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Possibility for detecting 14 typical odorants occurring in drinking water by employing human odor-binding protein OBP2a


Odor issues occurring in drinking water have been a big challenge to face for water suppliers globally, which highly commend to develop quick or on-site odor detection tools for the management of odor problems. Olfactory sensors based on odor-binding proteins (OBPs) have been utilized to analyze pollutants in food and air samples, while their application for the detection of typical odor-causing compounds in drinking water is rarely reported, partly due to the lack of knowledge about the binding properties of odorants. In this study, the binding affinity and mechanism of human odor-binding protein OBP2a to 14 typical odorants in water were first assessed using fluorescent competitive binding assays and molecular docking techniques. The 14 odorants include 7 aldehydes, 2 terpenes, 2 thioethers, bis(2-chloro-1-methylethyl) ether (DCIP), 2-ethyl-4-methyl-1,3-dioxolane (2E4MDL), and 2-isobutyl-3-methoxypyrazine (IBMP). The results showed that OBP2a could bind to 9 odorants (Ki = 29.91 μmol/L–48.36 μmol/L), including IBMP, 2-MIB, and six aldehydes (hexanal, heptanal, benzaldehyde, 2-octenal, decanal, and β-cyclocitral), among which stronger binding affinity for aldehydes is observed (Ki = 29.91 μmol/L–43.87 μmol/L). Molecular docking confirmed that Lys112 and Phe97 are major amino acid residues involved in the binding of the most target odorants. To be specific, IBMP and aldehydes can form hydrogen bonds with Lys112; aromatic ring-containing odorants such as IBMP and benzaldehyde can also form pi–pi stacking with Phe97. The binding affinity of OBP2a to fatty aldehydes including hexanal, heptanal, 2-octenal, decanal, and β-cyclocitral increased with the increase of hydrophobicity of aldehydes. The valuable information to the binding of OBP2a to typical odorants in this study would provide a theoretical foundation for the development of OBP-based odor detection biosensors to achieve quick detection in drinking water, further helping the improvement of water treatment processes in the water industry.

Graphical Abstract


  • OBP2a has a broad binding spectrum and binds preferentially to aldehydes

  • Lys112 and Phe97 are main amino acid residues involved in binding of 14 odorants

  • Hydrogen bond contributes to the compact binding of OBP2a and aldehydes

  • Hydrophobicity of aldehydes affects significantly their binding affinity to OBP2a


Taste and odor, as an important esthetic indicator of drinking water, has become an essential factor that affects consumers’ judge on water quality and safety [1,2,3]. Water utilities have to face a big challenge or difficulty to cope with taste and odor problems in drinking water worldwide [4, 5]. In China, serious water supply crisis due to offensive odors occurred in Wuxi City in 2007 [1, 6] and Lanzhou City in 2014 [7]. Moreover, a national investigation across China showed that 80% of the sampled surface source waters exhibited some odor issues, including swampy/septic, earthy/musty, fishy and chemical odors [8, 9].

It is reported that numerous odorants with different physio-chemical properties have been identified to cause odor problems in drinking water [5]. 2-Methylisoborneol (2-MIB) and geosmin (GSM), the main earthy/musty odorants in China, have odor thresholds of 10 ng/L and 4 ng/L, respectively [10]. Pyrazines can also contribute to earthy/musty odors in drinking water at low ng/L level [11]. Thioethers including dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS), the major cause of swampy/septic odor in water crisis of Wuxi City (2007), have odor thresholds of 10 ng/L and 30 ng/L, respectively [1]. Some aldehydes could cause grassy or fishy odor problems at concentrations of hundreds of ng/L–μg/L [12, 13]. Bis(2-chloro-1-methylethyl) ether (DCIP) and 2-ethyl-4-methyl-1,3-dioxolane (2E4MDL) exhibit chemical, solvent-like odors at ng/L level [4]. 2E4MDL has been involved in several serious odor incidents worldwide [4]. These odorants are frequently detected in source water, and some of which (e.g., 2-MIB, DCIP) were also present in the corresponding finished water with concentrations up to hundreds of ng/L [5]. To prevent the public from exposure to odor in drinking water, the newly issued Standards for Drinking Water Quality of China (GB5749-2022) regulate a limit of 10 ng/L for the earthy/musty odorants 2-MIB and geosmin, and a guideline of 30 ng/L for swampy/septic odorants DMDS and DMTS in appendix A. The widespread occurrence of odor problems in drinking water, low odor thresholds, as well as the requirements of odorless water from standards, highlight a growing need to develop rapid or on-site odorant detection tools for quick response to employ applicable measures or management strategies to cope with taste and odor problems in drinking water industry.

In general, the instruments of gas chromatography–mass spectrometry (GC–MS) or gas chromatography–triple quadrupole tandem mass spectrometry (GC–MS/MS) were employed for the simultaneous quantification of various odorants in water [5, 14]. However, these methods usually need complex pre-treatment and expensive instruments, which make impossible for quick or on-site detection [15, 16]. In recent years, with the continuous exploration and recognition on olfactory system and odor perception mechanism [17,18,19], some bionic olfactory biosensors have been successfully developed by adopting biomaterials to detect various odorants in different environments [20,21,22,23]. Compared to the biomaterials of olfactory tissues, olfactory cells and olfactory receptors, odor-binding proteins (OBPs) were thought to be good candidates due to their excellent biological stability and the broad binding spectrum [20, 24]. Specifically, odor-binding proteins from organisms such as pigs, cattle, and rats have been reported for the detection of ligands including octenol, carvone, and alcohol with detection limits down to femtomolar levels [23, 25,26,27].

For the olfactory perception, OBPs are thought to play a mediating role by providing hydrophobic pockets that bind odor molecules and transport them to olfactory receptors [24]. OBPs are abundantly expressed in the mucus of various species and can be divided into insect OBPs and mammalian OBPs [28], of which, two related OBPs have been identified in humans: OBP2a and OBP2b [28, 29]. OBP2a is mainly expressed in the nasal epithelium, lacrimal glands, and salivary glands, while OBP2b is expressed in the prostate and mammary glands [29]. Not only have they been identified as relevant transporter proteins in olfactory recognition, but they can also act as scavengers to neutralize toxic volatiles or to prevent saturation of odor receptors [30]. Previous studies have shown that OBP2a has a strong affinity for medium and long-chain fatty acids and aldehydes, and has been adopted for biosensor development [31, 32]. In contrast, the application of OBP2a for the detection of typical odorants in drinking water was limited, partly due to the lack of knowledge about the binding properties with odorants. It would be essential to assess the binding capacity and mechanism with OBPs for further development of biosensors to detect odorants.

In the present study, 14 typical odorants in drinking water were selected to assess the binding characteristics and mechanism with human odor-binding protein OBP2a. The 14 odorants include seven aldehydes, two terpenoids, two thioethers, bis(2-chloro-1-methylethyl) ether, 2-ethyl-4-methyl-1,3-dioxolane, and 2-Isobutyl-3-methoxy pyrazine (IBMP), which are either frequently detected in raw and finished drinking water, or involved in some odor incidents previously. Fluorescence competitive binding assay was employed to determine and screen the odorants with good binding capacity to OBP2a. Then by using molecular docking, the binding modes and mechanism of OBP2a to these odorants were first characterized. This study provides a theoretical basis for further development of olfactory protein-based biosensors for monitoring and early warning of odor issues, which will benefit the odor problem control and management in water industry.

Materials and methods

Chemicals and reagents

All odorant standards were obtained from Beijing Ruizhihanxing Technology Co. (China) at a minimum purity of 95% (most > 99%, see catalogue numbers and brand names of standards in Additional file 1: Table S1). Chromatographic-grade methanol was obtained from Fisher Scientific Co. (China). The fluorescent probes including N-phenyl-1-naphthylamine (1-NPN), 1-aminoanthracene (1-AMA) and 8-anilino-1-naphthalene sulfonic acid (ANS) were bought from Sigma-Aldrich Co. (USA) (purity > 97%). The fluorescent probes and all odorant standards were dissolved in methanol for a 1 mmol/L stock solution. Bicinchoninic acid assay (BCA) protein assay kit and phosphate buffered solution (PBS) of pH = 7.4, were purchased from Beijing Solarbio Science & technology Co. Ltd (China). Ultrapure deionized water (> 18 MΩ cm) was produced with a Milli-Q purification system. The detailed information for the 14 odorants is shown in Table 1.

Table 1 The detailed information for the 14 odorants [5, 12]

Expression and purification of OBP2a

The expression and purification of the active recombinant OBP2a was cloned from the full-length cDNA of humans in Cellregen Biotech Co., Ltd (China). Briefly, a DNA sequence encoding the OBP2a was cloned into the expression vector pET30a (+), transferred into E. coli BL21 (DE3) cells and expressed at 37 °C with induction of IPTG [33]. Harvested host cells are sonicated and centrifuged to obtain supernatant, which were purified by Ni affinity chromatography. Next, the molecular mass of recombinant OBP2a was determined to be 19 kDa by sodium dodecyl sulfate—polyacrylamide gel electrophoresis (SDS-PAGE) analysis (Additional file 1: Fig. S1). The concentration of recombinant OBP2a was determined to be 400 μg/mL by BCA, which was further dispensed into 1.5-mL Eppendorf tubes in a buffer of NaHCO3 (20 mM, pH 8.3) and stored at −20 °C.

Fluorescent competitive binding assays

Fluorescence competitive binding assays were performed on a fluorescence spectrophotometer F-7000 (Hitachi, Japan) in a 1-cm light path quartz cuvette. The slit widths used for excitation and emission were 10 nm.

Fluorescent probes including 1-NPN, 1-AMA, and ANS were tested for binding OBP2a and the procedure was as follows: OBP2a, fluorescent probe and a mixture of the two with equal masses (2 μmol/L) were scanned at an excitation wavelength of 290 nm to verify an interaction between the two. Then, a 2 μmol/L solution of recombinant protein in 100 mmol/L PBS, pH 7.4, was titrated with aliquots of 1 mmol/L fluorescent probe solutions in 2 μL increments, resulting in a final concentration of 2–20 μmol/L. The OBP2a/1-NPN, OBP2a/1-AMA and OBP2a/ANS mixture were excited at 337 nm, 256 nm and 378 nm, respectively, and the corresponding emission spectra are 350–600 nm, 270–550 nm, and 380–600 nm, respectively. The maximum saturation fluorescence intensity was determined by a non-linear regression between fluorescent probe concentration and the corresponding maximum fluorescence intensity using GraphPad Prism 7.0 software. The fitting model is "one-site specific binding", assuming that the protein is 100% active and the binding stoichiometry in the saturated state is 1:1 (protein: ligand). The binding constant of OBP2a to the fluorescent probe was calculated using Scatchard equation: B/F = (Bmax/Kd)–(B/Kd), where B is the concentration of receptor-ligand specific binding, F is the free ligand concentration, Kd is the binding constant and Bmax is the maximum saturation concentration of ligand binding (the total amount of receptor).

The binding constant of each fluorescent probe was calculated, and one with lower binding constant was selected to measure the binding affinity of OBP2a to 14 odorants. A mixture of equal masses of OBP2a and probe (2 μmol/L) was prepared for the assessment of binding properties of odorants, and the mixture was left to stand for two minutes prior to the determination of the maximum fluorescence intensity. For each odorant, 15 concentration levels (5–75 μmol/L) with each level differing 5 μmol/L were fully reacted and the maximum fluorescence intensity was recorded for each gradient. Three replicates were performed for each odorant. The dissociation constant of OBP2a to each odorant was calculated using the equation: Ki = IC50/(1 + [probe]/Kd), where [probe] is the free concentration of fluorescent probe, Kd is the binding constant of OBP2a to fluorescent probe and IC50 is the concentration of ligand added when the fluorescence intensity is reduced by half, which was calculated using non-linear regression (log [inhibitor] vs. normalized response, variable slope) (GraphPad Prism v.7.0).

Molecular docking

The three-dimensional structure of OBP2a with two mutations (99, from Cys to Ser; 112, from Lys to Asn) was downloaded from Protein Data Bank ( [34] as described by Arne Skerra et al. [29] (PDB entry code, 4run). Pymol software is utilized to prepare OBP2a structure without above amino acid site mutation for molecular docking analysis.

Schrödinger-Maestro v11.9, a protein preparation wizard was used to prepare and refined the OBP2a structure. The predicted protein was pre-processed as follows: bond order and charges are redistributed; missing hydrogen atoms are added and co-crystallized water molecules are removed. The pre-processed protein was optimized and minimized using the force field OPLS3e with the maximum heavy atom root-mean-square deviation to 0.30 Å [35].

The 2D structure of 14 odorants (ligands) was retrieved from PubChem online database ( [36]. The LigPrep tool embedded in Maestro was used to further prepare the minimized 3D structure. Force field OPLS3e is used to minimize ligand energy [37,38,39]. Complete stereoisomers at most 32 per ligand were generated by retaining specified chiralities in the desalting process.

The Deep Site, an online server for protein binding site prediction, was used to determine the active sites of the OBP2a. The docking was performed in Extra Precision mode (XP). Different docking scores were obtained by computing the synthetic effects of electrostatic forces, van der Waals forces, hydrophobic interactions, hydrogen bonds, and other non-covalent bonds [40, 41] as well as antagonistic effects with amino acid residues. Ten different poses were considered for each ligand, and the best pose was selected based on interaction mode and docking score to evaluate the binding affinity between OBP2a and odorant.

Results and discussion

Characterization of the binding of OBP2a to 1-NPN

The fluorescence spectra of OBP2a, probes (1-NPN, 1-AMA, ANS), and a mixture of equal amounts of OBP2a and one probe at an excitation wavelength of 290 nm are shown in Fig. 1 and Additional file 1: Fig. S2. The maximum emission wavelength of OBP2a were 328 nm (line a). The maximum emission wavelengths of 1-NPN, 1-AMA and ANS were 425 nm, 527 nm, and 479 nm (line b), respectively. The fluorescence intensity of OBP2a decreased and fluorescence burst occurred after mixing OBP2a and probes, and the fluorescence maximum emission peak of probes was blue-shifted (line c), indicating that there was an interaction between OBP2a and the probes, and an energy transfer occurred [42].

Fig. 1
figure 1

Fluorescence spectra of OBP2a (pink line), 1-NPN (light green line) and OBP2a/1-NPN mixture (2 μmol/L:2 μmol/L) (dark green line)

The binding curves of OBP2a with 1-NPN, 1-AMA and ANS are shown in Fig. 2 and Additional file 1: Fig. S2. Compared to 1-AMA and ANS, 2 μmol/L 1-NPN could generate a relative strong fluorescent signal (fluorescent intensity of 1474); the fluorescence intensity rapidly increased with the addition of 1-NPN and reached saturation at 12 μmol/L. The binding constants of 1-NPN, 1-AMA and ANS were determined to be 2.28, 14.76, 34.76, respectively. A lower binding constant reflects a stronger affinity between OBP2a and fluorescence probe. Thus, 1-NPN was selected for assays on the binding of OBP2a to candidate odorants (Fig. 2).

Fig. 2
figure 2

Binding curve and Scatchard plot (embedded plot) of the fluorescent probe 1-NPN with OBP2a

Binding properties of OBP2a to 14 odorants

The competitive binding curves and dissociation constants (Ki) of the 14 odorants with OBP2a are shown in Fig. 3 and Table 2, respectively. Good reproductivity for the binding of OBP2a and odorants were observed (relative standard deviations < 10%). A smaller Ki indicates a stronger binding affinity between OBP2a and the corresponding odorant. As indicated, OBP2a has a broad binding capacity for odorants, with dissociation constants for the 14 odorants ranged from 29.91 μmol/L to 63.06 μmol/L, among which 9 odorants including DMTS, IBMP, 2-MIB and six aldehydes (hexanal, heptanal, benzaldehyde, 2-octenal, decanal and β-cyclocitral) can reduce fluorescence intensity by more than 50%. In particular, OBP2a selectively binds to aldehydes with no more than 10 carbon atoms (Ki, 29–43 μmol/L), and the strongest binding capacity of aldehydes was for 2-octenal (Ki, 29.91 μmol/L) followed by decanal (Ki, 31.59 μmol/L) and β-cyclocitral (Ki, 34.79 μmol/L), which was in consist with literature that OBP2a appears to be specific for aldehyde-based compounds [28]. In contrast, OBP2a showed weak binding to 2-MIB and IBMP with dissociation constants of 43.89 μmol/L and 48.36 μmol/L, respectively, and almost no binding activity for geosmin, DCIP, DMDS, undecanal and 2-ethyl-4-methyl-1,3-dioxolane.

Fig. 3
figure 3

Competitive binding curves of 14 odorants towards recombinant OBP2a

Table 2 Binding affinities of 14 odorants to OBP2a

Analysis of binding modes and mechanism based on molecular docking

The predicted three-dimensional structure of wild-type OBP2a is shown in Fig. 4. OBP2a adopts the typical lipocalin structure, with a brief α-helix segment near the C-terminus and a β-barrel arrangement forming a basket around a binding cavity [43]. A stable hydrophobic cavity is formed by the hydrophobic and aromatic side chains of the amino acids.

Fig. 4
figure 4

Three-dimensional structure of OBP2a (a) and analysis of binding mechanisms of OBP2a to 2-ethyl-4-methyl-1,3-dioxolane (b), 2-octenal (c), and 2-isobutyl-3-methoxy pyrazine (d). Purple arrow, hydrogen bonding; green arrow, pi–pi stacking

Table 3 illustrates the molecular docking results of OBP2a with 14 odorants. The docking score characterizes the free energy of the binding of OBP2a with an odorant, and a lower score indicates a stronger binding ability for the odorant. In general, the docking scores of 14 odorants ranged from − 6.61 to − 3.04. β-Cyclocitral (score value, − 6.61), benzaldehyde (score value, − 5.63) and 2-ethyl-4-methyl-1,3-dioxolane (score value, − 5.55) exhibit relative high docking scores.

Table 3 Molecular docking of OBP2a with 14 odorants

The binding modes and mechanism of 14 odorants are shown in Fig. 4 and Additional file 1: Fig. S3. Arg55, Lys112 and Phe97 were the dominant residues participating in the binding of most odorants. 4 of the 14 odorants, including DCIP, DMDS, DMTS and 2-ethyl-4-methyl-1,3-dioxolane, rely on hydrophobic interactions to bind to OBP2a. The remaining 10 tested odorants can generate hydrogen bonds with amino acid residues within the hydrophobic pocket of OBP2a. To be specific, the hydroxyl groups on 2-MIB and geosmin can interact with Arg55 to generate hydrogen bonds, respectively. The oxygen atom on the IBMP branch and the formyl groups of all aldehydes interact with Lys112 to generate hydrogen bonds. In particular, except hydrophobic and hydrogen bonding interactions, aromatic ring-containing odorants, e.g., IBMP and benzaldehyde, also interact weakly with the benzene ring on the side chain of phenylalanine by pi–pi stacking interaction. IBMP, containing a double nitrogen-containing six-membered heterocyclic aromatic rings, exhibits earthy/musty/sweet odor with odor threshold < 10 ng/L [11]. Molecular docking shows that IBMP can form hydrogen bonds and pi–pi stacking with Lys112 and Phe97 located deep in the binding cavity of OBP2a.


In this study, some odorants including geosmin, 2-MIB, IBMP, DMDS, DMTS and 2E4MDL with odor threshold concentrations at low ng/L level (~ 10 ng/L) exhibit weak even no affinities. The weak binding affinities of these odorants indicates that there may be other information transfer proteins assisting odor molecules to cross the hydrophilic barrier of the nasal mucus to reach olfactory receptors [44]. Another possible reason might be that 1-NPN and some odorants bind to OBP2a at different active sites within the hydrophobic pocket. Interestingly, geosmin forms hydrogen bonds with the amino acid residue Arg55, while its binding affinity was lower than that of DCIP, DMDS, DMTS and 2E4MDL with only hydrophobic interactions. The microstructure of the hydrophobic inner cavity of OBP2a interacts with odorant molecules in multiple ways that strengthen or restrict the binding of odorants.

This study demonstrates that molecular docking is a useful tool to predict the binding affinity for odorants with similar compound structures and binding modes. According to Spearman correlation analysis, molecular docking scores of 14 odorants were weakly correlated with the corresponding dissociation constants (n = 14, r = 0.53) (Additional file 1: Fig. S4), while a higher positive correlation was observed for straight-chain aliphatic aldehydes, including hexanal, heptanal, 2-octenal, decanal and undecanal (n = 5, r = 0.7) (Additional file 1: Fig. S5). It was reported that Schrodinger's Glide docking module aims to predict binding patterns and make binding energy estimates based on the correct pose in the pocket, and is therefore more informative in predicting the binding affinity of substances with the same binding pattern [45]. The application of molecular docking can improve the efficiency of screening the specific binding receptor-ligand thus assisting in the development of biosensors.

The binding affinities of OBP2a to aldehydes varied significantly according to fluorescent competitive binding assays. The binding affinities for the fatty aldehydes including hexanal, heptanal, 2-octenal, decanal, and β-cyclocitral, were increased with the increase of hydrophobicity of aldehydes according to Pearson correlation analysis between dissociation constants and octanol–water partition coefficients (logKow) (n = 5, r = − 0.66) (Additional file 1: Fig. S6). These aldehydes mainly rely on hydrophobic and hydrogen bonding interactions with Lys112 in the protein binding pocket based on molecular docking. The addition of CH2 groups in aldehydes increases the hydrophobicity, thus strengthening the hydrophobic interaction [46]. Specially, the stronger affinity of benzaldehyde comes from its benzene ring structure by generating pi–pi stacking with Phe97, increasing its affinity substantially compared to other aliphatic aldehydes with higher hydrophobicity. In addition, undecanal exhibits the weakest affinity. A possible explanation was that the length of the undecanal molecule might exceed the size of OBP2a binding cavity, as described by Abduragimov, A. R. et al. (2000) [47]. The trailing CH2 group stretches in the hydrophobic cavity of the lipid transport protein as the alkyl chain lengthens, and an extremely long carbon backbone makes the molecule fold during binding or removes a portion of the odorant's structure from the protein binding cavity [47, 48].

The results of this study showed that OBP2a has a broad binding spectrum with the target odorants and binds preferentially to aldehydes, e.g., 2-octenal, decanal, β-cyclocitral, benzaldehyde, illustrating the possibility for detecting aldehydes in drinking water using OBP2a. Compared to other biomaterials, e.g., olfactory receptors, odor-binding proteins have better stability [49] and resistance [50], and are readily expressed and purified in heterologous systems [20], making them substantial advantages for the development of reproducible biosensors for the detection of odorants. Previously, various OBP-based biosensors have been developed. Lu et al. [32] developed an electrochemical impedance biosensor for the detection of benzaldehyde and docosahexaenoic acid in ng/L level using OBP2a; Sankaran et al. [51] used the odor-binding protein LUSH from Drosophila to construct a quartz crystal microbalance biosensor to detect alcohol with detection limits of < 5 mg/L. A limitation of using odor-binding proteins for the development of biosensors is their limited chemical space of known peptides suitable for chemical binding. Fluorescent competitive binding assays and molecular docking could assess the binding affinity and mechanisms of odor-binding protein to specific odorant. Protein engineering could increase the ranges of molecular recognition for odorant-binding proteins. In addition, considering OBP2a has a broad spectrum with odorants in drinking water, a comprehensive assessment including the capacity for analysis of the real water samples with different water matrices, accuracy and precision, selectivity and reproducibility, and anti-interference ability will be conducted for biosensors using OBP2a modified transducers for the detection of odorants.


  • According to fluorescent competitive binding assays, OBP2a has a broad binding spectrum to terpenes, pyrazines, thioethers, and aldehydes. Among the 14 target odorants, OBP2a showed good affinity for aldehydes.

  • Lys112 and Phe97 within the hydrophobic pocket of the protein proved to be dominant binding sites for most target odorants with high affinities. 8 odorants (IBMP and all aldehydes) interacted with Lys112 by hydrogen bonding; IBMP and benzaldehyde also produced π–π stacking with Phe97.

  • According to Pearson correlation analysis, hydrophobicity of aldehydes affects significantly their binding affinity to OBP2a.

Availability of data and materials

The data sets used and/or analyzed during the current study are available from the corresponding author on reasonable request.


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This work was supported by Funds for the National Natural Science Foundation of China (Grant Numbers 52100018, 52070185), the Fellowship of China Postdoctoral Science Foundation (2020M680709).

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Authors and Affiliations



XC: conducting experiment, analysis, writing, reviewing and editing; FQ: supervision, reviewing, editing; CW: conducting experiment, analysis, supervision, reviewing and editing; YQ: conducting experiment; YZ: conducting experiment, analysis; QG: analysis; QW: analysis; SW: Analysis; MY: supervision, reviewing, editing; JY: supervision, reviewing, editing. All authors read and approved the final manuscript.

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Correspondence to Chunmiao Wang or Jianwei Yu.

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Additional file 1:

Table S1. The Catalogue number and brand names of all standards. Figure S1. SDS-PAGE analysis of recombinant OBP2a. Figure S2. Fluorescence spectra a, c of OBP2a, fluorescence probes and OBP2a/ fluorescence probe mixture and binding curves and Scatchard plots b, d of fluorescent probes with OBP2a. Figure S3. Analysis of the binding patterns of 11 odorants with OBP2a. Figure S4. Spearman correlation analysis of docking scores and IC50. Figure S5. Relation of docking score with IC50 and dissociation constants of straight-chain aliphatic aldehydes. Figure S6. Relation of dissociation constants with molecular weight and octanol–water partition coefficient of aldehydes.

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Chang, X., Qiu, F., Wang, C. et al. Possibility for detecting 14 typical odorants occurring in drinking water by employing human odor-binding protein OBP2a. Environ Sci Eur 35, 44 (2023).

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