The interaction between phenylboronic acid derivatives and active ingredients with diphenol structure of traditional Chinese medicine

Many active ingredients of traditional Chinese medicine with important pharmacological effects always have glycol or diphenol structure, which lays a foundation for the combination with phenylboronic acid (PBA) derivatives to form cyclic boronic esters compounds. Herein, four important pharmacological active ingredients, namely baicalein, baicalin, gallic acid and protocatechuic acid, were chosen to study the interaction with PBA derivatives. Five PBA derivatives of 3-aminophenylboronic acid monohydrate (APBA), 3-acrylaminophenylboronic acid (AAPBA), poly(3-acrylaminophenylboronic acid) (PAAPBA), poly([poly(ethylene glycol) methacrylate-block-3-acrylaminophenylboronic acid]) (PEbPB), and poly[poly(ethylene glycol) methacrylate-random-3-acrylaminophenylboronic acid] (PErPB) were used. The interactions between five PBA derivatives and four active ingredients were explored by fluorescent spectrophotometer using the alizarin red (ARS) method. The fluorescent intensity of PBA derivative-ARS-active ingredient mixture was decreasing with the increasing concentrations of active ingredients. In comparison, the fluorescent intensity of PAAPBA, PEbPB, and PErPB showed an obviously decrease after active ingredients were added, while the fluorescent intensity of APBA and AAPBA showed a gradually decrease after active ingredients were added. These results indicated a stronger interaction between PBA polymers and active ingredients than that of APBA and AAPBA. Simultaneously, PEbPB and PErPB could enhance cellular uptake of baicalin in A549 cells. This research provided new strategies for improving the bioavailability and water solubility, extending the circulation time, and wider application of the active ingredients of traditional Chinese medicine in the prevention and therapy of diseases.


Introduction
It is well known that many traditional Chinese medicines with proven curative effects have been widely used in clinical practice and have made indelible contributions to human health (Nahin et al. 2007;Wang et al. 2016;Takayama and Iwasaki 2017;Yu et al. 2017). Under the general trend of international "herbal fever" in the world, active ingredients with special pharmacological effects of traditional Chinese medicine have attracted much attention owing to their pivotal role in the prevention and treatment of diseases (Yang et al. 2013;Liu et al. 2015).
Although many active ingredients of traditional Chinese medicine have important pharmacological effects, their inherent shortcomings limit their wide application. For example, baicalein shows antibacterial (Palierse et al. 2021), antiviral (Lalani et al. 2020), anticancer effects (Bie et al. 2017). However, baicalein has poor water solubility and is easily oxidized. Therefore, taking effective measures to improve water solubility and protect from oxidation is of great significance for the wide application of baicalein in the treatment of diseases.
Phenylboronic acid (PBA) derivatives are well known to combine with polyol and diphenol compounds to reversibly form cyclic boronic esters (Lorand and Edwards 1959; Van et al. 1984;Sadasivuni et al. 2013;Govindaraj et al. 2017). For example, Melavanki et al. investigated the interaction between boronic acids and sugar, and study the effect of structural change of sugars on binding affinity (Bhavya et al. 2020;Melavanki et al. 2020). As shown in Fig. 1, boronic acid is weak Lewis acid, and it will convert into tetrahedral boronate after boronic acid reacts with water. This process is responsible for the ability of boronic acid to bind reversibly with diols and other Lewis bases (Bhavya et al. 2020;Ni et al. 2012). The reversible boronic ester could be facilitated in the drug delivery systems. For example, phenyboronic acid and catechol can form boronate esters in a drug delivery system that can improve the stability and in response to acidic pH values and cis-diols (Li et al. 2014(Li et al. , 2012. The unique chemistry of PBA derivatives makes a foundation to design PBA derivatives-based drug delivery systems, which can greatly enhance molecular interactions by multivalent effects with polyol or diphenol compounds (Lan and Guo 2019). Many active ingredients have glycol or diphenol structure, which makes a chance to circumvent their own shortcomings by combining with PBA derivatives in drug delivery systems.

Synthesis of AAPBA
AAPBA was prepared using the modified method described by Lee et al. (2004). APBA (5.0 g, 3.22 × 10 -2 mol), NaHCO 3 (5.0 g, 6.0 × 10 -2 mol), THF (15 mL), and H 2 O (30 mL) were added to a round bottom flask in an ice bath. Acryloyl chloride (5.0 mL, 5.0 × 10 -2 mol) was added dropwise over 0.5 h, then the mixture was reacted for 2 h at 25 °C. Ethyl acetate was added to extract production three times and then the organic layer was concentrated under vacuum. The resulting solid residue was dissolved in distilled water (80 °C), and the insoluble residues were filtered off. Filtrate was kept at 4 °C overnight, then AAPBA was obtained as crystals.

Synthesis of PAAPBA
PAAPBA was synthesized through reversible additionfragmentation chain transfer (RAFT) polymerization using AAPBA as a monomer, AIBN as initiator, and CPADB as RAFT agent. AAPBA (955 mg, 5 mmol), CPADB (6.9 mg, 0.1 mmol), and AIBN (3.28 mg, 0.02 mmol) were dissolved in DMF (1.5 mL). The mixture was reacted at 70 °C for 24 h after purging with argon for 30 min. The tube was doused into an ice bath for 5 min to terminate the reaction. PAAPBA was obtained after precipitation into ethyl acetate, washed, and dried.

Preparation of polymer nanoparticles
Polymer nanoparticles were prepared using nanoprecipitation method. Briefly, 10 mg of polymers (PAAPBA, PEbPB, and PErPB, respectively) was dissolved in DMSO (1 mL). Deionized H 2 O (10 mL) was added dropwise under ultrasound. Then, the solution was transferred to a dialysis tube (MWCO 3500) and dialyzed against deionized H 2 O for 24 h. The organic solvent was removed by replacing deionized H 2 O every 3 h. Finally, three nanoparticles were obtained after freeze-drying the suspension, and they were simply named as NPS PAAPBA , NPS PEbPB , and NPS PErPB , respectively.

Study of the interaction between PBA derivatives and ARS
APBA, AAPBA, NPS PAAPBA , NPS PEbPB , and NPS PErPB , respectively, were diluted to different concentrations from 0.01 to 1.25 mmol/L. ARS (0.15 mmol/L) was added to the above solution with the same volume. The interaction

The interaction between PBA derivatives and active ingredients
PBA derivatives, ARS and four active ingredients molecules (baicalin, baicalein, gallic acid, and protocatechuic acid) were added to a series of tubes. The final concentrations of PBA derivatives and ARS were 1 mmol/mL and 0.1 mmol/ mL. Four active ingredients were diluted to obtain the final concentrations from 0.0 to 9.0 mmol/L in tubes, respectively. The interaction between PBA derivative and active ingredients was measured by a fluorescent spectrophotometer.

Cellular uptake
To evaluate cellular internalization, NIH3T3 and A549 cells, respectively, were seeded in 24-well plates at a density of 5 × 10 4 cells/well and incubated overnight. Then cells were co-incubated with baicalin, PEbPB@baicalin, and PErPB@ baicalin, respectively. After 6 h, cells were washed three times with PBS buffer, fixed with 4% paraformaldehyde, and sequentially stained with DAPI. The cellular uptake was determined by fluorescent microscopy.

Results and discussion
Preparation and characterization of PBA derivatives we prepared AAPBA and three polymers (PAAPBA, PEbPB, and PErPB) through RAFT polymerization using CPADB as chain transfer agent, and AAPBA/PEGMA as monomers (Fig. 2). The structures of AAPBA and three polymers were evaluated by 1 H NMR as shown in Fig. 3. In Fig. 3A of AAPBA spectrum, peaks at 5.7-6.5 ppm were assigned to protons on the double bond, and signals at 7.3 ~ 7.9 ppm were assigned to protons on the benzene ring. Compared with the spectrum of AAPBA, signals of the double bond in PAAPBA (Fig. 3B) completely disappeared, and peaks of phenyl group at 6.8 ~ 7.9 ppm were retained, confirming the successful preparation of PAAPBA. Simultaneously, the new resonance signals in PAAPBA at 0.8 ~ 2.4 assigned to protons from the main chain appeared. After copolymerization of PAAPBA and PEGMA, the new resonance signals appeared at 3.2 ~ 3.7 ppm assigned to protons from methylene in side chains of PEGMA, indicating PEbPB was synthesized successfully (Fig. 3C). In Fig. 3D, there were

The interaction between PBA derivatives and ARS
PBA derivatives can combine with ARS reversibly (Fig. 4A) (Springsteen et al. 2001). The bonding of ARS with boronic acid groups can cause changes in fluorescence intensity, which can be used as a universally applicable indicator to analyze the interaction between boric acid and diol/diphenol molecules (Li et al. 2014;Takahashi et al. 2018;Gennari et al. 2017). Compared with the weak fluorescence of ARS, the ARS-boronic acid compound showed strong fluorescence emission. When diol/diphenol molecules were added, the fluorescent intensity weakens due to the original ARS-boronic acid balance was broken.
The interaction between five PBA derivatives and ARS is studied by UV (Fig. 4) and fluorescent spectrophotometer (Fig. 5). In Fig. 4B and S1, UV spectra of APBA, ARS and APBA-ARS complex were measured. Compared with APBA and ARS, the absorption of APBA-ARS complex obviously increased at about 450 nm. The absorption at about 450 nm was increasing over the increasing concentrations of APBA (from 0.010 to 1.250 mmol/L) (Fig. 4C). These results indicated that APBA and ARS interacted and formed APBA-ARS compound. In Figs. 4D, E, there were similar phenomenon and similar peak types to APBA owing to the similar structure between AAPBA and APBA. These results indicated that AAPBA could interact with ARS and form AAPBA-ARS compound. For three polymers of PAAPBA (Figs. 4F, G), PEbPB (Figs. 4H, I), and PErPB (Figs. 4J, K), there were similar peak location and types among the three polymers, but different from that of APBA and AAPBA. These differences may be ascribed to the obvious different structures of three polymers to APBA and AAPBA. Compared with the spectra of polymers and ARS, the spectra of polymer-ARS compounds showed obvious absorption at 500-600 nm, and the absorption intensity of polymer-ARS compound obviously increased with the increasing concentrations of three polymers (the concentrations of AAPBA in the polymer are from 0.010 to 1.250 mmol/L). Simultaneously, the peak type changed from a single peak to double peak over the increasing concentrations of polymers. To dissolve polymers, PAAPBA, PAAPBA, and PAAPBA were dissolved in an alkaline solution. In alkaline solution, UV absorption spectrum of ARS was the double peak ( Figure S2). With the increasing concentrations of polymers, the concentration of alkaline solution increased, which led to the UV absorption spectrum changing from a single peak to double peak. Above all, these results indicated that three polymers could interact with ARS and form polymer-ARS compounds. Figure 5 showed the fluorescent spectra of APBA, AAPBA, PAAPBA, PEbPB, and PErPB. It was observed that all the fluorescent intensity increased with the increasing PBA derivatives concentrations with similar peak location at about 600 nm, indicating that five PBA derivatives can interact with ARS. For the fluorescent peak results, APBA and AAPBA showed similar phenomenon and three polymers showed similar phenomenon, which maybe resulted from their different structures. For APBA and AAPBA, the molecules existed alone in the solution and could freely combine with ARS. For PAAPBA, PEbPE, and PErPB, PBA molecules existed in the chain of polymers and its motion was limited by the polymer chain. Notably, there was a different pattern of fluorescent spectra and peak values at low and high concentrations of three polymers. According to Flory-Krigbaum dilute solution theory, molecular chains are far apart and independent of each other. In the dilute polymer solution, polymer monomolecular chain coils exist in isolation, and there is no overlap between each other. In a concentrated solution, polymer chains are close to each other or even penetrate each other, and physical cross-linking (entanglement) or interaction occurs between the molecular chains. Therefore, at lower concentrations of three polymers, polymers existed as a free chain. At higher concentrations, the polymer formed into nanoparticle, and PBA molecules gathered into the core of nanoparticles. Above all, these results indicated that five PBA derivatives were capable of combining with ARS.

The interaction between PBA derivatives and active ingredients
In recent years, active ingredients of traditional Chinese medicine have attracted much attention on the prevention and treatment of diseases (Liang et al. 2017;Kosuru et al. 2018;Sun et al. 2020). Many active ingredients own diphenol structures, which lays the foundation for complex PBA derivatives. The unique chemistry of PBA derivatives with diphenol compounds provides a way to design PBA-based drug delivery system. Herein, four common and important pharmacological active ingredients, namely baicalin (Fig. 6A), baicalein (Fig. 6B), gallic acid (Fig. 6C),  (Fig. 6D), were used to study the interaction with PBA derivatives. All four active ingredients show many important pharmacological effects. Baicalin has been used in the clinic for the treatment of pneumonia, cerebral ischemia, hepatitis, infection and tumors due to its antibacterial, anti-inflammatory, antihypertensive, anti-allergic properties. (Liang et al. 2017) Gallic acid has good anti-inflammatory and antioxidant effects and good pharmacological effects on cardiovascular system, nervous system diseases, diabetes, liver fibrosis, tumors, etc. (Khan et al. 2018;Bai et al. 2021;Choubey et al. 2018). Protocatechuic acid is a natural phenolic acid, which widely exists in our daily diet and herbs. Protocatechuic acid plays important roles in antioxidant, anti-inflammatory, anti-tumor, antiviral, and neuroprotective effects, protection from metabolic syndrome, and preservation of liver, kidneys, and reproductive functions (Krzysztoforska et al. 2019;Kakkar and Bais 2014;Song et al. 2020).
The interaction between PBA derivatives and ARS was in dynamic equilibrium (Springsteen et al. 2001). When diol/ diphenol molecules were added to PBA-ARS system, the fluorescent intensity weakened due to the original ARSboronic acid balance being broken (Fig. 7A). To verify the binding action between phenylboronic acid and four active ingredients, APBA and PErPB were chosen to measure the relative contents of free Boronic and boronate complex using 11 B NMR spectra. As shown in Fig. 7, all the 11 B NMR spectra consist of two peaks: free boronic peak and boronate complexed peaks. Compared with APBA, the boronate complexed peak intensity was higher for APBA-four compounds (Fig. 7B-E). The 11 B NMR spectra of PErPB and PErPB-four compounds showed similar phenomenon to APBA (Fig. 7F-I). These results indicated that there was more boric acid ester formed after four compounds were added into APBA and PErPB, respectively. Based on these results, we guessed that APBA, AAPBA, PAAPBA, PEbPB, and PErPB could bind with baicalin, baicalein, gallic acid, and protocatechuic acid. To study the interaction between PBA derivatives and baicalin, fluorescent spectra were measured using a fluorescent spectrophotometer and results were shown in Fig. 8. In Figs. 8A (Figs. 8N,  O), the fluorescent intensity obviously decreased (from hundreds to dozens) after baicalin was added. The results may be due to the stronger interaction between baicalin and PBA derivatives. The interaction between PBA derivatives and ARS led to high fluorescent intensity. After baicalin was added, the interaction between PBA derivatives and ARS was destroyed and replaced. Owing to the concentration of baicalin was much high than ARS, most PBA derivatives bound with baicalin leading to the fluorescent intensity decreased from hundreds to dozen. These results indicated that there was obvious interaction between baicalin and five PBA derivatives.
To study the interaction between PBA derivatives and baicalein, fluorescent spectra were measured using a fluorescent spectrophotometer and results were shown in Fig. 9. In Figs (Figs. 9N, O), the fluorescent intensity obviously decreased (from hundreds to dozens) after baicalein was added, indicating that strong interaction between four PBA derivatives and baicalein.
To study the interaction between PBA derivatives and gallic acid, fluorescent spectra were measured using a fluorescent spectrophotometer and results were shown in Fig. 10. In Figs. 10A,D,G,J, M, no fluorescence was found for the mixture of PBA derivatives and gallic acid, or the mixture of ARS and gallic acid. Figures 10B,E,H,K, N showed the fluorescent spectra of PBA derivatives-ARS-gallic acid mixture with the changing concentrations of gallic acid. In Figs. 10B, C, the fluorescent intensity decreased regularly  (Figs. 10N, O), the fluorescent intensity obviously decreased after gallic acid was added, indicating the strong interaction between these four PBA derivatives and gallic acid.
To study the interaction between PBA derivatives and protocatechuic acid, fluorescent spectra were measured using a fluorescent spectrophotometer and results were shown in Fig. 11. In Figs. 11A,D,G,J, M, no fluorescence was found for the mixture of PBA derivatives and protocatechuic acid, or the mixture of ARS and protocatechuic acid. Figures 11B,E (Figs. 11N, O), the fluorescent intensity decreased regularly over the increasing concentration of protocatechuic acid. In Figs. 11K, L, the fluorescent intensity obviously decreased after protocatechuic acid was added, indicating the strong interaction between PErPB and protocatechuic acid. In comparison, there were different changing regular of fluorescent spectra and fluorescent peak values for all the active ingredients with PBA derivatives. This phenomenon may be ascribed to the different structures of four active ingredients and five PBA derivatives. Generally, three polymers showed stronger interaction with four active ingredients than APBA and AAPBA.

Cellular uptake
Drug delivery systems have received much attention in recent years, especially for cancer treatment. Drug delivery system is an interface between the patient and the drug. In addition to improving the pharmacokinetics of the loaded poorly soluble hydrophobic drugs, the drug delivery system allowed cancer-specific drug delivery by inherent passive targeting phenomena and adopted active targeting strategies. Therefore, drug delivery system can improve the efficacy and safety of drugs by controlling the rate, time, and place of release of drugs in the body (Kumari et al. 2016). To explore the drug delivery capacity of phenylboronic acid derivatives, PEbPB and PErPB were chosen to act as the carriers. Baicalin with green fluorescence was chosen as a drug model. Baicalin was loaded into PEbPB and PErPB nanoparticles, respectively. Then these drug-loaded nanoparticles were co-incubated with NIH3T3 and A549 cells for 6 h, respectively. The cellular uptake of baicalin were shown in Fig. 12. For NIH3T3 and A549 cells, baicalin showed stronger green fluorescence in PEbPB@baicalin and PErPB@baicalin groups than that in baicalin groups, implying that PEbPB and PErPB could enhance cellular uptake of baicalin. In addition, there was stronger green fluorescence for A549 than NIH3T3, and there were many green fluorescent dots around HUVEs cells. The results implied that PEbPB and PErPB could decrease cellular uptake of baicalin in NIH3T3 and enhance cellular uptake of baicalin in A549 cells. The phenomenon may be beneficial to design drug delivery system to treat cancer disease.

Conclusion
Herein, we used five PBA derivatives of APBA, AAPBA, PAAPBA, PEbPB, and PErPB to study the interaction with active ingredients of traditional Chinese medicine. The chosen active ingredients with important pharmacological effect were baicalin, baicalein, gallic acid, and protocatechuic acid with diphenol structure. The interactions between five PBA derivatives and four active ingredients were explored by a fluorescent spectrophotometer. The fluorescent spectra showed that five PBA derivatives can combine with four active ingredients, testified by the decreasing fluorescent intensity over the increasing concentrations of four active ingredients. Three polymers of PAAPBA, PEbPB, and PErPB showed stronger interaction with four active ingredients than APBA and AAPBA. After adding four active ingredients, the fluorescent intensity of three polymers showed a sharp decrease compared with that of the PBA derivatives-ARS compound. Simultaneously, PEbPB and PErPB could act as carriers and load baicalin to obtain PEbPB@baicalin and PErPB@baicalin, and they all could decrease cellular uptake of baicalin in NIH3T3 cells and enhance cellular uptake of baicalin in A549 cells. This research provided new strategies for the wider application of active ingredients in the prevention and therapy of diseases by building drug delivery systems based on the interaction with PBA derivatives. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.