1 Structural Variables of Oxygen-Containing Functional Groups and the Physicochemical Properties of Lignans

Oxygen in lignan molecules can occur in different functional groups, including phenolic hydroxyls, aryl methyl ethers, methylenedioxy groups, aliphatic alcohols, carbonyl groups, lactones, and glycosides. When the functional state of an oxygen atom changes, the manner in which its lone pairs participate in conjugation and hydrogen bonding, the electron distribution of adjacent atoms, and the local spatial structure may also change, thereby further affecting the acid–base properties, redox behavior, solvation, and intermolecular interactions in the solid state.

These differences are first reflected in hydrogen-bond donor (HBD) and hydrogen-bond acceptor (HBA) properties. Phenolic hydroxyls and aliphatic alcohol hydroxyls can donate the O—H hydrogen in hydrogen bonding, and their oxygen atoms can also act as hydrogen-bond acceptors, although phenolic hydroxyl oxygens are relatively weak acceptors. Aryl methyl ethers no longer function as hydrogen-bond donors after loss of the O—H bond, but the ether oxygen can still act as a hydrogen-bond acceptor. Carbonyl oxygen is a pronounced hydrogen-bond acceptor but does not provide a hydrogen-bonding hydrogen. Lactones contain both a carbonyl oxygen and an ester single-bond oxygen, and cyclization also restricts the conformational freedom of the corresponding side chain.

Polarity primarily concerns the internal charge distribution and dipolar characteristics of a molecule; hydrophilicity reflects the tendency of a molecule to interact with water through hydrogen bonding, dipolar interactions, and other solvation interactions; actual aqueous solubility is also influenced by ionization state, crystal packing, lattice stability, temperature, and solution composition. Even when the number of oxygen atoms is the same, different functional states of those oxygen atoms can give rise to different physicochemical properties.

2 Phenolic Hydroxyl Groups: Hydrogen Bonding, Weak Acidity, and Electron Delocalization in Phenoxide Species

2.1 Phenolic hydroxyl groups participate in hydrogen bonding and acid–base equilibria

Phenolic hydroxyl groups on lignan aromatic rings have the typical Ar—OH structure. The O—H group can donate hydrogen bonds to water, alcohols, carbonyl groups, and other atoms bearing lone pairs; the lone pairs on oxygen can accept hydrogen bonds, although their basicity and hydrogen-bond accepting ability are influenced by conjugation with the aromatic ring.

Phenolic hydroxyl groups are weakly acidic:

Ar—OH ⇌ Ar—O⁻ + H⁺

In the phenoxide anion formed after deprotonation, the negative charge is distributed mainly on oxygen, while delocalization can also occur through overlap of the oxygen p orbital with the aromatic π system, with the ortho and para carbon atoms contributing to the corresponding resonance structures. Electron-donating or electron-withdrawing substituents on the aromatic ring can further alter the acidity of the phenolic hydroxyl group and the stability of the phenoxide anion.

When the solution pH approaches or exceeds the pKa of the corresponding phenolic hydroxyl group, the degree of ionization increases. Neutral phenols and phenoxide anions differ markedly in hydration, electrostatic interactions, and partitioning between two phases; consequently, the apparent solubility and distribution behavior of phenolic lignans may show pH dependence.

2.2 Stability of phenoxyl radicals affects radical reactivity

Phenolic hydroxyl groups can donate hydrogen atoms to certain radicals and form phenoxyl radicals, for example:

Ar—OH + ROO• → Ar—O• + ROOH

For many phenolic compounds, this overall reaction can proceed as hydrogen atom transfer (HAT), while the microscopic process may also exhibit proton-coupled electron transfer (PCET) characteristics. After formation of the phenoxyl radical, the unpaired electron can be delocalized between the oxygen atom and the aromatic π system. The O—H bond dissociation enthalpy (BDE) of the phenolic hydroxyl group, the aromatic substitution pattern, and the degree of resonance stabilization of the radical can all influence reaction thermodynamics and kinetics.[1,2] Hydrogen bonding between the solvent and phenolic hydroxyl groups can also alter the reaction rates of phenols with radicals.[2]

Experimental comparisons have been reported between lignan structures and radical reactivity. Eklund et al. measured the scavenging activity of 15 lignans against the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical. Lignans containing a 3,4-dihydroxyphenyl group, namely a catechol structure, exhibited relatively high radical-scavenging activity; the corresponding 3-methoxy-4-hydroxyphenyl, or guaiacyl, structures were somewhat less active. Increased benzylic oxidation decreased radical-scavenging activity in this experimental system.[3]

These results reflect radical reactivity in specific chemical systems. Radical-scavenging activity is also affected by the reaction medium, radical species, and reaction mechanism and is not equivalent to the overall redox effects observed in biological systems.

3 O-Methylation: Loss of Hydrogen-Bond Donor Ability and Changes in the Local Electronic Environment

3.1 Conversion of Ar—OH to Ar—OCH₃ changes the chemical state of oxygen

O-Methylation of a phenolic hydroxyl group can be represented as:

Ar—OH → Ar—OCH₃

After O-methylation, the original O—H bond is replaced by an O—CH₃ bond. The corresponding site no longer functions as a hydrogen-bond donor and no longer retains the proton-dissociation or O—H hydrogen-atom-transfer properties of the phenolic hydroxyl group; however, the aryl ether oxygen retains its lone pairs and hydrogen-bond acceptor ability.

This transformation also increases the local nonpolar surface area and alters the electronic and steric environment around the aryl oxygen. The methoxy oxygen remains conjugated with the aromatic ring; therefore, O-methylation simultaneously affects hydrogen-bonding properties, acid–base properties, local electron distribution, and the molecular surface.

Kato-Noguchi et al. identified (−)-matairesinol with the molecular formula C₂₀H₂₂O₆ and (−)-arctigenin with the molecular formula C₂₁H₂₄O₆.[4] The two compounds share the same dibenzylbutyrolactone core, with their principal structural difference located at the C-4 position of the aromatic ring: matairesinol bears a phenolic hydroxyl group at C-4, whereas arctigenin bears a methoxy group at the same position. Both compounds contain six oxygen atoms; however, matairesinol contains two phenolic hydroxyl groups and two methoxy groups, whereas arctigenin contains one phenolic hydroxyl group and three methoxy groups. O-Methylation at C-4 removes the O—H bond at this site, thereby eliminating one hydrogen-bond donor and one phenolic proton-dissociation site; the oxygen atom remains present as an aryl ether oxygen and can act as a hydrogen-bond acceptor.

Figure 1. Structural difference in phenolic O-methylation between matairesinol and arctigenin.

(−)-Matairesinol and (−)-arctigenin share the same dibenzylbutyrolactone core, and their principal aromatic-ring substitution difference occurs at C-4: matairesinol bears a phenolic hydroxyl group (—OH) at C-4, whereas arctigenin bears a methoxy group (—OCH₃) at C-4. Source: Kato-Noguchi et al.[4], Figure 8, licensed under CC BY 4.0.

3.2 O-Methylation simultaneously alters hydration and intermolecular interactions in the solid state

O-Methylation eliminates one O—H hydrogen-bond donor and introduces a methyl group to form Ar—OCH₃. The ability of the corresponding site to form donor-type hydrogen bonds with water is reduced, while the local nonpolar surface area increases; however, the partitioning behavior of the entire molecule remains jointly influenced by other hydroxyl, carbonyl, and ether oxygen groups as well as by the overall molecular shape.

Intermolecular interactions in the solid state also change accordingly. Removal of a phenolic hydroxyl group can weaken hydrogen bonding between the molecule and water, thereby reducing hydration at that site. Removal of the phenolic hydroxyl group may also weaken intermolecular hydrogen bonding within the crystal, lowering lattice stability and thereby reducing the lattice interactions that must be overcome for the molecule to pass from the solid state into solution.

Reduced hydration and decreased lattice stability exert effects in opposing directions on dissolution. The actual aqueous solubility after O-methylation depends on the net effects of solvation, lattice stability, ionization state, and other structural features of the molecule.

4 Methylenedioxy Groups: Bridging of Two Aryl Oxygens and Local Geometric Constraints

4.1 Methylenedioxy groups alter the functional state of two adjacent oxygen atoms

A methylenedioxy group is typically formed when two adjacent oxygen atoms on the same aromatic ring are bridged by a methylene group to form a 1,3-benzodioxole structure. Its core linkage is an —O—CH₂—O— bridge between adjacent positions on the aromatic ring, with both oxygen atoms attached to the same aromatic ring.

The sequential structural relationship among pinoresinol, piperitol, and sesamin illustrates this transformation. Ono et al. demonstrated that CYP81Q1 can catalyze the conversion of (+)-pinoresinol through piperitol to (+)-sesamin, with two methylenedioxy bridges formed sequentially during the process.[5]

Both aromatic rings of pinoresinol contain adjacent methoxy–phenolic hydroxyl combinations. Formation of one methylenedioxy group gives piperitol; subsequent formation of another methylenedioxy group on the second aromatic ring gives sesamin. As this conversion proceeds, free phenolic hydroxyl groups progressively disappear, while the aryl oxygens become fixed within methylenedioxy rings.

4.2 Methylenedioxy bridges constrain local geometry

The orientations of free phenolic hydroxyl and methoxy groups can adjust within the limits imposed by conjugation, steric effects, and intermolecular interactions. Once two adjacent aryl oxygen atoms are bridged by CH₂ to form a five-membered ring, the distance and relative orientation between the two oxygen atoms become constrained by the ring structure.

This geometric constraint can affect:

① the spatial relationship between the lone pairs of the two oxygen atoms and the aromatic π system;

② the manner in which solvent molecules approach the aryl oxygen region;

③ the steric volume around the ortho region of the aromatic ring;

④ local conformational freedom of the molecule.

The property changes that occur after formation of a methylenedioxy group therefore arise from both the disappearance of free phenolic hydroxyl groups and the local geometric constraint generated by the five-membered ring.

5 Alcohols, Carbonyl Groups, and Lactones: Side-Chain Oxidation State and Conjugated Systems

5.1 Oxidation of alcohols to carbonyl groups alters hydrogen bonding and bond dipoles

When a side-chain alcohol is oxidized, the oxidation state of the corresponding carbon atom increases. Common structural transformations include:

Primary alcohol R—CH₂OH → Aldehyde R—CHO

Secondary alcohol R₂CHOH → Ketone R₂C=O

When an alcohol is converted into an aldehyde or ketone, the O—H hydrogen-bond donor is lost, and the C—O single bond is converted into a C=O double bond. The carbonyl group has a relatively large bond dipole; the carbonyl oxygen acts as a hydrogen-bond acceptor, whereas the carbonyl carbon is electrophilic. Thus, this oxidation process simultaneously changes local charge distribution, hydrogen-bonding behavior, and chemical reactivity.

Formation of C=O strengthens local dipolar interactions, whereas loss of O—H removes one hydrogen-bond donor. These two changes can simultaneously affect polarity and solvation; therefore, the overall partitioning and dissolution behavior after alcohol oxidation also depends on the remaining functional groups and solid-state structure of the molecule.

5.2 Conversion of secoisolariciresinol to matairesinol is accompanied by lactone formation

Secoisolariciresinol contains two terminal alcohol groups in its side chain, whereas matairesinol possesses a γ-lactone structure. In their study of secoisolariciresinol dehydrogenase from Forsythia intermedia, Xia et al. demonstrated that the conversion of (−)-secoisolariciresinol to (−)-matairesinol proceeds through the corresponding lactol intermediate, followed by further oxidation to form the lactone.[6]

After lactone formation, the free alcohol hydroxyl groups originally present in the side chain no longer exist in the same form, and an ester carbonyl and an endocyclic lactone oxygen appear in the molecule. The number of hydrogen-bond donors decreases, while a carbonyl dipole and the hydrogen-bond acceptor properties of the carbonyl oxygen are introduced.

Cyclization also converts the originally open and relatively flexible side chain into a five-membered lactone ring, restricting the rotational freedom of the relevant single bonds. The oxidation state and cyclization state of the side chain therefore jointly influence molecular hydration, dipolar interactions, and solid-state packing.

5.3 α,β-Unsaturated carbonyl groups form continuous conjugated systems

When a carbonyl group is directly connected to an adjacent C=C bond, an α,β-unsaturated carbonyl system is formed:

O=C—C=C

The p orbitals of C=O and C=C overlap continuously, allowing π electrons to delocalize across the entire conjugated system. A resonance contribution can be represented as:

O=C—C=C ↔ ⁻O—C=C—C⁺

This electron delocalization distinguishes an α,β-unsaturated carbonyl group from an isolated carbonyl group and alkene. Conjugation generally alters ultraviolet absorption characteristics and gives the β-position corresponding electrophilic reactivity.

The neolignan futoenone contains a conjugated enone structure. In their original structural study, Ogiso et al. used the ultraviolet spectra of futoenone and its derivatives to suggest that an α,β-unsaturated carbonyl group is an important component of its chromophoric system.[7]

6 Glycosylation: Hydrated Surface Area, Hydrogen-Bond Networks, and Actual Solubility Behavior

6.1 Introduction of sugar groups increases hydrogen-bonding sites and solvent-accessible surface area

O-Glycosylation of lignans usually occurs at free hydroxyl groups. After formation of a glycosidic bond, the original O—H group of the aglycone is converted into a glycosidic oxygen linkage; however, the sugar moiety itself contains multiple free hydroxyl groups and an oxygen atom within the sugar ring. Consequently, the numbers of hydrogen-bond donors and hydrogen-bond acceptors, molecular weight, and polar surface area of the entire molecule all change.

Pinoresinol and pinoresinol diglucoside (PDG) can be used to compare this structural transformation. The PDG studied by Pu et al. was pinoresinol 4,4′-di-O-β-D-glucoside, in which one β-D-glucosyl group is introduced at each of the two phenolic hydroxyl positions of pinoresinol.[8]

After glycosylation:

① multiple sugar hydroxyl groups increase the number of sites capable of forming hydrogen bonds;

② molecular weight and molecular volume increase;

③ the molecular surface accessible to polar solvents expands;

④ overall conformational freedom and the solid-state hydrogen-bonding network also become more complex.

Pu et al. calculated the solvent-accessible surface area (SASA) of pinoresinol diglucoside (PDG) and pinoresinol (PINL) to be 938 Ų and 565 Ų, respectively, corresponding to an increase of 373 Ų after glycosylation.[8] Compared with pinoresinol, PDG contains two β-D-glucosyl groups. The multiple hydroxyl groups on these sugar moieties increase the number of sites capable of forming hydrogen bonds with water. The molecular surface model also shows several distinct hydrophilic regions on the surface of PDG. Glycosylation therefore alters both the solvent-accessible surface area and the hydrophilic/hydrophobic surface distribution of the molecule.

Figure 2. Hydrophilic/hydrophobic molecular surfaces of pinoresinol diglucoside and pinoresinol.

Panel A shows pinoresinol diglucoside (PDG), and panel B shows pinoresinol (PINL). Red regions represent hydrophilic surface areas, whereas blue regions represent hydrophobic surface areas. Source: Pu et al.[8], Figure 3, CC BY.

6.2 Hydration capacity and actual aqueous solubility are controlled by different factors

Pu et al. measured the kinetic solubilities of PDG and pinoresinol in phosphate-buffered saline (PBS, pH 7.4) as follows:[8]

When expressed as mass concentration in the original study, PDG was approximately 13.62 μg/mL higher than pinoresinol. When compared using the molar concentrations reported in the same paper, PDG was approximately 146 μM, whereas pinoresinol was approximately 240 μM.

The different rankings obtained with the two modes of expression arise mainly because glycosylation markedly increases the molecular weight of PDG. Mass concentration represents the mass of solute per unit volume of solution, whereas molar concentration represents the amount of substance per unit volume; the two cannot be used interchangeably when comparing solubility in terms of the number of molecules.

The study measured kinetic solubility after dilution of DMSO stock solutions into PBS. Kinetic solubility is influenced by the rate of precipitation, incubation time, and sample preparation method and has a different experimental meaning from thermodynamic equilibrium solubility measured after excess solid reaches solid–liquid equilibrium.[9]

Glycosylation increases hydrogen-bonding sites and solvent-accessible surface area, which favors hydration; actual aqueous solubility is additionally influenced by the number and type of sugar groups, glycosidic linkage positions, molecular weight, conformation, and intermolecular interactions in the crystal.

7 Oxygen-Containing Functional Groups Jointly Influence Polarity, Partitioning, and Aqueous Solubility

7.1 Solvation and solid-state stability jointly affect aqueous solubility

When a solid lignan dissolves in water, intermolecular interactions within the crystal must be overcome while water molecules simultaneously solvate the solute entering the liquid phase. Stronger interactions with water favor solvation, whereas a more stable crystal lattice increases the energetic cost of removing molecules from the solid.

Oxygen-containing functional groups can affect both processes simultaneously. For example:

① increasing the number of hydroxyl groups can increase opportunities for hydrogen bonding with water, while also potentially strengthening intermolecular hydrogen bonding within the crystal;

② O-methylation reduces the number of hydrogen-bond donors and decreases hydration at the corresponding site, while potentially weakening pre-existing lattice hydrogen bonds;

③ glycosylation introduces multiple hydroxyl groups and a larger polar surface, while substantially increasing molecular weight and potentially generating a complex solid-state hydrogen-bonding network.

Studies based on the general solubility equation (GSE) for organic nonelectrolytes also show that aqueous solubility is related to both the octanol/water partition coefficient and melting point. The revised GSE applied and validated by Ran et al. uses the octanol/water partition coefficient, Kow, and melting point as two parameters, reflecting properties related to partitioning between water and a nonpolar phase and the contribution of solid-state factors to dissolution, respectively.[10]

7.2 Ionization state changes partitioning and hydration

The octanol/water partition coefficient P primarily describes the equilibrium distribution of a neutral molecule between octanol and water. For ionizable compounds in which neutral and ionic forms coexist at a particular pH, the distribution coefficient D is generally used to describe the total partitioning of all chemical forms between the two phases.

For lignans containing phenolic hydroxyl groups, the proportion of phenoxide anions increases when the pH approaches or exceeds the pKa of the corresponding phenolic hydroxyl group. Ionic forms generally exhibit stronger hydration and electrostatic interactions and therefore partition differently from neutral forms. Consequently, the distribution coefficient of the same phenolic lignan can vary with pH.

O-Methylation removes the ionizable site associated with the corresponding phenolic hydroxyl group; most hydroxyl groups introduced through glycosylation are aliphatic alcohol hydroxyls, whose acid–base properties differ from those of phenolic hydroxyl groups. Functional-group type and ionization state therefore provide a better representation of actual aqueous-phase behavior than simply counting the total number of oxygen atoms or hydroxyl groups.

7.3 Major oxygen-containing structural transformations and their physicochemical effects

8 Representative Research Chemicals Related to Oxygen-Containing Functional Groups, Side-Chain Oxidation States, and Glycosylation of Lignans

Table 1. Lignans Related to Phenolic Hydroxyl, Methoxy, and Side-Chain Alcohol Groups

Table 2. Lignans Containing Methylenedioxy Groups, Lactones, and Multiple Oxygen-Containing Functional Groups

Table 3. Glycosylated Lignans and Comparative Aglycone Systems

Note: The products listed above are representative Aladdin research-related products. Specific applications should be determined according to product specifications, batch COAs, and the intended reaction or evaluation system. Additional product specifications, grades, and COA information can be searched on the Aladdin website using the product name/CAS number/catalog number.

References

[1] Wright JS, Johnson ER, DiLabio GA. Predicting the activity of phenolic antioxidants: theoretical method, analysis of substituent effects, and application to major families of antioxidants. J Am Chem Soc. 2001;123(6):1173–1183. DOI: 10.1021/ja002455u.

[2] Foti MC. Antioxidant properties of phenols. J Pharm Pharmacol. 2007;59(12):1673–1685. DOI: 10.1211/jpp.59.12.0010.

[3] Eklund PC, Långvik OK, Wärnå JP, Salmi TO, Willför SM, Sjöholm RE. Chemical studies on antioxidant mechanisms and free radical scavenging properties of lignans. Org Biomol Chem. 2005;3:3336–3347. DOI: 10.1039/B506739A.

[4] Kato-Noguchi H, Takahashi Y, Tojo S, Teruya T. Isolation and identification of allelopathic substances from Forsythia suspensa leaves, and their metabolism and activity. Plants. 2024;13(5):575. DOI: 10.3390/plants13050575.

[5] Ono E, Nakai M, Fukui Y, et al. Formation of two methylenedioxy bridges by a Sesamum CYP81Q protein yielding a furofuran lignan, (+)-sesamin. Proc Natl Acad Sci U S A. 2006;103(26):10116–10121. DOI: 10.1073/pnas.0603865103.

[6] Xia ZQ, Costa MA, Pelissier HC, Davin LB, Lewis NG. Secoisolariciresinol dehydrogenase purification, cloning, and functional expression: implications for human health protection. J Biol Chem. 2001;276(16):12614–12623. DOI: 10.1074/jbc.M008622200.

[7] Ogiso A, Kurabayashi M, Mishima H, Woods MC. Futoenone, a constituent of Piper futokadzura. Symposium on the Chemistry of Natural Products, symposium papers. 1967;11:221–228. DOI: 10.24496/tennenyuki.11.0_221.

[8] Pu Y, Cai Y, Zhang Q, et al. Comparison of pinoresinol and its diglucoside on their ADME properties and vasorelaxant effects on phenylephrine-induced model. Front Pharmacol. 2021;12:695530. DOI: 10.3389/fphar.2021.695530.

[9] Kerns EH, Di L, Carter GT. In vitro solubility assays in drug discovery. Curr Drug Metab. 2008;9(9):879–885. DOI: 10.2174/138920008786485100.

[10] Ran Y, Jain N, Yalkowsky SH. Prediction of aqueous solubility of organic compounds by the general solubility equation (GSE). J Chem Inf Comput Sci. 2001;41(5):1208–1217. DOI: 10.1021/ci010287z.

For more related articles, see below:

Molecular Targets, Mechanisms of Action, and Research Applications of Natural Products in the Regulation of Metabolic Disorders

Coumarins at a Glance: Structural Classification, Key Properties, and Experimental Selection (Tables 1–3)

Pyran and the “Pyran Family” Research Selection Guide: Key Structural Points, a Classification Roadmap, and Experimental Selection Navigation

Benzopyran Family at a Glance: From the Core Scaffold to Three High-Frequency Applications (Photochromism / Drug Scaffolds / Fluorescence) — with Product Selection Logic and Product Tables (Tables 1–3)

Benzofuran Heteroaromatic Building-Block Guide: From “Adding an Oxygen Atom” to More Controllable Scaffold Hops and Selection Navigation (Tables A–C)

Will glycosylation affect the biological activity of recombinant protein?

Plant proteomics and glycosylation experiments