Molecular dynamics assisted analysis on improved free radical removal ability of fullerene(C60)-curcumin aggregate

doi:10.21203/rs.3.rs-4463712/v1

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Molecular dynamics assisted analysis on improved free radical removal ability of fullerene(C₆₀)-curcumin aggregate

https://doi.org/10.21203/rs.3.rs-4463712/v1

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The self-aggregation of curcumin (Cur) on the surface of fullerene (C₆₀) was induced by ultrasonic solvent exchange method. Associating the characterization results of infrared spectrum, X-ray diffraction, and scanning electron microscopy, the molecular structure and morphology of two components in the aggregate (C₆₀/Cur) were determined unchanged essentially. By analyzing the dependence of related cosmetic effects on the aggregating ratio of C₆₀ and Cur, the monolayer saturation was found advantageous to improve the amphiphilicity and oxidation resistance. Especially, C₆₀/Cur showed a better performance to eliminate free radicals compared to counterpart of simply mixing C₆₀ and curcumin. Based on molecular dynamics simulations of the Forte module, the self-aggregation is proposed occurring through π-π stacking interactions between the benzene ring from curcumin and the spherical π bond of C₆₀, and a molecular configuration was offered for displaying the optimal spatial arrangement of C₆₀/Cur. Furthermore, the Mulliken charges on the phenolic hydroxyl groups of the curcumin molecules with original and aggregated state were calculated respectively using Gaussian software, and the charge values were found to transfer from negative to positive due to the aggregating. The dispersion of the electron cloud on the benzene ring was recognized responsible to enhance the electron absorption capacity of the phenolic hydroxyl group, thereby improving free radical removal ability of C₆₀/Cur.

Fullerene

curcumin

aggregate

antioxidation

cosmetics

Fullerenes have an obvious affinity for free radicals and perform strong antioxidation ability, which can prevent oxidative damage to cells, protect cells from excitotoxicity in vitro, and delay apoptosis[1]. Recently, fullerenes have been utilized as highly cutting-edge ingredient to enhance the effectiveness of cosmetics in anti-wrinkle, whitening, and anti-aging applications[2, 3]. However, original fullerenes are extremely hydrophobic, resulting in low solubility and easy aggregation in aqueous systems[4], limiting their widespread application in the cosmetic field. So the surface modification of fullerenes has been attracting much interests from academic and industrial fields.

Currently, the water solubility of fullerenes is typically enhanced by modulating surface hydrophilicity. One method of surface modification involves utilizing the covalent interaction of hydrophilic macromolecules, such as cyclodextrins, cellulose, glucose, and phosphine-containing compounds, with the fullerene carbon cage[5–7]. This process enhances the hydrophilicity of fullerenes by introducing hydrophilic groups to the surface, which also gives fullerenes new functions such as optimizing nuclear magnetic resonance spectroscopy, sensitizing photodynamics, and enabling cancer therapy[8–10]. Another approach to increase surface hydrophilicity is to utilize the porous structure of fullerenes to no-covalent adsorb some solubility modifiers[11, 12], such as amphiphilic polymers [13], polysaccharides[14], and peptides[15]. This method can be achieved by spontaneous contact of reactants under simple regulating external conditions, so preserve fullerenes with original carbon cage structure and related properties.

The surface modifications not only improve the hydrophilicity of fullerenes but endow them new functions for further applications in biomedicine, electrochemistry, and photocatalysis[16–20]. However, the compatibility functions aroused from surface modification have not been realized in cosmetics application of fullerenes yet [21], and even some original positive properties of fullerenes have been suppressed. Diana Dulić's researched on C₆₀ derivatives, obtained by combining fullerene and curcumin through the Binger effect, and demonstrated the products with moderate antioxidant activity due to the active react-sites of fullerenes inhibited during modification process [22]. It has been reported that the antioxidative and moisturizing properties of fullerenes, which are related to the efficacy of cosmetic products, are suppressed when the carbon-carbon double bonds are covered by modifiers[23]. So, enhancing oxidation resistance accompanied by enhancing hydrophilicity is a constructive aim in surface modification research of fullerenes as a cosmetic raw material.

In this paper, curcumin was chosen as a surface modifier to facilitate a self-aggregating onto the surface of fullerene through solvent exchange under ultrasonic conditions. The Infrared Visible Spectrum (IR), X-Ray Diffraction (XRD), Scanning electron microscope (SEM), and thermogravimetric analysis were used for analyzing the surface morphology and structure of the fullerenes-curcumin aggregate (C₆₀/Cur), and then the relationship between aggregated ratio and the surface hydrophilicity was determined. Based on the dependence of free radical removal ability of C₆₀/Cur on the aggregated ration, a molecular dynamics assisted mechanism analysis was implemented for clarifying the interaction between curcumin molecules and fullerene carbon cages under aggregating state. This study aimed at offering a theoretical foundation for the combination of fullerenes and polyphenols in batches, with potential applications in the field of cosmetics.

2.1 Reagents

Fullerene (C₆₀), curcumin (Cur), hexafluoroisopropanol, and 1,1-diphenyl-2-trinitrophenylhydrazine (DPPH) were purchased from Macklin (Shanghai, China), isopropanol was purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing China).

2.2 Aggregate synthesis

Add 150 mL of isopropanol with varying quantities of Cur to prepare Cur isopropanol solutions with different concentrations. Also, add 300 mg of C₆₀ to 50 mL of hexafluoroisopropanol. The Cur solution and C₆₀ solution are mixed at a volume ratio of 3:1, and the resulting mixture is immediately placed in the ultrasonic instrument. The ultrasonic at room temperature is used for 40 minutes Afterward, the sample is refrigerated at 4°C overnight for future use. The mixed solution was centrifuged at 7900 revolutions per minute for 10 minutes. The precipitate was collected, washed several times with ethanol and acetone to remove any residual solvent, and then dried to obtain brownish-yellow aggregate powder C₆₀/Cur-x (x = 200–500 mg) with different loads.

2.3 Physical characterization

All raw materials and aggregate products were tested using a multi-functional X-ray diffractometer (Bruker Axs D2, Karlsruhe, Germany). The scanning range of the X-ray diffractometer is set to (2 θ Value) from 10° to 80°, with a scanning speed of 0.08°/s. Use the FTIR spectrometer (NicoletTM iSTM10, Thermo Fisher, USA) to scan the sample and determine its infrared spectrum. Perform 32 scans within a range of 4000 ~ 1000 cm^− 1 with a resolution of 4 cm^− 1. SEM images of raw materials and aggregate products were obtained using a field emission scanning electron microscope (JSM 6700F JEOL, Japan). A TG-DTA/DSC synchronous thermal analyzer (STA449F3, Netzsch, Germany) was utilized to analyze the thermal stability of the sample. Set the termination temperature at 900°C, with a heating rate of 10 K/min under a N₂ atmosphere. Use the JC2000D1 contact angle measuring instrument to measure the contact angle of 6mg of C₆₀/Cur aggregate and 6mg of C₆₀ and Cur separately. Utilize distilled water and glycerin as the liquid samples for the contact angle test.

2.4 Evaluation of free radical clearance capacity

Take 1.0 mL of a 0.1 mg/mL Cur, C₆₀, C₆₀/Cur ethanol solution and mix it with 3.0 mL of a 0.08 mg/mL DPPH ethanol solution. The initial step involves mixing the sample with DPPH solution, resulting in a blank solution. This solution is then allowed to react at 37°C, while observing the UV spectrogram of the mixed solution as it changes over time. Absorbance was measured at 517nm. The DPPH radical clearance (D) is calculated by the following formula.

$$\text{D}\text{%}=\frac{{\text{A}\text{b}\text{s}}_{\text{b}\text{l}\text{a}\text{n}\text{k}} -{ \text{A}\text{b}\text{s}}_{\text{s}\text{a}\text{m}\text{p}\text{l}\text{e}}}{{\text{A}\text{b}\text{s}}_{\text{b}\text{l}\text{a}\text{n}\text{k}}} \times 100\text{%}$$

According to various time intervals, 3ml of Cur, C₆₀, C₆₀/Cur, and DPPH reaction solutions were taken, and their ultraviolet absorption spectra were measured using an ultraviolet spectrophotometer (UV-3600, SHIMADZU, Japan).

2.5 Evaluation of moisturizing capacity

Spread the sample on the glass slide using 3M double-sided adhesive tape, and measure its mass (M_n,0). Then wet the slide with the sample and measure its mass (M_n,1). Then put it in the dryer, remove the sample after 2 hours, and measure its mass (M_n,2). Calculate the moisture retention rate of the sample using the following formula:

Moisture retention rate=(M _n,2 - M _n,0) /(M _n,1 - M _n,0 )× 100% (2)

2.6 Molecular dynamics simulations

Molecular dynamics simulations are performed using the Forcite module. At 20.83×20.83×20.83 Å³ simulation box, put five C₆₀ molecules and five curcumin molecules into it, and add 50 ethanol molecules as the solvent. Each component utilizes the CVFF force field to describe its interactions, and the Lennard-Jones force field parameters between different molecules are determined using the Lorentz-Berthelot mixing rule. Short-range interactions are truncated at 10 Å, while long-range interactions are calculated using the PPPM algorithm. Periodic boundary conditions are applied in all three directions of the simulation box.

A relaxation process of 10 picoseconds was carried out at 300 Kelvin to eliminate the influence of the initial configuration. Next, a 2 ns dynamic simulation is conducted using the NpT ensemble to generate the dynamic trajectory of the system. The time step of the simulation is 1 femtosecond, the temperature is maintained at 300 Kelvin, and the pressure is 0.1 megapascals. The constant temperature is adjusted using the Nosé-Hoover method, and the constant pressure is adjusted using the Berendsen method. After the simulation system stabilizes, the configuration from the last 0.1 ns is averaged, and the stable configuration of C₆₀ interacting with curcumin is extracted. Next, calculate the Mulliken charge of the stable configuration. The charge calculation has been completed using the Gaussian software. First, geometric optimization was performed for the configuration. Using B3LYP density functional theory, the optimized configuration was obtained using the 6-311G (d, p) basis set. Then the Mulliken charge of the system was calculated using the same conditions.

3.1 Structure and composition

According to the XRD results (Fig. 1a), C₆₀/Cur mainly exhibits three characteristic diffraction peaks of C₆₀ at 11°, 17°, and 20°. This indicates that the crystal structure of the aggregate is essentially consistent with C₆₀ after aggregation. At the same time, a series of new peaks which did not exist previously in raw fullerenes, appear in the aggregate within the range of 15–35 °, which correspond to the XRD image features of pure curcumin very well. The infrared spectrum of C₆₀/Cur (Fig. 1b) is generally similar to that of pure curcumin, but some characteristic peaks of fullerenes (1182 cm^-1 and 1428 cm^-1) can also be identified. Combining XRD and IR results, it can be determined generally that the aggregate of curcumin and fullerene has been realized.

As shown in the thermogravimetric analysis (Fig. 1c), the mass of C₆₀ remains stably unchanged as the temperature increases to 750 ℃. While Cur begins to lose weight at around 200℃ due to converting into reactive organic precursors, and then completes oxidation with about 60% mass loss at 400 ℃[24].All of C₆₀/Cur aggregates with various loads behave similar mass-losing pattern with pure cur, and the loss percentage presents a linear dependence with the loading quantity of Cur. The consistency of mass-losing temperatures between free and aggregated Curs implies the aggregate tending to physical combination, which responds to no new peak observed in IR of C₆₀/Cur.

The fluorescence spectrum of the C₆₀/Cur-300 ethanol solution (Fig. 1d) exhibits obvious fluorescence characteristics of curcumin (with a 540 nm emission peak under 365 nm excitation). Furthermore, as the concentration of aggregates solution increasing, the emission intensity initially increases and then decrease, consistent with the fluorescence properties of a curcumin ethanol solution. The aggregation fluorescence quenching occurs when curcumin reaches a certain concentration[25]. The excitation peak of aggregates with different materials amounts increases with the increase of concentration, and then decreases. This further confirms the successful loading of curcumin on the surface of C₆₀.

3.2 Surface Characterization

According to the results of the field emission scanning electron microscope, the raw material C₆₀ is irregularly spherical (Fig. 2a), and curcumin is in the form of long rods (Fig. 2b). The overall shape of the aggregate remains similar to that of raw material C₆₀, and the amount of curcumin aggregate has no significant effect on it (Fig. 2c and d). A certain amount of curcumin can be observed on the surface of C₆₀/Cur-200, but most of it is the exposed surfaces of C₆₀.While the C₆₀/Cur-300 surface is enriched with a large amount of curcumin rods, and the C₆₀ surface is barely observed (Fig. 2e and f). It indicates that the surface aggregate of C₆₀/Cur-300 tends to reach saturation. In addition, the size of the curcumin aggregates on C₆₀ decreased significantly, which may be attributed to further dispersion during the ultrasonic process in the organic solvent.

By measuring the contact angle, it was found that the lipophilicity and hydrophilicity of C₆₀ were not optimal (Fig. 3a and e). However, after the surface was covered with 200mg of amphiphilic curcumin, the contact angles with water and glycerin were significantly reduced (Fig. 3b and f). Furthermore, the contact angles between saturated C₆₀/Cur-300 aggregate and water/glycerin decrease to 57° and 60° respectively (Fig. 3c and e), possessing significant amphiphilic properties. which have essentially reached the amphiphilicity of the pure curcumin sample (Fig. 3d and h). It indicates that the coverage of curcumin on the surface of the C₆₀/Cur-300 has nearly reached saturation. The surface of fullerene is porous and has a strong adsorption capacity for water molecules. The results of the moisturizing experiment (Fig. 4) indicate that fullerene has the most potent water-locking effect, while curcumin has the least effect. As the curcumin loading increases, the water retention capacity of fullerene gradually decreases because its surface porous structure has been covered. When the load reaches 500 mg, the moisture retention rate is close to that of pure curcumin, indicating that the surface of C₆₀ has been completely covered by curcumin.

3.3 Evaluation of antioxidant efficacy

As shown in the UV-visible spectrum of C₆₀/Cur-300 solution reacting with DPPH within 24 hours (Fig. 5a), the absorption peak at 519 nm corresponding to DPPH can be found to gradually decrease with increasing reaction time. After 24 hours of the reaction, the UV absorption peak of DPPH essentially disappeared, indicating that the reaction was essentially complete. The curves of free radical clearance (Fig. 5b) were determined based on the UV-visible spectra of several samples reacting with DPPH within 24 hours. As the curcumin loading increasing, the clearance rate of aggregates also gradually increased. When the load was increased to 300mg, the clearance rate of free radicals in the aggregate was 97%, which is higher nearly 20% than that of pure curcumin. In detail, the free radical clearing effect of C₆₀/Cur-300 and curcumin began to diverge after 30 minutes, with a maximum difference till 360 minutes. With the load increasing further, the aggregate's free radical clearance rate has been slightly improved. So, the C₆₀/Cur-300 was considered as an optimal aggregate for further research.

Direct physical mixing of C₆₀ and curcumin was conducted based on the raw material ratio of C₆₀/Cur-200 and C₆₀/Cur-300. The free radical clearance rate of the C₆₀ + Cur-200 and C₆₀ + Cur-300 mixtures within 24 hours was investigated by comparing aggregates (Fig. 6). The results indicated that the apparent radical clearance capacity of the simple mixture fell between that of pure C₆₀ and curcumin, and did not perform a higher clearance capacity than curcumin like the aggregate C₆₀/Cur. It can be inferred that the interaction between C₆₀ and Cur in C₆₀/Cur effectively enhances the aggregate's ability to remove free radicals.

3.4 Molecular dynamics simulations

In this study, the spontaneous aggregate state of C₆₀ and Curcumin was simulated using the Force module. In the ethanol environment, the spherical π bond of some C₆₀ molecules and the benzene ring of curcumin molecules are π-π stacking, forming a molecular aggregating configuration as shown in Fig. 7a. The benzene ring in the curcumin molecule and the carbon cage in the fullerene molecule can generate Van der Waals attraction through interaction. The C-H bond of benzene is directed towards the center of the carbon cage of fullerene, leading to mutual attraction and the formation of an aggregate (Fig. 7b). In general, π - π stack integration occurs as edge-to-face (T Shaped) (Fig. 8a) and offset face-to-face (F Shaped) (Fig. 8b) interactions. Various stacking mechanisms will impact the repulsion among π electrons and the attraction between the positive charge of the benzene ring and π electrons. The π-π stacking interaction between molecules can influence the intermolecular interaction energy, which can be analyzed using the theories of Morokuma[26], Ziegler and Rauk[27]:

$${\text{E}}_{int}={\text{E}}_{\text{e}\text{l}\text{e}\text{c}}+{\text{E}}_{\text{P}\text{a}\text{u}\text{l}\text{i}}+{\text{E}}_{\text{o}\text{r}\text{b}}+{\text{E}}_{\text{d}\text{i}\text{s}\text{p}}$$

E_elec the undisturbed Coulomb force interaction, E_Pauli represents Pauli repulsion energy (unstable interaction between orbits and any spatial repulsion), E_orb is usually conceptualized as the orbital interaction energy (electron pair bonding, polarization, and charge transfer), and E_disp accounts for the dispersion correction, which means the intermolecular Van der Waals force[28]. When the benzene ring is stacked on a surface composed of a large number of π bonds, the π electron clouds of the two monomers can induce a shift in each other. Due to the sacrifice of E_disp in π - π stacking, the T-shaped slip stacking arrangement is considered the preferred orientation. The increased stability of the benzene ring through T-type stacking disperses the electron cloud on the benzene ring and enhances the electron absorption capacity of the phenolic hydroxyl group, thereby further improving the reactivity of the hydroxyl group on the benzene ring. Kevin Carter Funk has shown that this non-covalent bond interaction has a significant impact on the stability of the benzene ring [29].

Curcumin is the primary naturally occurring yellow polyphenol found in turmeric rhizomes. The two hydroxyl groups on the benzene ring are believed to play a crucial role in its antioxidant properties[24, 30]. The antioxidant mechanism of polyphenols involves a hydrogen extraction reaction. This process begins with the phenolic hydroxyl of polyphenols losing a proton, which then combines with DPPH to form DPPH-H. As a result of the proton loss, polyphenols undergo rearrangement, and the double bond carbon adjacent to the benzene ring loses its proton and subsequently combines with DPPH-H (refer to Fig. 9a). Consequently, Gaussian software was utilized to calculate the Mulliken charge on the two phenolic hydroxyl groups of curcumin molecules with free and aggregated states respectively. The results showed that the charge of oxygen atoms on the two phenolic hydroxyl groups before aggregation was − 0.192 and − 0.232, respectively, and then changed to + 0.179 and + 0.105 after aggregation (Fig. 9b). This is mainly due to the π - π stacking interaction between the large spherical π bond formed by the fullerene cage and the benzene ring of curcumin after aggregation. The π bond of fullerene and the aromatic ring of curcumin will stack face to face. The charge penetration effect and resulting electrostatic interaction will help to reduce the Pauli repulsion, thereby increasing the total interaction energy and stabilizing the benzene ring. The electron cloud around the benzene ring is dispersed, leading to an increase in the charge of the oxygen atom in the phenolic hydroxyl group. This enhances the electron absorption capacity, making it easier for hydrogen to detach and combine with DPPH, thereby improving the aggregate's ability to clear free radicals.

In this study, fullerene-curcumin aggregates were prepared using the solvent exchange method under ultrasonic conditions to induce self-aggregation of curcumin on the surface of fullerenes. The surface hydrophilicity and lipophilicity of the aggregates were significantly improved with the increase in curcumin aggregation. The study demonstrated that the aggregates had a better radical removal ability compared to the simple mixtures of the two compositions when the encapsulation was close to saturation. Based on molecular dynamics simulations, it has been demonstrated that the edge-to-face stacking of aggregated curcumin with fullerenes increases the electron-withdrawal capacity on the phenolic hydroxyl groups of curcumin, thereby enhancing its overall radical clearance ability. The Amphipathic and improved oxidation resistance of C₆₀/Cur broaden the application domains of fullerenes in cosmetics, and offer a new insight for other specific modification research.

Funding: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sector.

Author contribution: The manuscript was written through contributions of all the authors.

Availability of data and material: All the data and electronic materials are available for the Origin and Gaussian program.

Competing interest policy: The authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

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No competing interests reported.

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Molecular dynamics assisted analysis on improved free radical removal ability of fullerene(C₆₀)-curcumin aggregate

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1. Introduction

2. Experimental