Green synthesis of new and natural diester based on gallic acid and polyethylene glycol

Background Antioxidant polyphenols like gallic acid (GA) and its esters called “gallates”, which have health advantages for humans, have grown in significance in maintaining a healthy lifestyle and eating a significant amount of secondary plant phytochemicals. Here, for the first time, we suggest a green synthesis of a brand-new, all-natural diester based on gallic acid and polyethylene glycol. Methods This di-gallate is created in a single step without the use of a solvent (solid-solid reaction). This reaction has a potential yield of up to 90%. The bathochromic shift of the absorption bands from 277 nm to 295 nm in the UV-VIS spectra was caused by the addition of PEG to gallic acid. To confirm the structure of this di-gallate; Fourier-transform infrared (FTIR) spectroscopy, proton and carbon nuclear magnetic resonance ( 1H and 13C NMR), the thermal stability identified by thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were all used to thoroughly analyze the manufactured product. Results and conclusions The acquired results, when compared to the literature spectrums, supported the establishment of the di-ester structure and created new opportunities for a large number of applications.


Introduction
The introduction "Our food should be our medicine and our medicine should be our food", Hippocrates' description, though thousands of years old, used to emphasize the importance of nutrition to prevent or cure disease.A sizable group of bioactive phytochemicals known as polyphenols are now receiving more and more attention from the scientific community as well as, most notably, from the general public due to their abundance in foods like fruits, vegetables, and beverages, the regular consumption of which is thought to be good for human health.In addition to their chemopreventive and anticancer potential, studies have demonstrated that polyphenols and their derivatives can reduce the risks of diabetic, cardiovascular, and neurological illnesses. 1,2Although more recently, the produce, cosmetic and pharmaceutical industries have been using "polyphenols" for their antioxidant properties.According to their chemical structure, polyphenols possess at least two benzene rings and at least one or more hydroxyl substituents characterizing the phenyl system, 3 and they are classified into four subclasses, including flavonoids, phenolic acids, stilbenes, and lignans. 4garding phenolic acids compounds, are non-flavonoid polyphenolic compounds characterized by a carboxyl group linked to phenyl ring, 5 this group includes cinnamic acids (Caffeic, ferulic and p-coumaric acids) and benzoic acids (gallic, vanillic, and syringic acids).When it comes to gallic acid (GA, 3,4,5-tri-hydroxylsbenzoic acid), is the most common hydroxybenzoic acid, that originates from plants and can be produced by acid hydrolysis of hydrolyzable tannins 6 or synthesized from shikimic acid, 7 these compound present different pharmacological activities, mostly antioxidant activity.The GA alkyl esters (gallates) are an an vital elegance of herbal phenolic compounds that may be extracted from flora or synthesized through esterification of gallic acid with the corresponding alcohol in the presence of the catalyst. 8,9These alkyl gallates, specifically those with more than seven carbon atoms in the side-chain, have greater favorable and potent activities than gallic acid itself, 10,11 that have precious biological effects, such as anti-microbial, anti-inflammatory, 12 antitumor, 13 antifungal, 14 or prevention of gastrointestinal diseases, diabetes and even cardiovascular diseases. 15In many cases, these GA alkyl esters are useful as food additives 16,17 or beauty additives. 18e to its high latent heat capacity, 19 biocompatibility, solubility in aqueous solutions, and other promising biological properties, polyethylene glycol (PEG) is a synthetic polymer, a polyether made from ethylene glycol, considered as a biopolymer, is widely used in medical and pharmaceutical applications.The strong polarity of PEGs is a result of the terminal hydroxyl and ether groups. 20,21Because the low molecular weight PEGs have more hydroxyl groups than their structure would suggest, they are less soluble in water and other solvents as their molecular weight rises.PEG is easily chemically altered and can connect to other molecules and surfaces.PEG modifies the solubility and enlarges the connected molecules' size when it is attached to other molecules. 22Polyethylene glycol esters are also used in the medical, cosmetic and food industries.It is made from polyethylene glycol and the corresponding acid.
Here, we present GA/PEG composites which combine the activities of gallic acid and polyethylene glycol « di-gallate of polyethylene glycol », through a rather simple and green esterification process and with progressed properties.The method is based on using only reagents, i.e. solvent-free reaction in a simple Erlenmeyer flasks ("solid-solid reaction").This method increases the yield of the reaction compared to the standard reaction.The structure of the synthesized di-gallate was confirmed by different analyses: FTIR, 1 H NMR, 13 C NMR, UV-Vis, TGA and XRD.We noted the absence of the acid function and the presence of the ester function confirming that the two reagents; gallic acid and polyethylene glycol are associated.The goal of such research work is to develop an efficient and rapid method of the synthesis of GA/PEG composites to be able to acquire di-gallate composites and their changeable properties.

Apparatus
Fourier Transformed Infrared (FTIR) Spectroscopy; The FTIR spectra were checked in utilizing a Brucker Tensor-27.The scan was done between 4000 and 400 cm-1.All spectra were baseline-corrected with Opus software.

Scanning Electron Microscopy (SEM);
The analysis of nanocomposites powder image was carried out Quanta FEG 250 instrument (FEI, Hillsboro).
Thermogravimetric analysis (TGA); it was carried out on a Setsys Evolution analyzer (ATG Setaram, Caluire, France) equipped with an aluminium cell, using aluminium pans to encapsulate the samples.Typically, samples were heated at a constant rate of 10 °C/min from room temperature up to 550 °C, under a helium flow of 50 mL/min.The thermal decomposition temperature was taken at the onset of significant (≥5%) weight loss from the heated sample.

Materials
The gallic acid was purchased from BioChem, Polyethylene glycol glycol 2000 (PEG, M = 2000g/mole) was obtained from Fluka, sulfuric acid was purchased from Emsure and dichloromethane was obtained from Sigma Aldrich.

Synthesis of of polyethylene glycol diester
In this work, we performed a green esterification synthesis using the solid-solid reaction.PEG-digallates was prepared from gallic acid (0.02mol) and polyethylene glycol (PEG) (0.01mol) at 110°C using sulfuric acid (0.5ml) as a catalyst for 12 hours.Thereafter, 50mL of dichloromethane was added and filtred to remove non-reagent amounts of gallic acid and polyethylene glycol.The synthetic scheme is shown in Figure 1.

Results and discussion
The solid-solid reaction can increase the yield of the esterification reaction until 20% compared with the standard esterification reactions.The final product was analyzed by IR-FT, 1 H NMR and 13 C NMR spectroscopy and compared with literature spectra.
Vibrational characterization by FTIR spectroscopy FTIR analysis was used to identify changes or structure inter¬actions caused by the addition of PEG to Gallic acid.The FT-IR spectra of gallic acid, PEG and PEG-digallate complex treated using Origin Pro (v.2018), are given in Figure 2. As proven in PEG-digallate, the peaks around 3000-3500 cm-1 represented the stretching and bending vibration of -OH, indicating the attendance of hydroxyl groups.The high absorption peaks at 2876, 1707 and 1279 cm-1 , were assigned to methylene (=CH 2 ), carboxyl (-COOH), and phenol (-OH); groups in pure PEG, respectively; at the same time as the peaks at 1609 and 1104 cm-1 were attributed to aromatic ester and ether groups. 23It is worth noting that, the FT-IR spectra of the obtained complex, pure PEG and GA are relatively similar with several other peaks observed around 1700-1000 cm-1 which are attributed to C-O bond stretching vibration and O-H bond bending vibration into gallic acid.In contrast, changes in the stretching frequency of the current groups indicated the interaction of PEG with the gallic acid functional group.

NMR analysis
Nuclear magnetic resonance (NMR) spectroscopy was used to determine the molecular structure and chemical composition of the complex PEG-digallate synthesized.
The 1 H-NMR spectra of PEG-digallate and gallic acid have been acquired in DMSO-d6 at a 5 mM concentration.As proven in Figures 3 & 4, the chemical shift of every proton for gallic acid and the obtained product were studied in DMSOd 6 earlier than and after reaction with PEG.The signals at 6.95, 8.13, and 8.31 ppm correspond to the ortho protons of the benzene ring, the para and meta hydroxyl (O-H) protons, respectively.Notably, the disappearance of the acidic group (COOH) proton signal and the presence of the polyethylene glycol proton CH2-CH2 are indicated at 3.51 ppm.Additional signals appear at 3.4 and 3.6 ppm, corresponding to protons at the ends of the polymer chains (protons c and b; Figure 3).Comparing the 1 H-NMR spectra of gallic acid and PEG-digallate, we note that at 12.26 ppm there is no signal corresponding to the -COOH proton of the acid functional group, but there is a signal corresponding to the -OH proton of the hydroxyl functional group at 8.13 and 8.31 ppm, confirming that gallic acid has reacted with the acid function and that the resinous hydroxyl function is free.Furthermore, comparing the 1 H NMR spectra of PEG-digallate and pure polyethylene glycol (From literature 24 ), we take a look at the same strong signal at 3.51 ppm, corresponding to the CH 2 -CH 2 of polyethylene glycol.
The CH 2 -CH 2 protons at the ends of the polymer chains maintain the same signal at 3.4 and 3.6 ppm.
Based at the evaluation of 13 C-NMR spectra of the obtained complex PEG-digallate and gallic acid (Figure 5 and 6), which revealed the presence of a carbon signal (O=C-O) at 167.94 ppm corresponds to the ester function.Another peak at    Comparing these results with the literature demonstrates the presence of signals for all carbons corresponding to gallic acid. 25The results in Figure 6 are compared with the 13 C-NMR spectrum of polyethylene glycol esters, showing the same strong signal at 70.24 ppm, corresponding to the CH 2 -CH 2 carbons of polyether, and the shift peak of C-O-ester carbons appearing at 60, 68 ppm. 26ray diffraction analysis X-ray diffraction (XRD) analysis was performed to examine the physical state of the obtained PEG-digallate complex.Figure 7 displays the results of the XRD examinations of gallic acid, PEG, and the composite PEG-digallates.Gallic acid was discovered to have numerous strong diffraction peaks of crystallinity between a diffraction angle of 2 θ = 7-50°in the X-ray diffractogram, indicating that it is exists as a crystalline substance.27,28 Pure PEG displayed two distinct high intensity peaks at approximately 2 θ = 19.20°and23.20°with a few minor peaks at: 2 θ =26.35, 36.18,39.84 and 45.29°, respectively.Complex development, where the typical PEG peaks of the composite PEG-DG are present with slightly decreased strength and the elimination of huge diffraction peaks, where it is no longer feasible to detect the characteristic peaks of the composite, both contributed to a partial amorphization.The findings showed that gallic acid is no longer a crystalline substance and that the PEG complex it was successfully formed into still exists in a semi-amorphous condition.

Scanning electron microscopy
Figure 8 displays SEM images of the complex PEG-Digallate, pure PEG, and gallic acid; they were taken with zooms up to 500, 50, and 10 μm, respectively.Smaller, regular-shaped crystals with a surface that appeared smooth define the pure gallic acid, which also has smaller, more crystalline forms (Figure 8, A).In contrast to the PEG-digallate composite, pure PEG has an extremely smooth and flat surface and very regular compact structure (Figure 8, B).Scanning electron micrographs reveal that the surface morphology of the PEG-digallate (Figure 8, C) composite was amorphous; this may have contributed to the absence of crystal structures (confirming the X-ray diffraction results) and may be related to the uniform dispersion of GA in the PEG matrix.

UV-Vis absorption measurement
The UV-Vis absorption spectra of PEG, GA and PEG-digallate dissolved in DMSO were recorded at room temperature, as shown in Figures 9 & 10.The absorption spectral changes of PEG-digallate in the presence and absence of PEG are shown in Figure 9. From the obtained spectrum, the absorbance maximum of gallic acid was discovered at 277 nm, which was related to the absorption of aromatic amino acids.The peaks of PEG-digallate increased with the addition of GA, and a red shift at 295 nm was noted.The outcomes suggested that polyethylene glycol and gallic acid had formed a novel combination.A transparent compound in a spectral domain, when taken in an isolated state, can become absorbent if it is put in the presence of a species with which it interacts by a mechanism of the donor-acceptor (D-A) type.According to the UV-Vis absorption  spectra of PEG in DMSO, there was no discernible absorption between 200 and 800 nm, demonstrating that pure polyethylene glycol has a relatively low total absorption in the UV region (Figure 10).Important for electrochromic applications is this observation.

Thermal analysis (TGA)
Thermal gravimetric analysis (TGA) was used to investigate the thermal degradation of PEG-digallate.The TG and DTG curves of PEG, GA, and PEG-digallate were shown in Figure 11 from room temperature to 600 C in a N2 environment at a heating rate of 10°C/min.The Figure 11 C confirms that the surface medicine was successful.PEG-digallate TG and DTG curves (Figure 11 C) differ from those of PEG and GA (Figure 11 A & B).The PEG TG and DTG curves show that PEG has a single breakdown phase that begins at 350 °C and ends about 405 °C.Thermal breakdown of PEG is anticipated to occur at both the backbone chains -C-O-and -C-C-bonds.Figure 11 (A) illustrates that in GA, which has three decomposition phases, the compound is stable up to 87°C when the first mass loss (8.27%) occurs up to 105°C due to hydration water.Gallic acid is stable beyond this temperature until 200°C, with no mass loss and no endo or exothermal processes.The second phase (36.38%) occurs between 220-264°C, and the DTA curve shows an endothermic peak (250°C).The oxidation of organic matter immediately causes the third mass loss (18.22%) and two related exothermic peaks (at 400 and 428°C, respectively).The final residue of gallic acid breakdown was 1.5% of the total original mass (carbon residue).The TG and DTG curves of PEG-digallate in Figure 11C show that the majority of weight loss occurs below 100 °C, which can be attributed to trapped water.Thermal deterioration of composites, on the other hand, occurs primarily around 210 °C and 420 °C, which can be attributed to the GA layer stably coating on PEG.By counting the weight loss from 200 °C to  700 °C, the TG curve reveals that the amount of GA coated on PEG is around 3 to 4 wt.%.Because to the reduction in surface area for alterations, the aggregated PEG-digallate explains the limited amount of coated GA.

Conclusions
The goal of using a non-solvent and environmentally friendly approach to prepare the specified composite PEG-digallate was discovered to have been achieved.This solid-solid reaction is a synthetic method that maximizes the yield of the reaction (90%) and minimizes waste of chemical reagents.A successful production of PEG-digallate was confirmed by the 1 H and 13 C NMR analyses using DMSO-d 6 as the solvent by the development of new peaks brought on by the interaction of the PEG matrix and gallic acid.The presence of the C-O ether and C-O ester carbons at the end of the chain and hydrogen bonding between the -OH and -COOH groups both appear to play a role in these interactions.Gallic acid absorbs more when PEG is added, with the highest absorption occurring at 295 nm, confirming the UV-Vis spectra of PEG-digallate.We also observed the structure and compact shape of the produced complex, which can support the amorphous composite, by XRD patterns and SEM images.Thermal analysis (TG-DTA) has been shown to be effective approaches for assessing the thermal behaviour of gallic acid.TGA investigated the temperature and enthalpy of melting and solidifying processes to demonstrate the chemical and structural stability of PEG-digllate, which could be maintained even after 200 heat cycles.As a result, this surface modification process paves the way for new applications requiring various types of PEG matrix with gallic acid.
Results and discussion (NMR analysis): "we take a look at the same …" in the scientific writing, it is better to avoid first person pronoun, i.e. we etc.

7.
Results and discussion (NMR analysis): Based at the evaluation of 13C-NMR.Replace at by on 8.
Figure 4 needs to be replaced by a high resolution image.Peak integration should be displayed in all NMR spectres.9.
Figure 5 needs to be replaced by a high resolution image 10.
I recommend moving Figure 10 to the supporting information (supplementary information).11.
Figure 11.Thermogravimetric analysis TGA of (A) gallic acid, (B) PEG and (C) PEG-digallate (please check this figure and replace it by the adequate figure).Thermogravimetric curves are not presented in this manuscript. 12.
In Thermal analysis (TGA) ( line 10 ) and in conclusion : authors used the abbreviation DTA whereas there is no DTA curves 13.

Questions
In the background: authors have mentioned "Here, for the first time, we suggest a green synthesis of a brand-new, all-natural diester based on gallic acid and polyethylene glycol "in my opinion the diester presented here is not all-natural because it is synthetized by a synthetic polymer PEG and Gallic acid (natural compound) , furthermore the synthetic method des not respect the green chemistry rules (high temperature,toxic concentrated acid) I recommend to change this statement. 1.
Introduction (para 3): "This method increases the yield of the reaction compared to the standard reaction".Could you provide one or two references? 2.
In the last paragraph of the introduction "Here, we present GA/PEG composites" this macromolecule is not a composite "see the definition of composite".Please correct this in the whole paper.

3.
Introduction : para 3 : " Here, we present GA/PEG composites which combine the activities of gallic acid and polyethylene glycol « di-gallate of polyethylene glycol »" what authors mean by activities ?Do you mean synergetic effect, if it is the case please correct the sentence? 1.
Could the authors justify the choice of reaction temperature?2.
No need to include NMR spectra of gallic acid because it is available in many works ( example: DOI:10.1371/journal.pone.0140242 1. Figure 11.Thermogravimetric analysis TGA of (A) gallic acid, (B) PEG and (C) PEG-digallate ( please check this figure and replace it by the adequate figure).There is no Thermogravimetric analysis TGA in this manuscript.

2.
In Thermal analysis (TGA) (line 10): authors used the abbreviation DTA; meanwhile there is no DTA curves.

3.
English of the paper should be corrected professionally.

4.
Please provide more details about potential applications of this macromolecule.5.

Are sufficient details of methods and analysis provided to allow replication by others? Partly
If applicable, is the statistical analysis and its interpretation appropriate?

Not applicable
Are all the source data underlying the results available to ensure full reproducibility?Yes Are the conclusions drawn adequately supported by the results?Yes Competing Interests: No competing interests were disclosed.
Reviewer Expertise: The whole article have been read, checked and experted I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

General comments:
The manuscript contains several grammatical and technical errors.1.
The discussion part seems to be lacking in contrast with the abundance of results obtained.

Specific comments:
What is the concentration of sulfuric acid catalyst used for the synthesis of polyethylene glycol diester in the study?The mentioned volume of 0.5ml is for what total volume?Needs further clarification.

1.
Suggest mentioning the ratio of GA/PEG synthesized in the study.2.
Suggest improving the resolutions for Figure 1, Figure 4, Figure 5.

3.
Organize all figures to be after the mentioned text.4.
Change the sub-titles in the Results and Discussion to be in terms of the result from the analyses and not the name of the analysis itself.Example: 'Scanning electron microscopy' to be changed to 'Electron micrographs of the samples'.
At the conclusion section, the authors mentioned 'TGA investigated the temperature and enthalpy of melting and solidifying processes to demonstrate the chemical and structural stability of PEG-digllate, which could be maintained even after 200 heat cycles.'Is there any evidence for the claimed 200 heat cycles done?

7.
It is suggested for the authors to strengthen the discussion section by comparing the data analysis obtained from the study with previous studies reported earlier.
Please also see the attached file linked here containing additional review comments.

If applicable, is the statistical analysis and its interpretation appropriate? Yes
Are all the source data underlying the results available to ensure full reproducibility?Partly Are the conclusions drawn adequately supported by the results?Partly

Figure 7 .
Figure 7. X-ray diffraction patterns of pure gallic acid, pure PEG and the composite PEG-DG.

Figure 9 .
Figure 9. UV-Vis spectra of gallic acid and PEG-digallate in DMSO as solvent.

Reviewer Report 12
September 2024 https://doi.org/10.5256/f1000research.153173.r243716© 2024 Sivalingam A. This is an open access peer review report distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.The author(s) is/are employees of the US Government and therefore domestic copyright protection in USA does not apply to this work.The work may be protected under the copyright laws of other jurisdictions when used in those jurisdictions.Azhagu Madhavan SivalingamNatural Products & Nanobiotechnology Research Lab, Department of Community Medicine, Saveetha Medical College, Saveetha Institute of Medical and Technical Sciences, Chennai, Tamil Nadu, India 1.The abstract does not sufficiently explain the rationale behind in this study 2. The Introduction contains several disjointed statements that make it difficult to follow the main purpose of the study, and clarity of these sentences.3. The methodology mentions the use of analytical equipment and providing specific models or manufacturing and specificity makes.4. The whole phytochemical compound benefits or evaluates the significance of the findings.5.The conclusion does not elaborate on the potential mechanism.Biorefinery.2023.Publisher Full TextIs the work clearly and accurately presented and does it cite the current literature?YesIs the study design appropriate and is the work technically sound?NoAre sufficient details of methods and analysis provided to allow replication by others?YesIf applicable, is the statistical analysis and its interpretation appropriate?YesAre all the source data underlying the results available to ensure full reproducibility?YesAre the conclusions drawn adequately supported by the results?YesCompeting Interests: No competing interests were disclosed.Reviewer Expertise: Plant based nanotechnology, Pharmacology, I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.Reviewer Report 30 April 2024 https://doi.org/10.5256/f1000research.153173.r261846© 2024 Wan Azlee N.This is an open access peer review report distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.