Synergistic Antioxidant Performance of Lignin and Quercetin Mixtures

Research Article pubs.acs.org/journal/ascecg

Synergistic Antioxidant Performance of Lignin and Quercetin Mixtures Di Liu,† Ying Li,† Yong Qian,*,†,‡ Yang Xiao,† Shengjun Du,† and Xueqing Qiu*,†,‡ †

School of Chemistry and Chemical Engineering and ‡State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510640, P. R. China S Supporting Information *

ABSTRACT: A natural, effective, and inexpensive hindered phenolic antioxidant mixture was prepared by blending lignin into quercetin. The antioxidant performance of lignin and quercetin mixture was analyzed by determining the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity and a low-cost and highefficiency ratio was found to be 4:1 (w/w). After UV radiation for 4 h, the DPPH scavenging ratio of the quercetin/lignin mixture decreased only 13.8%, while that of quercetin and lignin decreased 42.9% and 28.6%, respectively. The UV and fluorescence analysis indicated that quercetin molecules inserted into the lignin to weaken its aggregation and form new conjugated structures. Adding lignin may provide a green alternative to the expensive quercetin or synthetic antioxidants used in food, cosmetics, and pharmaceuticals. KEYWORDS: Lignin, Quercetin, Antioxidant, Synergistic effect, UV radiation

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INTRODUCTION Natural antioxidants are widely used in food, cosmetics, and pharmaceuticals due to their high activity, low toxicity, and green properties,1−4 and thus they are more popular than synthetic antioxidants. As one of the most excellent natural antioxidants, quercetin is a typical flavonoid with benefits on human health including antioxidative, anticancerous, antiinflammatory, and antidiabetic activities.5−8 However, quercetin tends to degrade under UV radiation and other severe environments.9 Therefore, several efforts have been invested to maintain its activity. The photodegradation behavior of quercetin in creams would be effectively released when it is protected by cellulose.10 Zheng and Chow prepared a chemically stable and readily water-soluble solid complex of quercetin with hydroxypropyl-b-cyclodextrin (HP-b-CD) by spray drying, and it displayed excellent thermal stability.11 Zhang et al. encapsulated quercetin in the chitosan nanoparticles by ionic gelating of chitosan with tripolyphosphate anions.12 The nano complexes improved the bioavailabilty of quercetin in pharmaceutical formulation.12 Therefore, mixing quercetin with other substances will significantly improve its stability.13−15 Lignin, the unique aromatic polymer in plants, is a natural polyphenolic antioxidant as well as a natural broad-spectrum sun blocker.16−19 Studies on its antioxidative protections for materials, cosmetics, foods, and pharmaceuticals attract considerable attention, especially under UV radiation.20−23 ́ KoŠiková et al. investigated the protective effect of lignin antioxidants against H2O2-induced oxidative damage of DNA in human carcinoma cells and male rats, as well as the stabilization © 2017 American Chemical Society

effects during the processing of polypropylene (PP) composites and thermo-oxidative aging of styrene−butadiene vulcanizates.20 Their work indicated that the lignin preparations show great potential as antioxidants in human diets and polymer blends.20 Lately, Gadioli et al. compared lignin with industrial Irganox 1010 as the primary stabilizer in formulations for PP during accelerated aging and confirmed the better antioxidant performance of lignin due to its cross-linking macromolecule nature.21 Aguiébéghin et al. found that ethanol extracts obtained from biorefinery grass lignins could compete with commercial rosemary antioxidant extracts for food and packaging matrices.22 Moreover, self-assembling polysaccharide films were applied as active hydrogels for controlled release of antioxidant molecules.22 In a previous study, it was found that lignin from different technical resources could significantly boost the performance of sunscreen lotions and dramatically improved their photostability.24 The synergistic effect between lignin and chemical sunscreen actives was also due to the antioxidant protection of lignin.24,25 Shahidi et al. also found the synergistic antioxidant effect between lignin units like 3tertiary-butyl-4-hydroxyanisole (BHA) and 2,6-ditertiary-butyl4-methylphenol (BHT).26 Synergistic antioxidant effects between quercetin and small molecules such as vitamins E, C, and rutin have been demonstrated.27,28 Lignin not only has excellent antioxidant ability but also has good photostability. As lignin and quercetin coexist in many plants such as pine in the whole lifecycle, there Received: July 8, 2017 Published: August 2, 2017 8424

DOI: 10.1021/acssuschemeng.7b02282 ACS Sustainable Chem. Eng. 2017, 5, 8424−8428

ACS Sustainable Chemistry & Engineering

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RESULTS AND DISCUSSION Antioxidant Activities of Lignin, Quercetin, and Their Mixtures. DPPH radical assay was performed to estimate the radical scavenging ability of lignin, quercetin, and their mixtures. After addition of lignin and quercetin, the purple DPPH solution turned yellow and the absorbance peak at 517 nm decreased. Figure 1a shows the IP of different

might be some cooperation between them, especially under UV radiation. In this work, the antioxidant performance of quercetin, lignin, and their mixtures were investigated by determining their free radical scavenging abilities. Their synergistic antioxidant behavior was conformed and their combination antioxidant effect after UV radiation was further studied. The results demonstrate that low-cost lignin has the potential to partially replace the expensive quercetin in natural antioxidant applications.

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Research Article

EXPERIMENTAL SECTION

Materials. The lignin used in this work was extracted from pine wood and purified according to the procedure described in ref 18. Simply, the pine chips were cooked by ethanol/water mixture (65:35, vol/vol) under 180 °C for 60 min. The catalyst was 1.5 wt % sulfuric acid (98 wt %). After filtering with a nylon cloth, the filtrate was poured into water to precipitate lignin. The physicochemical properties of lignin and its 2D heteronuclear single quantum coherence (HQSC) NMR spectrum are shown in Table S1 and Figure S1, respectively, in the Supporting Information. Quercetin and 1,1-diphenyl-2-picrylhydrazyl (DPPH) were purchased from SigmaAldrich (Shanghai, China). Deionized water with resistivity > 18 MΩ/ cm was obtained from a Millipore water-purification system. Other reagents and solvents were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), without further purification. Evaluation of the Antioxidant Activities of Lignin, Quercetin, and Their Mixtures. DPPH was used as the radical generator. The antioxidant activities of the lignin and quercetin were determined based on the radical-scavenging capability. Lignin, quercetin, and their mixtures, aqueous dioxane solutions (3.8 mL) with different concentrations (5−500 mg/L), were mixed with 11.80 mL of a 6.1 × 10−5 mol/L DPPH methanol solution at 25 °C for 16 and 60 min, respectively.18 The mixtures of lignin and quercetin were prepared by blending quercetin dioxane/water (90:10, vol/vol) solution with lignin dioxane/water (90:10, vol/vol) solution under different ratios. The concentrations of DPPH radicals at 0, 16, and 60 min were monitored at 517 nm (λmax) using a UV−vis spectrometer (Shimadzu Co., Japan). The inhibition percentage (IP) of the DPPH radical was calculated using the following equation:

IP =

A 2 − A1 + A 0 × 100% A2

(1)

where A2 = absorbance at 0 min; A1 = absorbance at 16 or 60 min; and A0 = absorbance of the blank solution. Characterizations. The molecular weights of lignin were measured by Agilent 1100 series gel permeation chromatography (Agilent Technologies Corp., U.S.A.) with PLgel 5 μm 500 Å columns. The mobile phase was tetrahydrofuran (THF) with a flow rate of 1 mL/min. Elemental analysis of lignin was measured using an Elementar Vario EL cube (Elementar, German). The fixed carbon content and moisture of lignin were measured by thermogravimetric analysis (Q500, TA, U.S.A.), and the ash content was determined by muffle furnace (P330, German) with lignin ashing at 800 °C. The 1H NMR spectra of lignin and quercetin and their mixtures were recorded on a NMR spectrometer (Bruker, AVANCE HD III 600, Germany). The data were processed using the Bruker Topspin-NMR software. For 2D HQSC NMR of lignin, ∼100 mg of lignin was dissolved in 0.6 mL of dimethyl sulfoxide (DMSO-d6). The widths of the spectra were 5 000 and 20 000 Hz, respectively, for the 1H and 13C dimensions. The UV−vis absorption spectra of quercetin, lignin, and their mixtures before and after irradiating were recorded in range of 250−450 nm with the UV−vis spectrophotometer (UV-2450, Shimadzu Co., Japan). The aqueous dioxane was scanned at the same wavelength as baseline. Emission fluorescence spectra of quercetin, lignin, and their mixtures were recorded on a fluorescence spectrophotometer (F-4500, Hitachi Co., Japan).The excitation wavelengths of lignin and quercetin were 291 and 370 nm, respectively, when excited at 375 nm.

Figure 1. (a) DPPH inhibition percentage (IP) of lignin solution at different concentrations; (b) DPPH IP of quercetin solution at different concentrations; (c) DPPH IP of quercetin/lignin mixtures at different ratios (wL + wQ = 100 mg/L).

concentrations of lignin. Lignin at low concentration has a good free radical scavenging ability, and the best concentration of lignin lies between 150 and 200 mg/L. After 60 min, its reaction with DPPH was still going on. With the increase of the concentration of lignin, the free radical scavenging rate was first increased and then decreased a little after 250 mg/L. Usually, the antioxidant ability is not directly proportional to the 8425


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significantly, while that of the mixture decreased a little, as shown in Figure 3a. Taking the scavenge time of 30 min as

concentration. One possible explanation for lignin is that the high concentration promotes the aggregation of lignin and reduces the exposure of reactive groups to DPPH.29 Because quercetin provides active hydrogen to DPPH rapidly, it has much better antioxidant performance than lignin and its free radical scavenging rate is as high as 90% even at 15 mg/L, as shown in Figure 1b. However, the poor stability and high cost of quercetin hinder its application. Lignin is thus applied to blend with quercetin to reduce the cost and keep the stability. As expected, the IP of lignin/quercetin mixture increased with the proportion of quercetin. However, the IP reached a maximum of 94.2% at 60 min when the proportion of quercetin was beyond 0.2, which is even better than that of pure quercetin, as shown in Figure 1c. Blending with lignin can not only lower the cost dramatically but also maintain the antioxidant activity of quercetin. In addition, the DPPH scavenging kinetics of lignin/quercetin mixtures under different ratios were measured, as shown in Figure 2. The scavenging

Figure 3. (a) DPPH scavenging kinetics of quercetin and lignin mixtures before and after UV radiation for 4 h; (b) decreased rate of DPPH radicals after UV radiation on lignin, quercetin, and their mixture at 30 min.

Figure 2. DPPH scavenging kinetics of quercetin/lignin mixtures at different ratios (wL + wQ = 100 mg/L).

processes of lignin/quercetin mixtures can be fitted by the second-order exponential decay functions except the mixture with ratio of 1:1, which is fitted by the normal exponential decay function. As shown in Table 1, the decay times of the

example, the free radical scavenging activities of quercetin and lignin were reduced by 42.9% and 28.6%, respectively, while that of the mixture reduced only 13.8%, as shown in Figure 3b. The detailed calculation procedure is present in Table S2. The phenolic hydroxyl groups of antioxidants were easy to be oxidized into quinoid structures after UV radiation, and their free radial scavenging activity weakened.9 However, the threedimensional macromolecular structure of lignin is beneficial for protecting its phenolic hydroxyl groups from photodeactivation.24,30 Therefore, the activity reduction of lignin is less significant than that of quercetin. When quercetin was blended with lignin, the macromolecular structure of lignin protected quercetin and quercetin inversely protected lignin to some degree too. Thus, the decrease of the antioxidant activity of their mixture was less than half of that of both lignin and quercetin. Photostable Mechanism of Lignin/Quercetin Mixture. To explore the photostable mechanism of lignin/quercetin mixture, the UV spectra of lignin, quercetin, and their mixture before and after UV radiation were obtained, as shown in Figure 4. After 4 h irradiation, the characteristic peak of lignin around 280 nm had no change; only the absorbance decreased a little. However, the characteristic peak of quercetin at 375 nm underwent a gradual blue-shift to 295 nm, indicating the breakage of its cinnamoyl-conjugated structures.9,31 Without the stabilization of big cinnamoyl conjugated structure, quercetin could not trap the free radicals effectively as before;

Table 1. Parameters for Fitting the DPPH Scavenging Kinetic Curves of Quercetin/Lignin Mixtures with Different Ratios mass ratio (L/Q)

A0

k1

Γ1 (s)

k2

Γ2 (s)

R2

1:1 3:1 4:1 6:1 8:1 11.5:1 15.6:1 1:0

0.03 0.04 0.04 0.04 0.05 0.10 0.10 0.19

0.05 0.10 0.22 0.10 0.14 0.16 0.18 0.19

24.63 45.82 55.42 434.41 464.81 378.58 520.89 760.38

0.10 0.02 0.09 0.08 0.05 0.05 0.04

45.82 246.73 46.20 50.89 37.70 48.16 100.74

0.9994 0.9998 0.9999 0.9999 0.9999 0.9995 0.9999 0.9999

mixtures with ratios 3:1 and 4:1 are close to that of the mixture with the ratio of 1:1. When the ratio of the mixture is more than 6:1, the decay time is several times longer, which indicates the free radical scavenging rate is much slower. Antioxidant Activities of Lignin, Quercetin, and Their Mixtures before and after UV Radiation. DPPH antioxidants, especially small molecules such as quercetin, are photo-unstable and need protection. After UV radiation for 4 h, the antioxidant activities of quercetin and lignin both reduced 8426


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the aggregation of lignin. The proposed mixing structure of quercetin and lignin is shown in Figure 6. Formation of lignin/ quercetin complexes was beneficial for the antioxidant protection of both lignin and quercetin, especially after UV radiation.

Figure 4. UV spectra of lignin, quercetin, and their mixture before and after UV radiation for 4 h.

thus, its antioxidant ability decreased. When quercetin was blended with lignin, the characteristic peak of lignin shifted from 288 to 294 nm, indicating that new conjugated aromatic structures were formed. The conjugated structures between aromatic rings of quercetin and lignin were also demonstrated in their 1 H NMR spectra, as shown in Figure S2. Comparatively, the aromatic rings in quercetin are electronrich, while those in lignin are electron-poor. When lignin and quercetin form conjugated structures, the electron cloud density of aromatic rings in lignin increases and the electron cloud density of aromatic rings in quercetin decreases. Therefore, most of the aromatic hydrogen in lignin shifts to high field and that in quercetin shifts to lower field after mixing. After being irradiated by UV for 4 h, the characteristic peak of quercetin disappeared and the conjugated aromatic peak redshifted to 300 nm. It means more conjugated structures were formed, although the original cinnamoyl-conjugated structures of quercetin were still broken. Therefore, the antioxidant activity could be reserved in maximum. The fluorescence quenching of lignin dioxane/water solution in the presence of quercetin was observed and conformed the aromatic combination, as shown in Figure 5. Lignin exhibited a

Figure 6. Mechanism illustration of the interaction between lignin and quercetin in solution.

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CONCLUSION Lignin and quercetin were mixed as a natural, effective, and low-cost hindered phenolic antioxidant. The best ratio of lignin and quercetin was 4:1, and its free radical scavenging ability was even better than pure quercetin. Because quercetin effectively inserted into lignin and formed conjugated structures, their antioxidant property could be preserved to a maximum extent under long-time UV radiation. The work provides potentials for more efficient antioxidant application for lignin and reduction of the antioxidant cost of quercetin.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02282. Physicochemical properties of lignin; UV absorbance of initial and remaining DPPH radicals after being scavenged by lignin, quercetin, and their mixture; 2D HQSC NMR spectra of lignin; 1H NMR spectra of lignin, quercetin, and their mixtures; and fluorescence spectra of lignin in dioxane aqueous solution with increasing concentration of quercetin (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 5. Fluorescence spectra of lignin, quercetin, and their mixture in dioxane aqueous solution.

ORCID

Xueqing Qiu: 0000-0001-8765-7061

characteristic fluorescence peak at 480 nm when excited at 375 nm, while the fluorescence of quercetin could be neglected. When quercetin was added into lignin dioxane/water solution, the fluorescence intensity of lignin decreased significantly. The gradual quenching process of lignin dioxane/water solution was also observed by adding different amounts of quercetin, as shown in Figure S3. This indicates that quercetin molecules successfully inserted into lignin macromolecules and weakened

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21606089), the Guangdong Province Science and Technology Research Project 8427


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of China (2014B050505006), and The Fundamental Research Funds for the Central Universities (2015ZM149)

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Synergistic Antioxidant Performance of Lignin and Quercetin Mixtures

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