Abstract

Marein, systematically named 4′-(β-D-glucopyranosyloxy)-2′,3,3′,4-tetrahydroxychalcone and possessing the molecular formula C₂₁H₂₂O₁₁, represents a naturally occurring chalconoid glucoside with a molar mass of 450.39 grams per mole. This compound functions as an anthochlor pigment, exhibiting characteristic yellow coloration in biological systems. The molecular structure consists of an okanin aglycone moiety glycosidically linked to a β-D-glucopyranose unit at the 4′-hydroxy position. Marein demonstrates moderate water solubility due to its glycosidic nature and exhibits typical phenolic reactivity including acid-base properties and susceptibility to oxidative transformations. The compound's spectroscopic profile includes characteristic UV-Vis absorption maxima between 380-420 nanometers and distinctive NMR chemical shifts that facilitate structural identification. Found primarily in Coreopsis maritima, marein serves as a model compound for studying chalconoid glycoside chemistry and natural pigment behavior.

Introduction

Marein constitutes a significant member of the chalconoid glycoside class, a subgroup of flavonoid derivatives characterized by their open-chain structure and frequent occurrence as plant secondary metabolites. As the 4′-O-glucoside of okanin, marein represents a biologically relevant conjugation product that modifies the solubility and reactivity of the parent chalcone. The compound's systematic name, 2′,3,3′,4-tetrahydroxy-4′-{[(2''S'',3''R'',4''S'',5''S'',6''R'')-3,4,5-trihydroxy-6-(hydroxymethyl)oxan-2-yl]oxy}chalcone, precisely describes its stereochemical configuration and functional group arrangement. Chalconoid glycosides like marein participate in various biochemical pathways and contribute to plant coloration mechanisms through their anthochlor pigment characteristics. The structural features of marein, including multiple phenolic hydroxyl groups and a glycosidic linkage, provide interesting case studies for investigating hydrogen bonding networks, electronic delocalization, and glycoside hydrolysis kinetics.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Marein exhibits a well-defined molecular architecture consisting of two primary components: the chalcone-derived okanin aglycone and the β-D-glucopyranose moiety. The chalcone framework demonstrates a trans-configuration about the ethylene bridge, with the two aromatic rings adopting a nearly coplanar arrangement due to conjugation with the carbonyl group. Bond angles at the carbonyl carbon approximate 120 degrees, consistent with sp² hybridization, while the glycosidic linkage exhibits tetrahedral geometry with bond angles near 109.5 degrees. The glucopyranose unit maintains the characteristic chair conformation (⁴C₁) typical of β-D-glucosides, with all hydroxyl groups in equatorial positions except the anomeric center.

Electronic structure analysis reveals extensive conjugation throughout the chalcone system, with the highest occupied molecular orbital (HOMO) primarily localized on the electron-rich phenolic rings and the lowest unoccupied molecular orbital (LUMO) concentrated on the carbonyl and ethylene functionality. The HOMO-LUMO gap measures approximately 3.5 electronvolts, corresponding to the compound's absorption characteristics in the near-UV region. Resonance structures involving the carbonyl group and adjacent ethylene bond contribute to charge delocalization, while intramolecular hydrogen bonding between the 2′-hydroxy and carbonyl groups stabilizes the planar conformation. The glucosyl moiety does not participate significantly in the conjugated system but influences solubility and intermolecular interactions.

Chemical Bonding and Intermolecular Forces

Covalent bonding in marein follows predictable patterns for chalconoid glycosides, with carbon-carbon bond lengths in the aromatic rings measuring between 1.38-1.42 angstroms and carbon-oxygen bonds ranging from 1.36-1.43 angstroms. The glycosidic C-O bond length measures 1.43 angstroms, typical for β-glucosidic linkages. Bond dissociation energies for phenolic O-H bonds approximate 86 kilocalories per mole, while the glycosidic bond requires approximately 73 kilocalories per mole for homolytic cleavage.

Intermolecular forces dominate marein's solid-state behavior and solution properties. The compound exhibits extensive hydrogen bonding capacity through its eight hydroxyl groups (three phenolic, four alcoholic, and one anomeric), with hydrogen bond strengths ranging from 4-8 kilocalories per mole. Dipole-dipole interactions contribute significantly to molecular association, with a calculated molecular dipole moment of approximately 4.2 Debye resulting from the polarized carbonyl group and multiple hydroxyl functionalities. Van der Waals forces influence packing in the crystalline state, while π-π stacking interactions between chalcone systems occur at distances of 3.5-3.8 angstroms. The compound's calculated octanol-water partition coefficient (log P) of -0.82 indicates moderate hydrophilicity, primarily due to the glucosyl moiety.

Physical Properties

Phase Behavior and Thermodynamic Properties

Marein typically presents as a yellow crystalline solid at ambient conditions, with crystal morphology varying from needle-like to prismatic structures depending on crystallization conditions. The compound melts with decomposition between 195-205 degrees Celsius, with the exact decomposition temperature dependent on heating rate and sample purity. No boiling point is reported due to thermal instability at elevated temperatures. The density of crystalline marein measures 1.52 grams per cubic centimeter, as determined by X-ray crystallography.

Thermodynamic parameters include a heat of fusion of 28.5 kilojoules per mole and a heat of combustion of -8950 kilojoules per mole. The specific heat capacity at constant pressure measures 1.2 joules per gram per degree Kelvin at 25 degrees Celsius. Solubility characteristics demonstrate marked dependence on solvent polarity, with water solubility of approximately 5.2 milligrams per milliliter at 20 degrees Celsius. Solubility increases significantly in polar organic solvents such as methanol (42 milligrams per milliliter) and dimethyl sulfoxide (180 milligrams per milliliter) but remains low in non-polar solvents like hexane (0.02 milligrams per milliliter). The refractive index of solid marein is 1.65 at 589 nanometers.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic absorption bands corresponding to functional groups present in marein. The carbonyl stretch appears at 1645 reciprocal centimeters, while phenolic O-H stretches produce broad absorption between 3200-3400 reciprocal centimeters. Alcoholic O-H stretches from the glucosyl moiety appear at 3350 reciprocal centimeters, and aromatic C-H stretches occur near 3050 reciprocal centimeters. The glycosidic C-O-C vibration produces a distinctive band at 1070 reciprocal centimeters.

Proton nuclear magnetic resonance spectroscopy in deuterated dimethyl sulfoxide shows the following characteristic chemical shifts: chalcone vinyl protons at 7.65 ppm (d, J = 15.5 Hertz, H-α) and 7.72 ppm (d, J = 15.5 Hertz, H-β); aromatic protons between 6.20-7.85 ppm; anomeric proton at 5.10 ppm (d, J = 7.2 Hertz, H-1′); and glucosyl protons between 3.20-3.85 ppm. Carbon-13 NMR signals include the carbonyl carbon at 192.5 ppm, chalcone ethylene carbons at 144.8 ppm (C-α) and 122.5 ppm (C-β), aromatic carbons between 115-165 ppm, and glucosyl carbons with the anomeric carbon at 101.2 ppm.

UV-Vis spectroscopy in methanol solution shows absorption maxima at 212 nanometers (ε = 18,500 liters per mole per centimeter), 258 nanometers (ε = 12,300 liters per mole per centimeter), and 388 nanometers (ε = 22,800 liters per mole per centimeter). Mass spectrometric analysis exhibits a molecular ion peak at m/z 450.39 and characteristic fragment ions at m/z 288 [M-glucose]⁺, 153 [A-ring + carbonyl]⁺, and 135 [B-ring]⁺.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Marein demonstrates reactivity patterns characteristic of both phenolic compounds and glycosides. The phenolic hydroxyl groups undergo typical acid-base reactions with pKa values of 7.2 (2′-OH), 8.9 (3-OH), 9.4 (3′-OH), and 10.1 (4-OH), as determined by potentiometric titration. Glycosidic hydrolysis follows first-order kinetics with respect to marein concentration, with a rate constant of 3.2 × 10⁻⁵ per second at pH 7.0 and 25 degrees Celsius. Acid-catalyzed hydrolysis proceeds via specific acid catalysis with kH⁺ = 0.18 liters per mole per second at 25 degrees Celsius.

Oxidative reactions proceed readily due to the electron-rich nature of the phenolic system. Hydrogen peroxide oxidation follows second-order kinetics with k₂ = 8.7 liters per mole per second at pH 7.4 and 25 degrees Celsius, producing quinoid intermediates that subsequently polymerize. Photochemical degradation under UV irradiation (300-400 nanometers) follows pseudo-first-order kinetics with a quantum yield of 0.03 at 350 nanometers. Thermal decomposition above 195 degrees Celsius proceeds through multiple pathways including glycosidic cleavage, chalcone isomerization to flavanone, and oxidative coupling reactions.

Acid-Base and Redox Properties

The acid-base behavior of marein reflects its multiple ionizable groups, with buffer capacity maximized between pH 7.0-10.5. Titration experiments reveal four distinct equivalence points corresponding to the four phenolic hydroxyl groups. The compound exhibits greatest stability in the pH range 5.0-7.0, with degradation rates increasing significantly outside this range. Protonation occurs primarily at the carbonyl oxygen under strongly acidic conditions, with a protonation constant of 2.3.

Redox properties include a standard reduction potential of +0.71 volts versus the standard hydrogen electrode for the quinone/semiquinone couple. Cyclic voltammetry shows two reversible one-electron oxidation waves at +0.45 volts and +0.68 volts, corresponding to sequential oxidation of the ortho-dihydroxy systems. The compound demonstrates antioxidant activity through hydrogen atom transfer mechanisms, with a bond dissociation energy of 78.5 kilocalories per mole for the 2′-O-H group. Electrochemical oxidation produces stable radical intermediates that dimerize through C-C coupling at the 3-position.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of marein typically employs either total synthesis from appropriate precursors or enzymatic glycosylation of okanin. The most efficient chemical synthesis begins with 2,4,6-trihydroxyacetophenone and 2,3,4-trihydroxybenzaldehyde through Claisen-Schmidt condensation. The condensation reaction proceeds in ethanol-water mixture (4:1 v/v) with sodium hydroxide catalyst (2.0 molar equivalents) at 0-5 degrees Celsius for 4 hours, yielding okanin with 65-70% efficiency after recrystallization from aqueous methanol.

Glycosylation of okanin employs protected glucose donors under Koenigs-Knorr conditions. The preferred method uses acetobromoglucose (1.2 molar equivalents) with silver carbonate (2.5 molar equivalents) as promoter in anhydrous dichloromethane at room temperature for 12 hours, achieving 55-60% yield of protected marein. Subsequent deprotection with sodium methoxide in methanol (0.1 molar) at 0 degrees Celsius for 30 minutes provides marein with overall yield of 35-40% from okanin. Purification typically involves column chromatography on silica gel with ethyl acetate-methanol-water (100:16.5:13.5 v/v/v) as eluent, followed by crystallization from aqueous acetone.

Industrial Production Methods

Industrial production of marein relies primarily on extraction from natural sources, particularly Coreopsis maritima, rather than synthetic routes due to economic considerations. The extraction process employs ethanol-water mixtures (70-80% ethanol v/v) at 50-60 degrees Celsius for 4-6 hours, followed by filtration and concentration under reduced pressure. The crude extract undergoes purification through column chromatography using polyamide or Sephadex LH-20 media, with final purification by preparative high-performance liquid chromatography using C18 stationary phase and water-methanol gradient elution.

Process optimization focuses on maximizing yield while minimizing degradation, with typical production scales of 100-500 grams per batch. Economic analysis indicates production costs of approximately $120-150 per gram for purified marein, primarily due to chromatographic purification steps. Environmental considerations include solvent recovery systems with >95% recovery efficiency and waste stream treatment through anaerobic digestion. Current production volumes remain limited to laboratory and pilot plant scales due to specialized applications rather than bulk industrial use.

Analytical Methods and Characterization

Identification and Quantification

Identification of marein employs multiple complementary techniques to confirm structural identity and isomeric purity. High-performance liquid chromatography with diode array detection provides reliable separation from related chalconoids using C18 columns (250 × 4.6 millimeters, 5 micrometer particle size) with mobile phase consisting of 0.1% formic acid in water (A) and 0.1% formic acid in acetonitrile (B) in gradient mode: 0-5 minutes 10% B, 5-25 minutes 10-50% B, 25-30 minutes 50-100% B. Retention time typically falls between 18.5-19.2 minutes under these conditions.

Quantitative analysis utilizes external standard calibration with UV detection at 388 nanometers, providing a linear range of 0.1-100 micrograms per milliliter with correlation coefficients exceeding 0.999. The limit of detection measures 0.03 micrograms per milliliter and the limit of quantification is 0.1 micrograms per milliliter. Method validation demonstrates accuracy of 98-102% recovery and precision with relative standard deviation less than 2% for intra-day analysis and less than 3% for inter-day analysis. Alternative quantification methods include mass spectrometric detection using selected ion monitoring of m/z 450.2→288.1 transition, which provides improved specificity for complex matrices.

Purity Assessment and Quality Control

Purity assessment of marein requires evaluation of multiple parameters including chemical purity, isomeric purity, and absence of specific impurities. Chemical purity determination by HPLC typically exceeds 98% area percentage for reference standard material. Common impurities include okanin (0.5-1.0%), marein isomers with different glycosylation patterns (0.2-0.8%), and decomposition products such as quinoid derivatives (0.1-0.5%). Isomeric purity confirmation requires chiral chromatography to verify the β-configuration of the glycosidic linkage, with Chirpak IC-3 columns (150 × 4.6 millimeters, 3 micrometer particle size) using acetonitrile-water (85:15 v/v) with 0.1% formic acid as mobile phase.

Quality control specifications for reference standard material include loss on drying not more than 2.0% at 105 degrees Celsius, residue on ignition not more than 0.2%, and heavy metals content not more than 20 parts per million. Spectroscopic conformity requires UV-Vis spectrum in methanol showing λmax at 388 ± 2 nanometers with A388/A258 ratio of 1.82-1.88. Stability studies indicate that marein remains stable for at least 24 months when stored at -20 degrees Celsius in amber glass containers under inert atmosphere, with degradation not exceeding 5% under these conditions.

Applications and Uses

Industrial and Commercial Applications

Marein serves primarily as a reference compound and research chemical rather than finding extensive industrial application. Its use as a chromatographic reference standard for identification and quantification of chalconoid glycosides represents the most significant commercial application. Specialty chemical suppliers provide marein for research purposes at purity levels from 95% to 99%, with annual global production estimated at 5-10 kilograms. The compound's intense yellow coloration suggests potential as a natural dye, though economic factors limit commercial exploitation for this purpose.

In analytical chemistry, marein functions as a model compound for studying glycoside hydrolysis kinetics and chalconoid reactivity patterns. Its well-characterized spectroscopic properties make it useful for method development in HPLC-DAD and LC-MS analysis of phenolic glycosides. The market for marein remains highly specialized, serving primarily academic and research institutions rather than industrial consumers. Pricing reflects the compound's specialty status, with costs ranging from $100-500 per milligram depending on purity and quantity.

Research Applications and Emerging Uses

Research applications of marein center on its role as a representative chalconoid glucoside for fundamental studies of glycoside chemistry and natural product behavior. Investigations include mechanistic studies of glycosidic bond cleavage under various conditions, photochemical behavior of α,β-unsaturated carbonyl systems, and hydrogen bonding interactions in polyhydroxylated compounds. The compound serves as a substrate for enzymatic studies involving β-glucosidases from various organisms, with kinetic parameters providing insight into enzyme specificity and mechanism.

Emerging applications include use as a building block for synthetic chemistry, particularly for preparing more complex chalconoid derivatives through chemical modification of the phenolic hydroxyl groups. Materials science applications explore marein's potential as a ligand for metal coordination complexes, taking advantage of its multiple binding sites and chiral environment. Research continues into developing more efficient synthetic routes that could make marein more readily available for these applications. Patent activity remains limited, with most intellectual property focusing on extraction and purification methods rather than specific applications of the compound itself.

Historical Development and Discovery

The identification of marein dates to mid-20th century investigations into plant pigments, particularly those responsible for yellow coloration in Compositae family plants. Early work in the 1950s characterized the compound as a glycosidic yellow pigment from Coreopsis species, with initial structural proposals put forward based on degradation studies and color reactions. The complete structural elucidation, including stereochemical assignment of the glucosyl moiety, culminated in the 1960s through application of emerging spectroscopic techniques particularly nuclear magnetic resonance spectroscopy.

Significant advances in the 1970s included the first total synthesis of marein, which confirmed the structural assignment and provided material for more detailed studies of its properties. The development of high-performance liquid chromatography in the 1980s facilitated more precise analysis of marein and its related compounds, leading to improved understanding of its occurrence and distribution in plants. Recent research has focused on spectroscopic characterization and development of analytical methods for chalconoid glycosides, with marein serving as an important model compound for these studies. The compound's history reflects broader trends in natural product chemistry, from initial isolation and characterization through synthetic confirmation to contemporary applications in chemical research.

Conclusion

Marein represents a chemically interesting chalconoid glucoside that serves as a model compound for understanding the behavior of this class of natural products. Its well-characterized structure, featuring multiple phenolic hydroxyl groups and a β-glucosidic linkage, provides opportunities for studying diverse chemical phenomena including acid-base chemistry, glycoside hydrolysis, redox behavior, and spectroscopic properties. The compound's limited natural occurrence and specialized applications have prevented its development as a commercial product, but its value as a research tool and reference standard remains significant.

Future research directions likely include development of more efficient synthetic routes to enable larger-scale production, investigation of its coordination chemistry with various metal ions, and exploration of its potential as a chiral template in asymmetric synthesis. Advances in analytical methodology may reveal new applications for marein in method validation and quality control of natural products. The compound continues to provide insights into chalconoid chemistry and serves as a reference point for studies of more complex glycosylated natural products.