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Article

Biocatalytic Reduction of α,β-Unsaturated Double Bonds of Curcuminoid Derivatives by Exserohilum rostratum

by
Jânison Nazareno Pastana
1,
Victória Lopes Ribeiro
2,
Mayra Suelen da Silva Pinheiro
3,
José Edson de Sousa Siqueira
2,
Luana Cardoso Oliveira
1,
Heriberto Rodrigues Bitencourt
2,
Taícia Pacheco Fill
3,
Andrey Moacir do Rosario Marinho
1,2,* and
Patrícia Santana Barbosa Marinho
1,2,*
1
The Federal University of Southern and Southeastern Pará, Folha 31, Quadra 07, Lote especial, s/n, Marabá 68507590, Brazil
2
Federal University of Pará, Rua Augusto Corrêa, 01, Guamá, Belém 66075110, Brazil
3
State University of Campinas, Avenida Oswaldo Cruz, 301, City University Zeferino Vaz, Campinas 13083876, Brazil
*
Authors to whom correspondence should be addressed.
Chemistry 2025, 7(5), 143; https://doi.org/10.3390/chemistry7050143
Submission received: 28 June 2025 / Revised: 29 August 2025 / Accepted: 31 August 2025 / Published: 3 September 2025
(This article belongs to the Section Biological and Natural Products)
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Abstract

Turmeric is a spice that has gained significant popularity in global cuisine. Beyond its culinary applications, it possesses significant medicinal properties, including antioxidant, anti-inflammatory, and antibacterial properties, which are attributed to its majority compound, curcumin. In this study, we synthesized three curcuminoid derivatives via the Claisen–Schmidt method (1E,4E)-1-(2-methoxy-phenyl)-5-(3-methoxy-phenyl)-pent-1,4-dien-3-one (2a), (1E,4E)-1-(2-methoxy-phenyl)-5-(3,4,5-trimethoxy-phenyl)-pent-1,4-dien-3-one (2b), and (1E,4E)-5-phenyl-1-(2-methoxy-phenyl)-pent-1,4-dien-3-one (2c). The synthetic compounds were hydrogenated in the olefinic double bond (CH=CH) by biotransformation catalyzed by the fungus Exserohilum rostratum given (CH2-CH2) 3a, 3b, and 3c. All compounds were identified by NMR and MS. The compounds were evaluated for their antibacterial properties against Gram-positive and Gram-negative bacteria, with the results indicating good activity, highlighting that the bioreduction from 2a to 3a led to an improvement of up to eight times in the observed activity against S. typhimurium of 250 to 31.25 µg/mL. Additionally, compounds 2a, 2b, 3a, and 3b are not previously documented in the literature.

1. Introduction

Secondary metabolites from plants and microorganisms are the focus of studies in natural product chemistry, and they are of particular interest to medicinal chemistry when it comes to the study of isolated substances and their effects on the body [1,2]. The objective of these pharmaceutical, biological, and medicinal studies is to obtain bioactive compounds through natural or synthetic means. In these studies, the metabolism and mechanisms of action at the molecular level are analyzed, and relationships are constructed between the chemical structure and pharmacological activity [3,4,5]. Curcumin (CUR) (Figure 1) is a natural ketone derived from turmeric (Curcuma longa). It has been the subject of extensive investigation in a variety of pharmacological models in vivo and in vitro. Curcumin is a widely used spice, primarily in Asia [6,7,8].
CUR has been demonstrated to possess significant antioxidant, anti-inflammatory, antibacterial, antifungal, and antiviral properties, in addition to its capacity as a cancer-chemopreventive agent [9,10,11]. This electrophilic molecule contains two Michael acceptor units (α,β-unsaturated carbonyl) that are responsible for cytoprotection against oxidative damage. In addition to these units, the molecule has hydroxyl groups that react with reactive oxygen species. These groups donate radical hydrogen, thereby neutralizing possible cellular damage resulting from the presence of these intermediates [12]. Nevertheless, the molecule’s effectiveness is constrained by its instability and low bioavailability [13]. A body of research has been conducted on the structure and activity of CUR derivatives, with the objective of producing molecules that exhibit enhanced biological activity [14]. This necessitates the continuous dedication of substantial efforts to the structural modification of CUR, with a primary focus on alterations to its β-diketone structure and the substitution pattern of its aromatic rings [11]. Dibenzalacetone (DBA) (Figure 1) is a monocarbonyl derivative of CUR that has demonstrated a variety of biological activities, including antiparasitic, anti-inflammatory, and sunscreen properties [15,16,17]. The structural modification of dibenzalacetone by microorganisms can result in the formation of bioactive compounds through environmentally friendly methods.
The utilization of microorganisms in the process of obtaining new substances from compounds of interest is predicated on the fact that these living beings employ their biological systems to produce changes in chemical compounds that are not their natural substrates [18]. These alterations, brought about by the agency of microorganisms, are designated as biotransformation. The aforementioned processes are considered environmentally sustainable due to their reduced energy consumption, decreased waste accumulation, mild operating conditions, and reduced reliance on organic solvents. In addition to producing chiral products, using not only regiospecific or stereospecific reactions but also enantiospecific reactions, from a commercial point of view, can be cheaper and more direct than using analogous chemical substances [18,19,20].
Fungi, in addition to producing a wide variety of chemical substances of interest, are also widely used as biocatalysts, especially endophytes, which deserve attention as promising sources of biocatalysts with numerous applications. These organisms are a little explored source in microbial biotransformations [21].
The present study aimed to observe the improvement in the antimicrobial activity against Gram-positive and Gram-negative bacteria of synthetics curcuminoid derivatives and their products of the bioreduction of the α,β-unsaturated double bond to carbonyl by the fungus E. rostratum. An improvement of up to eight-fold in antimicrobial activity was observed. Compounds 2a, 2b, 3a, and 3b are also new to literature.

2. Materials and Methods

2.1. Instrumental

The 1D and 2D nuclear magnetic resonance spectra were obtained on a Bruker Ascend 400 NMR spectrometer (400 MHz). The masses of the compounds were obtained using a Thermo-Fisher (Waltham, MA, USA) Orbitrap HRLC-MS spectrometer. The biotransformation reactions were carried out in a laminar flow hood (Pachane, PA320, Piracicaba, Brazil), an autoclave (Prismatec, CS-75, Carlos Gomes, Brazil), an orbital shaker (Marconi, MA 140, Piracicaba, Brazil), and a rotary evaporator (Quimis, Q344M2, Diadema, Brazil).

2.2. Synthesis of Substances

2.2.1. (3E)-4-(2-Methoxy-Phenyl)-but-3-en-2-One (1)

In a dried 250-mL Erlenmeyer flask, 50 mL (~681 mmol) of acetone (Tedia Brazil®, Rio de Janeiro, Brazil) and 10 mL of 10% sodium hydroxide solution were added and maintained under stirring at room temperature. In a dried 50-mL Beaker, 3.67 g (27 mmol) of ortho-anisaldehyde (Sigma-Aldrich Brasil, São Paulo, Brazil) was dissolved in 30 mL of methanol; this solution was then added in the Erlenmeyer flask and kept under stirring for 40 min at room temperature. During the reaction, a color change to pale yellow was observed. After, the reaction was acidified with a 20% acetic acid solution until acid pH. After, the mixture was extracted with chloroform (Tedia Brazil®) (50 mL, 3×). The organic phase was washed three times with distilled water (50 mL each) and then dried over sodium sulfate. After removal of the drying agent by filtration, the solvent was removed to obtain the product as yellow crystals (3.94 g, 22.4 mmol, 83% yield). 1H NMR (400 MHz; CDCl3) δH: 6.90 (d, 1H, H-3; J = 8.4 Hz), 7.35 (m, 1H, H-4), 6.97 (t, 1H, H-5, J = 7.7 Hz), 7.52 (d, 1H, J = 8.8 Hz, H-6), 7.87 (d, 1H, J = 16.4 Hz, H-7), 6.64 (d, 1H, J = 16.4 Hz, H-8), 3.88 (s, 3H, C2-OCH3), and 2.07 (s, 3H, C10-CH3). 13C NMR (100 MHz; CDCl3) δC: 123.2 (C1), 156.2 (C2), 111.1 (C3), 131.7 (C4), 127.7 (C5), 128.2 (C6), 138.7 (C7), 120.7 (C8), 199.1 (C9), 55.4 (C2-OCH3), and 27.0 (C10-CH3).

2.2.2. (1E,4E)-1-(2-Methoxy-Phenyl)-5-(3-Methoxy-Phenyl)-Pent-1,4-Dien-3-One (2a)

In a dried 250-mL Erlenmeyer flask 3.50 g (20 mmol), substrate 1 was dissolved in 20 mL of sodium hydroxide 10%. Then, 2.5 mL (20 mmol) of meta-anisaldehyde (Sigma-Aldrich Brasil, São Paulo, Brazil), dissolved in 20 mL of methanol, was added. The mixture was stirred at room temperature for 3 h. After, the mixture was transferred to a separatory funnel, where extraction with chloroform (Tedia Brazil®, Rio de Janeiro, Brazil) was performed (50 mL, 3×). The organic phase was washed three times with distilled water (50 mL each) and then dried over sodium sulfate. After removal of the drying agent by filtration, the solvent was removed to obtain the product as a brown viscous oil (0.76 g, 2.6 mmol, 13% yield). 1H NMR (400 MHz; CDCl3) δH: 6.93 (d, 1H, H-3; J = 8.2 Hz), 7.33 (t, 1H, H-3; J = 7.8 Hz), 6.97 (m, 1H, H-5), 7.62 (dd, 1H, H-5; J = 8.0 and 1.5 Hz), 8.08 (d, 1H, H-7; J = 16.1 Hz), 7.14 (d, 1H, H-8; J = 16.4 Hz), 3.85 (s, 3H, C2-OCH3), 7.14 (d, 1H, H-2′), 7.00 (d, 1H, H-4′, J = 7.6 Hz), 7.37 (t, 1H, H-5′; J = 8.0 Hz), 7.22 (d, 1H, H-6′; J = 7.6 Hz), 7.64 (d, 1H, H-7′; J = 15.8 Hz), 6.99 (d, 1H, H-8′; J = 15.8 Hz), 3.91 (s, 3H, C3-OCH3). 13C NMR (100 MHz; CDCl3) δC: 123.8 (C1), 158.7 (C2), 111.3 (C3), 128.9 (C4), 120.9 (C5), 130.3 (C6), 138.8 (C7), 125.8 (C8), 189.5 (C9), 55.4 (C2-OCH3), 55.6 (C3′-OCH3), 142.9 (C1′), 116.2 (C2′), 160.0 (C3′), 113.4 (C4′), 128.9 (C5′), 121.6 (C6′), 136.5 (C7′), and 131.9 (C8′). HRESIMS m/z 295.1331 [M+H]+.

2.2.3. (1E,4E)-1-(2-Methoxy-Phenyl)-5-(3,4,5-Trimethoxy-Phenyl)-Pent-1,4-Dien-3-One (2b)

In a dried 250-mL Erlenmeyer flask 3.50 g (20 mmol), 1 was dissolved in 20 mL of 10% sodium hydroxide. To this solution was added 3.92 g (20 mmol) of 3,4,5-trimethoxybenzaldehyde (Sigma®), which had initially been dissolved in 30 mL of methanol (Tedia Brazil®, Rio de Janeiro, Brazil). The mixture was stirred for 3 h. After, the mixture was extracted with chloroform in a separatory funnel (50 mL, 3×). The organic phase was washed three times with distilled water (50 mL each) and then dried over sodium sulfate. After removal of the drying agent by filtration, the solvent was removed to obtain the product as a cadmium yellow viscous material (6.74 g, 20 mmol, 95% yield). 1H NMR (400 MHz; CDCl3) δH: 6.93 (d, 1H, H-3; J = 8.2 Hz), 7.36 (t, 1H, H-4; J = 7.8 Hz), 6.97 (t, 1H, H-5; J = 7.6 Hz), 7.60 (d, 1H, H-5; J = 1.8 Hz), 8.07 (d, 1H, H-7; J = 16.1 Hz), 7.15 (d, 1H, H-8; J = 16.0 Hz), 3.89 (s, 3H, C2-OCH3), 6.84 (s, 2H, H-2′/H-6′), 7.64 (d, 1H, H-7′; J = 15.8 Hz), 6.99, (d, 1H, H-8′; J = 15.8 Hz), and 3.91 (s, 9H, C3′-OCH3/ C4′-OCH3/ C5′-OCH3). 13C NMR (100 MHz; CDCl3) δC: 123.8 (C1), 158.7 (C2), 111.3 (C3), 128.9 (C4), 120.9 (C5), 130.3 (C6), 138.7 (C7), 125.2 (C8), 189.4 (C9), 55.6 (C2-OCH3), 56.3 (C3′-OCH3/C4′-OCH3), 61.1 (C5′-OCH3), 105.7 (C2′/C6′), 143.1 (C1′), 140.3 (C7′), and 131.9 (C8′). HRESIMS m/z 355.1545 [M+H]+.

2.2.4. (1E,4E)-5-Phenyl-1-(2-Methoxy-Phenyl)-Pent-1,4-Dien-3-One (2c)

Next, 3.50 g (20 mmol) of 1 was dissolved in 20 mL of 10% sodium hydroxide solution in a dried 250-mL Erlenmeyer flask, and then 2.1 mL (20 mmol) of benzaldehyde (Sigma-Aldrich Brasil, São Paulo, Brazil), dissolved in 20 mL of methanol (Tedia Brazil®, Rio de Janeiro, Brazil), was added. The mixture was stirred at room temperature for 3 h. Subsequent to this period, the reaction was transferred to a separatory funnel, where extraction with chloroform (Tedia Brazil®, Rio de Janeiro, Brazil) was performed (50 mL, 3×). The organic phase was washed three times with distilled water (50 mL each) and then dried over sodium sulfate. After filtration, the solvent was removed to obtain the product as a cadmium yellow viscous material (2.5 g, 8.4 mmol, 42% yield). 1H NMR (400 MHz; CDCl3) δH: 6.94 (d, 1H, H-3; J = 8.2 Hz), 7.37 (m, 1H, H-4), 6.99 (t, 1H, H-5; J = 7.8 Hz), 7.62 (m, 1H, H-6), 8.09 (d, 1H, H-7; J = 16.1 Hz), 7.14 (d, 1H, H-8; J = 16.1 Hz), 3.92 (s, 3H, C2-OCH3), 7.63 (m, 2H, H-2′/H-6′), 7.41 (m, 3H, H-3′/H-4′/H-5′), 7.64 (d, 1H, H-7′; J = 15.8 Hz), and 6.99, (d, 1H, H-8′; J = 15.8 Hz). 13C NMR (100 MHz; CDCl3) δC: 123.8 (C1), 158.7 (C2), 111.3 (C3), 128.4 (C4), 120.9 (C5), 130.4 (C6), 138.8 (C7), 125.4 (C8), 189.6 (C9), 55.6 (C2-OCH3), 143.0 (C1′), 128.9 (C2′/C6′), 127.8 (C3′/C5′), 126.4 (C4′), 135.1 (C7′), and 131.9 (C8′). HRESIMS m/z 265.1230 [M+H]+.

2.3. Biotransformation Reactions

A strain of the fungus Exserohilum rostratum was cultivated in a Petri dish containing Potato-Dextrose-Agar (PDA, Kasvi®, São José dos Pinhais, Brazil) Medium [22]. For each of the biotransformation reactions, the mineral Medium Czapek broth (pH 7.3) was prepared and distributed 200 mL of Medium, per flask, into six 500 mL-Erlenmeyer and sterilized in an autoclave at 121 °C for 15 min. Three Erlermeyer flasks were utilized for the MFS (Medium + Fungus + Substrate), and the remaining Erlermeyer flasks containing Medium plus Fungus (MF), Medium (M), and Medium plus Substrate (MS) were used as controls. The Erlenmeyer flasks were placed in an orbital shaker at 32 °C and 120 rpm. On the third day of biomass growth, in sterile conditions in a laminar flux, 30 mg of the substrate solubilized in 1000 µL of dimethyl sulfoxide (Sigma-Aldrich Brasil, São Paulo, Brazil) was added to each MFS and MS flask. After the addition of the substrate, the flasks were left for 2 days under stirring. Then, filtration was employed to obtain the broths, and liquid–liquid partition was carried out with EtOAc (3 × 75 mL) (Tedia Brazil®, Rio de Janeiro, Brazil) of the aqueous solutions (M, MF, MFS, and MS). Subsequently, the aqueous phases were discarded, and the EtOAc phases were dried with anhydrous sodium sulfate (Na2SO4) (Neon®, Suzano, Brazil) and concentrated in a rotary evaporator (Quimis®, Diadema, Brazil) to obtain the reaction extracts. The organic extracts were then subjected to a TLC and 1H NMR. This procedure was repeated individually for each of the substrates 2a, 2b, and 2c.

2.4. Fractionation of EtOAc Extracts to Obtain Biotransformation Products

The MFS-EtOAc phase of the biotransformation of curcumin analogues was fractionated by CC using silica gel (Silicycle®, Quebec City, QC, Canada, 70-230 mesh) and a mix of the solvents (Tedia Brazil®, Rio de Janeiro, Brazil) hexane, EtOAc, and MeOH in an increasing polarity gradient as the stationary phase. The resulting fractions were monitored by TLC (silica gel, Silicycle®, Quebec City, QC, Canada). This procedure was made individually for each of the substrates 2a, 2b, and 2c. As a result, the biotransformation products 3a (11.2 mg, 12% yield), 3b (12 mg, 13% yield), and 3c (14.5 mg, 16% yield) were obtained.

2.4.1. 1-(2-Methoxy-Phenyl)-5-(3-Methoxy-Phenyl)-Pent-3-One (3a)

1H NMR (400 MHz; CDCl3)-δH: 6.84 (d, 1H, H-3; J = 8.5 Hz), 7.19 (t, 1H, H-4; J = 7.8 Hz), 6.89 (dd, 1H, H-5; J = 7.5 and 1.2 Hz), 7.11 (dd, 1H, H-6; J = 7.5 and 1.6 Hz), 2.87 (q, 4H, H-7/H-7′; J = 5.1 Hz), 2.70 (q, 4H, H-8/H8′; J = 5.1 Hz), 3.79 (s, 3H, C2-OCH3), 6.74 (m, 3H, H-2′/H-4′/H-6), 7.19 (t, 1H, H-5′; J = 7.8 Hz), and 3.81 (s, 3H, C3′-OCH3). 13C NMR (100 MHz; CDCl3)-δC: 129.4 (C1), 157.5 (C2), 110.3 (C3), 127.6 (C4), 120.8 (C5), 130.1 (C6), 25.2 (C7), 43.0 (C8), 209.9 (C9), 55.3 (C2-OCH3), 55.3 (C3′-OCH3), 142.9 (C1′), 159.8 (C3′), 111.5 (C4′), 129.6 (C5′), 120.8 (C6′), 29.9 (C7′), and 44.4 (C8′). HRESIMS m/z 299.1642 [M+H]+.

2.4.2. 1-(2-Methoxy-Phenyl)-5-(3,4,5-Trimethoxy-Phenyl)-Pent-3-One (3b)

1H NMR (400 MHz; CDCl3)-δH: 6.83 (d, 1H, H-3; J = 8.2 Hz), 6.87 (dd, 1H, H-4; J = 7.5 and 0.9 Hz), 7.10 (dd, 1H, H-5; J = 7.7 and 1.6 Hz), 7.18 (ddd, 1H, H-6; J = 7.9, 1.7 and 1.5 Hz), 2.88 (t, 2H, H-7; J = 7.4 Hz), 2.83 (t, 2H, H-7′; J = 7.6 Hz), 2.70 (m, 4H, H-8/H-8′), 3.81 (s, 3H, C2-OCH3), 6.38 (s, 2H, H-2′, H-6′), 3.80 (s, 3H, C3′-OCH3), and 3.83 (s, 6H, C4′-OCH3, C5′-OCH3). 13C NMR (100 MHz; CDCl3)-δC: 129.3 (C1), 157.5 (C2), 110.4 (C3), 127.6 (C4), 120.6 (C5), 130.1 (C6), 25.1 (C7), 43.1 (C8), 210.0 (C9), 55.3 (C2-OCH3), 56.2 (C3′-OCH3, C4′-OCH3), 61.0 (C5′-OCH3), 105.4 (C2′/C6′), 137.1 (C1′), 153.3 (C3′/C5′), 136.4 (C4′), 29.8 (C7′), and 44.6 (C8′). HRESIMS m/z 341.1331 [M-H2O+H]+.

2.4.3. 1-(2-Methoxy-Phenyl)-5-(Phenyl)-Pent-3-One (3c)

1H NMR (400 MHz; CDCl3)-δH: 6.89 (d, 1H, H-3; J = 8.4 Hz), 7.23 (m, 4H, H-4, H-3′, H-4′, H-5′), 6.94 (d, 1H, H-5; J = 7.4 Hz), 7.17 (d, 1H, H-6; J = 7.3 Hz), 2.76 (m, 4H, H-7, H7′), 2.94 (m, 4H, H-8, H8′), 3.87 (s, 3H, C2-OCH3), and 7.33 (m, 2H, H-2′/H-6′). 13C NMR (100 MHz; CDCl3)-δC: 129.3 (C1), 157.5 (C2), 110.3 (C3), 127.5 (C4), 120.6 (C5), 130.1 (C6), 25.1 (C7), 43.0 (C8), 210.1 (C9), 55.3 (C2-OCH3), 141.2 (C1′), 128.4 (C2′/C6′), 128.6 (C3′/C5′), 126.1 (C4′), 29.9 (C7′), and 44.5 (C8′). HRESIMS m/z 269.1547 [M+H]+.

2.5. Minimum Inhibitory Concentration (MIC) Assay

For the antimicrobial test, the compounds (2a, 2b, 2c, 3a, 3b, and 3c) were tested against B. subtilis, S. aureus, E. coli, and S. typhimurium bacteria. For each bacterium, in 96-well plates, 100 μL of BHI (Brain Heart Infusion) broth (Kasvi®, São José dos Pinhais, Brazil) was added to each well. Subsequently, 100 μL of the solutions containing the samples to be tested were added to the first well, and this solution was homogenized. Subsequent serial dilutions were performed until the penultimate well was full, given concentrations ranging from 250 to 3.91 μg/mL for each compound. The final line of the plate was utilized as a negative control. In each well, 5 μL of the bacterial suspension at 1.0 × 105 FCU/mL was added, and the plates were then incubated at 37 °C for 24 h. The results were read by adding 10 μL of 2% solution of TTC (2,3,5-triphenyltetrazolium chloride, Neon®, Suzano, Brazil). DMSO was used as a negative control and tetracycline and amoxicillin were used as positive controls.

3. Results

3.1. Synthesized Compounds

Through Claisen–Schmit condensation synthesis, three dibenzalacetone derivatives (1E,4E)-1-(2-methoxy-phenyl)-5-(3-methoxy-phenyl)-pent-1,4-dien-3-one (2a), (1E,4E)-1-(2-methoxy-phenyl)-5-(3,4,5-trimethoxy-phenyl)-pent-1,4-dien-3-one (2b), (1E,4E)-5-phenyl-1-(2-methoxy-phenyl)-pent-1,4-dien-3-one (2c) were obtained. Compounds 2a and 2b are not yet reported in the literature. All compounds were characterized by NMR and MS.
For compounds 2a, 2b, and 2c, the E configuration for the double bonds was confirmed by analyzing the signals of the hydrogen atoms H-7/H-8 and H-7′/H-8′ in the 1H NMR spectra. The E configuration was assigned based on the coupling constants J = 16 Hz of their doublets around δH 7.15–8.20, which is diagnostic for E double bonds. The signals pertaining to the hydrogens of the aromatic rings between δH 6.50 and 7.50 are also observed. Finally, the signals relating to the hydrogens of the methoxyl groups were observed at δH 3.80–3.91 (3H; s). In the 13C NMR spectra of the compounds, signals related to the carbonyl group (δC 188.0–190.0), signals related to the methoxyl groups (δC 55.0–56.0), and other signals related to the aromatic rings were observed. The structures were confirmed through the use of HSQC and HMBC correlations, as well as Mass Spectrometry data (see Supplementary Material). These data have shown that the structures are (1E,4E)-1-(2-methoxy-phenyl)-5-(3-methoxy-phenyl)-pent-1,4-dien-3-one (2a), (1E,4E)-1-(2-methoxy-phenyl)-5-(3,4,5-trimethoxy-phenyl)-pent-1,4-dien-3-one (2b), and (1E,4E)-5-phenyl-1-(2-methoxy-phenyl)-pent-1,4-dien-3-one (2c). The 2c compound was compared with the literature [23]. Products 2a and 2b are new to the literature. Scheme 1 shows the general equation for the formation of synthesis products.

3.2. Characterization of Biotransformation Products

The synthetic dibenzalacetone derivatives (2a, 2b, and 2c) were subjected to biotransformation by the fungus E. rostratum, where a biocatalytic reduction of the carbon-carbon double bond of the α,β-unsaturated carbonyl units to the corresponding saturated ketones was observed, leading to the bioreduction products 3a, 3b, and 3c, where compounds 3a and 3b are new to the literature. The structures of the compounds were determined by NMR and MS.
The biotransformation products 1-(2-methoxy-phenyl)-5-(3-methoxy-phenyl)-pent-3-one (3a), 1-(2-methoxy-phenyl)-5-(3,4,5-trimethoxy-phenyl)-pent-3-one (3b), and 1-(2-methoxy-phenyl)-5-(phenyl)-pent-3-one (3c) were characterized by 1H and 13C NMR and two-dimensional NMR experiments (HSQC, HMBC, and COSY), in addition to the use of Mass Spectrometry, were employed to demonstrate the successful acquisition of bioreduction products from the double bonds of the α,β-unsaturated ketone system (see Scheme 2).
In the 1H NMR spectra of the biotransformation products 3a, 3b, and 3c, the absence of signals for the α,β-unsaturated double bond hydrogens (H-7/H-7′ and H-8/H-8′) was observed. However, the presence of two triplet signals around δH 2.70–2.90, referring to hydrogens of adjacent methylene groups, was observed. In the 13C NMR spectrum of the biotransformation products, the absence of signals for HC=CH of the α,β-unsaturated carbonyl is observed, and a signal around δC 210.0, typical of a saturated carbonyl group, is present. Furthermore, signals for methylene carbons (δC 25 to δC 45) have been observed. These data confirm the reduction of the HC=CH double bond to the CH2-CH2 system. The HSQC and HMBC data for the bioreduction products 3a, 3b, and 3c were obtained and confirmed the structures of the compounds. The 3c NMR data were compared with the literature and show complete similarity [24]. The bioreduction products 3a and 3b are new to the literature.

3.3. Antimicrobial Activity

The curcuminoid derivatives obtained by Claisen–Schmidt synthesis (2a, 2b, and 2c) and their bioreduction products (3a, 3b, and 3c) were evaluated in vitro against Gram-positive and Gram-negative bacteria. It was observed that the reduction of the HC=CH double bond to CH2-CH2 given 2a to 3a resulted in an enhancement in antimicrobial activity of four times against S. aureus and eight times against S. typhimurium bacteria. The results are presented in Table 1.

4. Discussion

A series of studies with monocarbonyl curcumin analogues have demonstrated the potential for the discovery of therapeutic agents [15]. The synthesis of their analogues is obtained by classical and combinatorial synthesis methods but also by biotransformation processes using living microorganisms or isolated enzymes causing structural modifications. The principal synthetic pathway is through the Claisen–Schmidt condensation reaction. Dibenzalacetone (DBA) is a curcumin analogue that is synthesized in high yield from acetone and benzaldehyde in a basic Medium at room temperature. It has the potential to treat several types of diseases [16,17]. In this study, we obtained three asymmetric derivatives of DBA, two of which are novel (2a and 2b) and have not been previously described in literature. The biotransformation of DBA and its derivatives by microorganisms can result in the formation of bioactive products through environmentally friendly methods. The fungus E. rostratum is a thermophilic microorganism that is ubiquitous in soils and plants [25]. It has been associated with virulence and resistance to antifungal agents [26]. We investigated the capacity of the fungus E. rostratum as a biotransformation agent, examining its potential to produce novel molecules from monocarbonyl derivatives of curcumin. Our findings demonstrated that the fungus exhibited chemoselective reduction properties, effectively hydrogenating the HC=CH double bond of the α,β-unsaturated carbonyl system without reducing the C=O double bond. The bioreduction process resulted in the formation of three compounds, two of which were identified as novel (3a and 3b). In addition to the formation of new molecules using the microorganism E. rostratum, a significant improvement in the activities of the bioreduction products in relation to their precursors was also observed. The results obtained in the present work corroborate the data obtained by us in previous works where we showed that the fungus E. rostratum behaves as a bioreducing agent in chalcone derivatives [27]. The amount of research has demonstrated that biotransformation products have enhanced antimicrobial activities, as evidenced by the work of Hao et al. [28]. In their study, the researchers biotransformed the flavonoids naringenin and hesperidin, and as a result, the products obtained exhibited a significant improvement in the antimicrobial activity of the compounds. The monoterpene α-Phellandrene was also biotransformed by a series of microorganisms, and the major product obtained, 5-p-menthene-1,2-diol, demonstrated notable antimicrobial activity [29]. Furthermore, the biotransformation of acanthoic acid by Xylaria sp. enhanced the activity against E. coli by eightfold [30].
The tested compounds showed good antimicrobial activity. It was observed that the bioreduction process of the CH=CH double bond improved the antimicrobial activity, with an emphasis on the reduction of the CH=CH bond of 2a forming 3a (CH2-CH2); for the bacterium S. aureus, a four-fold improvement in activity was observed, while for the bacterium S. thyphimurium, the result of bioreduction was even more significant, with an eight-fold improvement in activity observed from 2a to 3a.
The synthesized curcuminoid derivatives have two aromatic rings linked by an α,β-unsaturated carbonyl system. Fungal biotransformation by E. rostratum reduced the HC=CH double bonds, disrupting conjugation and increasing molecular flexibility due to greater rotational freedom around the C-7/C-8 and C-7′/C-8′ bonds. This reduction significantly altered the activity of the compounds; the greater conformational flexibility likely facilitates better binding to enzymes crucial for bacterial survival, leading to increased efficacy. However, further studies aiming to unravel a possible structure–activity relationship using computational techniques, as well as studies that aim to observe the improvement in cellular permeability by bioreductions, still need to be carried out, so that we can reach a better conclusion on how the improvement in activity observed by us is affected by the structure–activity relationship. These data allow us to conclude that biotransformation by microorganisms is a viable approach for the synthesis of bioactive compounds.

5. Conclusions

The compounds (1E,4E)-1-(2-methoxy-phenyl)-5-(3-methoxy-phenyl)-pent-1,4-dien-3-one (2a), (1E,4E)-1-(2-methoxy-phenyl)-5-(3,4,5-trimethoxy-phenyl)-pent-1,4-dien-3-one (2b), and (1E,4E)-5-phenyl-1-(2-methoxy-phenyl)-pent-1,4-dien-3-one (2c) were obtained in good yield via Claisen–Schmidt condensation. In biotransformation reactions, the fungus E. rostratum exhibited chemoselective reduction of α,β-unsaturated double bonds in curcuminoid derivatives given the compounds 1-(2-methoxy-phenyl)-5-(3-methoxy-phenyl)-pent-3-one (3a), 1-(2-methoxy-phenyl)-5-(3,4,5-trimethoxy-phenyl)-pent-3-one (3b), and 1-(2-methoxy-phenyl)-5-(phenyl)-pent-3-one (3c). The compounds were evaluated in vitro against Gram-positive and Gram-negative bacteria, exhibiting good activity, particularly the bioreduction of 2a to 3a and demonstrated improvement of up to eight times in the antibacterial activity. Compounds 2a, 2b, 3a, and 3b have not been previously documented in the literature. The work demonstrates that the acquisition of compounds through bioreduction methods by microorganisms is a promising strategy for the synthesis of new bioactive compounds.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/chemistry7050143/s1, Figure S1: 1H NMR spectrum compound 2a (CDCl3, 400 MHz); Figure S2: 13C NMR spectrum compound 2a (CDCl3, 100 MHz); Figure S3: HMBC spectrum compound 2a; Figure S4: HRESIMS (+) fullscan spectrum compound 1; Figure S5: 1H NMR spectrum compound 2b (CDCl3, 400 MHz); Figure S6: 13C NMR spectrum compound 2b (CDCl3, 100 MHz); Figure S7: HMBC spectrum compound 2b; Figure S8: HMBC spectrum compound 2b; Figure S9: HRESIMS (+) fullscan spectrum compound 2b; Figure S10: 1H NMR spectrum compound 2c (CDCl3, 400 MHz); Figure S11: 13C NMR spectrum compound 2c (CDCl3, 100 MHz); Figure S12: HSQC spectrum compound 2c; Figure S13: HMBC spectrum compound 2c; Figure S14: HRESIMS (+) fullscan spectrum compound 2c; Figure S15: 1H NMR spectrum compound 3a (CDCl3, 400 MHz); Figure S16: 13C NMR spectrum compound 3a (CDCl3, 100 MHz); Figure S17: HSQC spectrum compound 3a; Figure S18: HMBC spectrum compound 3a; Figure S19: HRESIMS (+) fullscan spectrum compound 3a; Figure S20: 1H NMR spectrum compound 3b (CDCl3, 400 MHz); Figure S21: 13C NMR spectrum compound 3b (CDCl3, 100 MHz); Figure S22: HSQC spectrum compound 3b; Figure S23: HMBC spectrum compound 2b; Figure S24: HRESIMS (+) fullscan spectrum compound 3b; Figure S25: 1H NMR spectrum compound 3c (CDCl3, 400 MHz); Figure S26: 13C NMR spectrum compound 3c (CDCl3, 100 MHz); Figure S27: HSQC spectrum compound 3c; Figure S28: HMBC spectrum compound 3c; and Figure S29: HRESIMS (+) fullscan spectrum compound 3c.

Author Contributions

Conceptualization, H.R.B., A.M.d.R.M. and P.S.B.M.; methodology, J.N.P., J.E.d.S.S., H.R.B., T.P.F., A.M.d.R.M. and P.S.B.M.; validation, V.L.R., J.N.P., M.S.d.S.P. and L.C.O.; formal analysis, A.M.d.R.M. and P.S.B.M.; investigation, V.L.R., J.N.P., M.S.d.S.P., J.E.d.S.S. and L.C.O.; resources, H.R.B., T.P.F., A.M.d.R.M. and P.S.B.M.; data curation, V.L.R., J.N.P. and M.S.d.S.P.; writing—original draft preparation, A.M.d.R.M. and P.S.B.M.; writing—review and editing, A.M.d.R.M. and P.S.B.M.; visualization, J.N.P., A.M.d.R.M. and P.S.B.M.; supervision, A.M.d.R.M. and P.S.B.M.; project administration, A.M.d.R.M. and P.S.B.M.; and funding acquisition, A.M.d.R.M. and P.S.B.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Council for Scientific and Technological Development (CNPq), grant number 308631/2021-8; 310540/2022-4 and 406269/2022-0.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors express their gratitude to the Dean of Research and Postgraduate Studies at UFPA (PROPESP/UFPA), the Federal University of Southern and Southeastern Pará (Unifesspa), and Coordination for the Improvement of Higher Education Personnel—CAPES.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
CCColumn Chromatography
COSYCOrrelated SpectroscOpy
CURCurcumin
DBADibenzalacetone
EtOAcEthyl acetate
HMBCHeteronuclear Multiple Bond Correlation
HREIMSHigh-Resolution Electron Ionization Mass Spectrometry
HRLC-MSHigh Resolution Liquid Chromatography-Mass Spectrometry
HSQCHeteronuclear Single Quantum Coherence
MMedium
MFMedium plus Fungus
MFSMedium plus Fungus plus Substrate
MICMinimum Inhibitory Concentration
MSMedium plus Substrate
NMRNuclear Magnetic Resonance
PDAPotato Dextrose Agar

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Chemistry 07 00143 g001
Figure 1. Chemical structure of curcumin (CUR) and dibenzalacetone (DBA).
Figure 1. Chemical structure of curcumin (CUR) and dibenzalacetone (DBA).
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Chemistry 07 00143 sch001
Scheme 1. Products obtained in Claisen–Schmidt synthesis reactions.
Scheme 1. Products obtained in Claisen–Schmidt synthesis reactions.
Chemistry 07 00143 sch001
Chemistry 07 00143 sch002
Scheme 2. Biotransformation reaction of compounds 2a, 2b, and 2c giving 3a, 3b, and 3c.
Scheme 2. Biotransformation reaction of compounds 2a, 2b, and 2c giving 3a, 3b, and 3c.
Chemistry 07 00143 sch002
Table 1. Result of antimicrobial activity.
Table 1. Result of antimicrobial activity.
SubstanceMIC (µg/mL)
Gram-PositiveGram-Negative
B. subtilisS. aureusE. coliS. typhimurium
2a>250250>250250
2b>250250>25062.50
2c250250>250125
3a>25062.50>25031.25
3b>250125>25062.50
3c>250>250>25062.50
Tetracycline3.913.913.913.91
Amoxicillin3.913.913.913.91
DMSO>250>250>250>250
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MDPI and ACS Style

Pastana, J.N.; Ribeiro, V.L.; Pinheiro, M.S.d.S.; Siqueira, J.E.d.S.; Oliveira, L.C.; Bitencourt, H.R.; Fill, T.P.; Marinho, A.M.d.R.; Marinho, P.S.B. Biocatalytic Reduction of α,β-Unsaturated Double Bonds of Curcuminoid Derivatives by Exserohilum rostratum. Chemistry 2025, 7, 143. https://doi.org/10.3390/chemistry7050143

AMA Style

Pastana JN, Ribeiro VL, Pinheiro MSdS, Siqueira JEdS, Oliveira LC, Bitencourt HR, Fill TP, Marinho AMdR, Marinho PSB. Biocatalytic Reduction of α,β-Unsaturated Double Bonds of Curcuminoid Derivatives by Exserohilum rostratum. Chemistry. 2025; 7(5):143. https://doi.org/10.3390/chemistry7050143

Chicago/Turabian Style

Pastana, Jânison Nazareno, Victória Lopes Ribeiro, Mayra Suelen da Silva Pinheiro, José Edson de Sousa Siqueira, Luana Cardoso Oliveira, Heriberto Rodrigues Bitencourt, Taícia Pacheco Fill, Andrey Moacir do Rosario Marinho, and Patrícia Santana Barbosa Marinho. 2025. "Biocatalytic Reduction of α,β-Unsaturated Double Bonds of Curcuminoid Derivatives by Exserohilum rostratum" Chemistry 7, no. 5: 143. https://doi.org/10.3390/chemistry7050143

APA Style

Pastana, J. N., Ribeiro, V. L., Pinheiro, M. S. d. S., Siqueira, J. E. d. S., Oliveira, L. C., Bitencourt, H. R., Fill, T. P., Marinho, A. M. d. R., & Marinho, P. S. B. (2025). Biocatalytic Reduction of α,β-Unsaturated Double Bonds of Curcuminoid Derivatives by Exserohilum rostratum. Chemistry, 7(5), 143. https://doi.org/10.3390/chemistry7050143

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