1, characterized in that, S3 includes the following steps: S31. In Materials Studio, use the Amorphous Cell module to construct a chitosan oligosaccharide-nanofiltration membrane-water ternary model.

7. The method for simulating a chitosan oligosaccharide nanofiltration process according to claim 6, characterized in that, In S31, the number of output configurations is set to 10, and the configuration with the lowest energy is selected for subsequent analysis.

8. The method for simulating the chitosan oligosaccharide nanofiltration process according to claim 1, characterized in that, S4 includes the following steps: S41. In Materials Studio, fix the chitosan oligosaccharide and nanofiltration membrane, and use the software's Forcite module to optimize the structure of the chitosan oligosaccharide-nanofiltration membrane-water ternary model. S42. In Materials Studio, use the software's Forcite module to perform molecular dynamics simulations on the optimized configuration obtained in S41. S43. In Materials Studio, remove the fixation of chitosan oligosaccharide and re-optimize the geometry of the final structure obtained in S42. S44. In Materials Studio, perform molecular dynamics simulations on the geometry-optimized structure from S43.

9. The method for simulating a chitosan oligosaccharide nanofiltration process according to claim 8, characterized in that, In S41, simultaneously select both chitosan oligosaccharide and nanofiltration membrane in the chitosan oligosaccharide-nanofiltration membrane-water ternary model, and check the FixCartesian position; In S42, the molecular dynamics simulation ensemble is NVT, with a total time of 20 ps; In S43, select chitosan oligosaccharide under the chitosan-nanofiltration membrane-water ternary model, and uncheck Fix Cartesianposition; In S44, the molecular dynamics simulation ensemble is NVT, with a total time of 1500 ps.

10. The method for simulating a chitosan oligosaccharide nanofiltration process according to claim 1, characterized in that, In S5, the diffusion trajectory to be investigated requires selecting the centroid of chitosan oligosaccharide. Then, the three-dimensional coordinates of the centroid of chitosan oligosaccharide during the diffusion process within the nanofiltration membrane are extracted using a Perl script to obtain the diffusion trajectory. In S5, the MSD data of chitosan oligosaccharides in the ternary system are obtained by using the analysis function of the Forcite module. The diffusion coefficient can be obtained by combining the EINSTEIN formula. When analyzing, it is necessary to set the object to be analyzed as chitosan oligosaccharides with the corresponding degree of polymerization and select the trajectory file of the last 1000 ps for analysis. In S5, Perl scripts are used to calculate interaction energies, including total interaction energy, electrostatic interaction energy, and van der Waals interaction energy. In S5, the analysis function of the Forcite module can be used to obtain the RDF diagrams of chitosan oligosaccharide-water and chitosan oligosaccharide-membrane material in the ternary system. The trajectory file with the last 1000 ps is selected for analysis. In S5, a Perl script is used to calculate hydrogen bonds between substances. The calculated hydrogen bond data includes the number of hydrogen bonds, the length of hydrogen bonds, and the angle of hydrogen bonds. During the calculation, the angle of hydrogen bonds is specified to be 120°-180°, and the length of hydrogen bonds does not exceed the cutoff radius given in the RDF plot.

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