Computational Design for The Development of Monomer Selectivity to α-Mangostin in Molecularly Imprinted Polymer

Winasih Rachmawati^1,2, Aliya Nur Hasanah¹_, Fauzan Zein Muttaqin², Muchtaridi Muchtaridi¹*

¹Department of Pharmaceutical Analysis and Medicinal Chemistry, Faculty of Pharmacy,

Universitas Padjadjaran, Jatinangor, Indonesia.

²Department of Pharmaceutical Analysis and Medicinal Chemistry, Faculty of Pharmacy,

Universitas Bhakti Kencana, Bandung, Indonesia.

*Corresponding Author E-mail: winasih.rachmawati@bku.ac.id, aliya.n.hasanah@unpad.ac.id, fauzanzein@bku.ac.id, muchtaridi@unpad.ac.id

ABSTRACT:

α-mangostin is the largest content in Garcinia mangostana rind, which has a wide range of biological activities and pharmacological properties. The extraction process to separate α-mangostin from complex matrices requires selectivity. A novel method of molecularly imprinted polymer (MIP) has characterization high selectivity, high stability, and low cost. MIP uses as a selective sorbent with adsorption method that α-mangostin has the higher binding capacity and specific recognition with MIP. The computational approach was developed to study monomer selectivity towards α-mangostin as a template for rational MIP design. The purpose of this research is to study molecular interaction between template and monomer and monomer template ratio optimization in computational design to find the best pre-polymerization complex for MIP preparations. The structure of α-mangostin and nine functional monomers was drawn using Marvin Sketch and then optimized by Hyperchem 8.0.10 software. Monomer positions are placed on the template structure in various complex ratios. Each conformation was calculated using a semi-empirical PM3 simulation method to obtain the lowest bond free energy. The results showed that the α-mangostin-methacrylic acid complex with 1:6 molar ratio had the most stable structure, the most hydrogen bonds, and the highest ∆G was -27.5114588 kcal/mol. This study presented a method of selecting numerous functional monomers and determining appropriate monomer ratios with a template to obtain MIP for α-mangostin.

KEYWORDS: α-mangostin, computational, methacrylic acid, molecularly imprinted polymer.

INTRODUCTION:

Garcinia mangostana L. (mangostin) is a tropical fruit that is popular in Asia¹. The active compounds in mangostin pericarp are xanthones known as α-mangostin². α-mangostin is the major constituent in terms of bioactive properties and is used for therapeutic applications as antibacterial³, antioxidant^4,5,    anticancer^6-8, antidyslipidemic⁹, and anti-inflammation¹⁰. It is also used to treat breast cancer^11,12. The separation of α-mangostin can be done through organic solvent extraction¹³, supercritical carbon dioxide-mediated hydrothermal extraction¹⁴, microwave-assisted  extraction ¹⁵, and ultrasonic extraction¹⁶. Liquid extraction is commonly used to isolate AM from Garcinia mangostana. However, these methods has poor selectivity. Molecularly imprinted polymer (MIP) is a technique that can overcome this problem, because it produces a sorbent material that can selectively bind specific target compounds in matrices selectively¹⁷.

Fig. 1: 2D Structure of α-mangostin (AM)

Molecularly imprinted polymers (MIP) are synthetic macromolecular materials¹⁷. This performance depends on many different parameters such as functional monomer ratio, crosslinker, temperature, and type of porogenic solvent. It provides predisposed polymer nanostructure materials by forming a specific cavity similar to the shape of the template¹⁸. The ratio between template and functional monomers is an important factor to consider when developing an effective MIP with good affinity¹⁷. During the pre-polymerization stage, the electrostatic force will induce the maximum attractive force between the functional group of the functional monomer and the template. The functional monomer allows interaction with the functional group of the target molecule to form a special binding site through covalent or non-covalent interaction. Non-covalent interaction mechanisms include: hydrophobic interactions, hydrogen bonds, van der Waals forces, and electrostatic interactions¹⁹. After polymerization and removal of the template molecule, a permanent memory of the imprinted species is formed, allowing the resulting polymer to selectively recombine imprinted molecules of closely related compounds¹⁸.

The selection of the best printing conditions is based on trial and error by experimental practices through the variation of functional monomers with template in solvents. A reasonable MIP design requires better methods and an understanding of the basic mechanics of the printing process. It determines the intermolecular interactions developed by MIP by analyzing the activity at the molecular level in the pre-polymerized mixture. Computational methods used to study electronic structures are becoming update in MIP design and evaluation^20,21. The computational technique is needed to reduce cost and time²². This class of methods includes semi-empirical, ab initio, and density functional strategies^23-25.

MATERIALS AND METHODS:

Hardware:

The hardware used is a personal computer with an Intel Core i9-9900K @3.6Ghz (16 CPUs), 64GB RAM.

Software: The software used is as follows: Marvin Sketch 18.28²⁶ and HyperChem 8.0.10 (Hypercube Inc.) ²⁷.

Materials:

α-mangostin (AM) as a template, acrylic acid (AA), methacrylic acid (MAA), methyl methacrylic acid (MMA), itaconic acid (IA), 2-vinyl pyridine (2VP), 4-vinyl pyridine (4VP), acrylamide (AAM), methacrylamide (MAM), and allylamine (AL) are used as functional monomers.

Methods:

Molecular modelling:

2D molecular models were drawn using Marvin Sketch 18.28 whereas 3D molecular models were built using Hyperchem 8.0.10 software. Apply the force field of conformationally stable isomers for geometric optimization and generate electrostatic potentials for setting up the input file in the next run simulations of molecular modeling.

Geometric optimization:

Applying the Restricted HartreeFock (RHF) semi-empirical method based on the theory of molecular orbitals, complex formation capacity of AA, MMA, IA, 2VP, 4VP, AAM, AA, MAM, and AL with α-mangostin as template molecule in the gas phase were optimized. By minimizing the binding energy of the molecule, the PM3 method is used in the ground state to optimize the structure of the template and the functional monomer composite molecule. The conjugate gradient process (PolakRibier) is used for geometric optimization, using a convergence value set at 0.01Kcal/(Å mol). The simulation was performed to determine the molecular structure changes in geometry that can provide a stable configuration during the period with a total energy minimization gradient-average root-squared near zero²⁸.

Energy calculations:

The calculation in the computational method has been carried out by using the Hyperchem software package, which was based on the PM3 semi-empirical method to locate the most stable template-monomer complexes. The interaction energies and ∆G were calculated through the equation (1):²⁹

∆G = G(template–monomer complex) – [G(template) + nG(monomer)] (1)

Where, ∆G = energy after optimization G (template–monomer complex) = binding energy of template monomer complex; G (template) = binding energy of template; nG (monomer) = energy of monomer ratio.

The moderate vibration frequency confirms that the structure is stable and allows the Gibbs free equation to be calculated. This model is useful to explore the influence of H-bond on polymer chain growth during the complexation of stable template polymerization functional monomer

RESULTS AND DISCUSSION:

Molecular simulation studies are very effective in modeling the different variables that affect the performance of the final MIP. In order to accurately study the influence of the main parameters on the common heterogeneity phenomenon in MIP, it becomes feasible to fully reduce the errors and changes of the secondary parameters. The attempts are different, some of which use a virtual monomer library and screened the template to choose the best monomer that interacts with the template.

The nine functional monomers libraries in Fig.2 are the original choices used in the MIP synthesis and contain a range of acidic, basic, and neutral monomers, which can interact with α-mangostin as template through non-covalent interactions.

Fig. 2: The 2D chemical structure monomers used for the computational study

The optimized structure of α-mangostin and nine monomers is carried out using the Hyperchem software. α-mangostin has six hydrogen bond acceptor (O atom) and three position as hydrogen bond donor (H atom at -hydroxyl functional groups). While, in methacrylic acid has two hydrogen bond acceptor and one hydrogen bond donor in the structure. The appropriateness of the interaction according to an automatic assignment of functionalities of interest on the template and the monomer as either hydrogen bond donors or acceptors¹⁹. The wire mesh reflects the enormity of the electrostatic potential (represented here as pink and green spheres). The stronger wire-mesh corresponded to a more negatively charged atom to form a hydrogen bond by sharing or donating the electron charges to the electron acceptor such as oxygen atoms³⁰. The optimized structure of template-monomer complexes was analyzed further to determine the interaction between template and functional monomer²⁴.

Fig. 3: Optimized structure of the template α-mangostin (a) and methacrylic acid (b) showing atomic charge on atoms

The hydrogen (H) of methacrylic acid has a relatively large positive charge (+0.227), while the oxygen (O) of α-mangostin has an electron source by -0.246 (Figure 3). The hydrogen bond getting stronger if the electronegativity of the atom attracted by H has become stronger. Every stable and optimal template monomer structure will produce strong non-covalent interactions.

Selection of Functional Monomer:

Computer studies pre-polymerized functional groups representing functional monomers, which will interact with various forms of template molecules to bind to the template³². The ratio of template – functional monomer is important to be identified before the pre-polymerization step. The ideal positions of the monomer donor/acceptor atoms are plotted on the compounds selected below as "sites-points" whose positions relative to each other and the monomers interact with template are controlled¹⁹. Selected functional monomers that exhibit chemical functions are designed to interact with template molecules through covalent or non-covalent chemistry³⁶. Gibbs free energy that gains the complexes (∆G) is presented in Table 2. It was observed that three monomers, which are methacrylic acid, acrylamide, and methyl methacrylic acid formed the most stable complex with α-mangostin in different ratio. Methacrylic acid was found to possess the strongest affinity for α-mangostin due to its acidic functional group that is able to imprint α-mangostin molecules. The hydrogen from carboxylic groups (-COOH) of methacrylic acid is an excellent hydrogen bond donor that can participate in proton acceptor of α-mangostin (oxygen from C=O or -OH). The ∆G of template-monomer complexes and a typical binding conformation between α-mangostin template and monomers are presented in Table 1. Hydrogen bond interactions are usually classified as strong (>15 kcal/mol), moderate (3-14 kcal/mol), and weak (<3 kcal/mol)³³. The ∆G of methacrylic acid was -27.5114588 kcal/mol, indicating strong hydrogen bonding. The highest binding energy of monomer showed the strongest complexes with α-mangostin and represent the best candidates for polymer preparation. The interaction between the monomer-template is explained automatically and then its position is determined so that the minimum interaction energy is measured.

Table 1. Change in the Gibbs free energy (ΔG) due to the complex formation between the template and the functional monomers in different mole ratios of monomer

Monomer	∆G (Kcal/mol)	Classification	Ratio
Methacrylic acid (MAA)	-27.5114588	Strong	1:6
Acrylamide (AAM)	-23.1938681	Strong	1:4
Methyl methacrylic acid (MMAA)	-19.7310847	Strong	1:6
Acrylic acid (AA)	-18.9649975	Strong	1:6
Itaconic acid (IA)	-14.1299594	Moderate	1:4
Methacrylamide (AAM)	-9.7697174	Moderate	1:2
Allylamine (AL)	-9.3124800	Moderate	1:1
4-vinyl pyridine (4VP)	-8.0878483	Moderate	1:2
2-Vinyl pyridine (2VP)	-7.0478911	Moderate	1:2

Template-monomer complex molar ratio optimization

The expected pre-polymerization mechanism of α-mangostin and methacrylic acid between the six possible hydrogen bonding sites is illustrated in Table 2. All possible interactions between functional monomers and template molecules are carried out in a 1:1 to 1:6 molar ratio. Mole ratio optimization of template monomer complex was carried out in order to test the influence produced by the increase in the functional monomer concentration for the selected template-monomer complexes.

The optimal conformation of α-mangostin and methacrylic acid was obtained at a conformational ratio of 1:6. The bond distance between hydrogen bond donor/acceptor from two structure was calculated. The expected bond length were found in a range 1.809-1.8328 A° confirming the formation of intramolecular hydrogen bond, within the hydrogen bond range 1.7 -3.5 A° ^33,37 as indicated in Table 2. Thus, hydrogen bonding is the main contributor to the stabilization of the pre-polymerization complex ^{17, 34, 35}. The calculated binding energy increases as the number of monomers used increases. The higher the binding energy value, the more stable the composite formed; therefore, the optimal molar ratio for preparing the MIP is 1:6. By visualizing the interaction monomers in different site of α-mangostin, an indication of possible monomer combinations that can be used to develop higher-affinity MIPs is given^36,38.

Table 2. Optimized structures of AM And MAA pre-polymerization in different ratio

Ratio AM-MAA	Structure	Bond distance (A°)	Binding energy (Kcal/mol)
1:1		1.809	-5,3939214
1:2		1.8110 1.8123	-9,8011146
1:3		1.8116 1.8084 1.8136	-14,7799565
1:4		1.8097 1.8032 1.8156 1.8123	-18,4397349
1:5		1.8102 1.8036 1.8121 1.8233 1.8199	-22,7564886
1:6		1.8121 1.8203 1.8328 1.8244 1.8187 1.8180	-27,5114588

CONCLUSION:

Pre-polymerization of molecularly imprinted materials for α-mangostin (AM) were design using Hyperchem for computational simulations. The rational computational MIP design predicted that methacrylic acid was the most suitable functional monomer for the synthesis of α-mangostin MIP.  This study proves the importance of studying the intermolecular interaction between template, functional monomer and template-monomer ratio for the synthesis of α-mangostin MIP, and it can be applied in practice.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

ACKNOWLEDGEMENTS:

We gratefully acknowledge the Rector of Universitas Padjadjaran and the Minister of Research and Higher Education for funding this project through the Grant of Dissertation Doctorate Research 2020 (PDD) from Ministry of Research and Technology/BRIN 1827/UN6.3.1/LT/2020 and Academic Leadership Grants: 1959/UN6.3.1/PT.00/2021 from Universitas Padjadjaran.

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Received on 28.07.2021           Modified on 01.10.2021

Accepted on 03.11.2021         © RJPT All right reserved

Research J. Pharm. and Tech. 2022; 15(8):3663-3668.

DOI: 10.52711/0974-360X.2022.00614