Abstract

Nisinic acid, systematically named (6Z,9Z,12Z,15Z,18Z,21Z)-tetracosa-6,9,12,15,18,21-hexaenoic acid, is a very long-chain polyunsaturated fatty acid with the molecular formula C₂₄H₃₆O₂. This ω-3 fatty acid features six cis-configured carbon-carbon double bonds positioned at carbons 6, 9, 12, 15, 18, and 21 from the carboxylic acid terminus. The compound exhibits characteristic properties of highly unsaturated carboxylic acids, including low melting points, susceptibility to oxidative degradation, and limited solubility in aqueous media. Nisinic acid demonstrates significant interest in lipid chemistry as an elongated homolog of docosahexaenoic acid and as an intermediate in the biosynthetic pathways of shorter-chain polyunsaturated fatty acids. Its extended conjugated system confers distinctive spectroscopic signatures and unique chemical reactivity patterns among natural fatty acids.

Introduction

Nisinic acid represents a specialized class of very long-chain polyunsaturated fatty acids found primarily in marine organisms. The compound belongs to the ω-3 fatty acid family, characterized by the presence of a double bond three carbons from the methyl terminus. With six double bonds in its 24-carbon chain, nisinic acid exhibits one of the highest degrees of unsaturation among naturally occurring fatty acids. The systematic IUPAC name, (6Z,9Z,12Z,15Z,18Z,21Z)-tetracosa-6,9,12,15,18,21-hexaenoic acid, precisely defines its stereochemical configuration and bond positions. The CAS registry number 68378-49-4 identifies this specific stereoisomer. Although less extensively studied than shorter-chain polyunsaturated fatty acids, nisinic acid holds significance in understanding the metabolic pathways and physical properties of highly unsaturated lipid molecules.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of nisinic acid features a carboxylic acid functional group at one terminus and a methyl group at the opposite end, connected by a 24-carbon chain containing six cis-configured double bonds. The cis configuration at each double bond introduces significant chain bending, resulting in a non-linear molecular geometry with approximate overall dimensions of 3.2 nanometers in length. The carbon atoms at double bond positions exhibit sp² hybridization with bond angles of approximately 120 degrees, while saturated carbon segments maintain sp³ hybridization with tetrahedral bond angles of 109.5 degrees. The electronic structure demonstrates conjugation effects, particularly between the C6-C7, C9-C10, C12-C13, C15-C16, C18-C19, and C21-C22 double bonds, though the methylene-interrupted pattern limits full electronic delocalization throughout the molecule.

Chemical Bonding and Intermolecular Forces

Covalent bonding in nisinic acid follows typical patterns for unsaturated carboxylic acids. Carbon-carbon single bonds measure approximately 1.54 Å in length, while carbon-carbon double bonds measure approximately 1.34 Å. The carbon-oxygen bonds in the carboxylic acid group measure 1.36 Å for the C-O single bond and 1.23 Å for the C=O double bond. The molecule exhibits significant dipole moment due to the polar carboxylic acid group, estimated at 1.7-1.9 Debye. Intermolecular forces include hydrogen bonding between carboxylic acid groups with bond energies of approximately 8-10 kcal/mol, van der Waals interactions along the hydrocarbon chain, and potential π-π stacking interactions between double bond systems. The extensive unsaturation reduces London dispersion forces compared to saturated analogs but introduces additional dipole-induced dipole interactions.

Physical Properties

Phase Behavior and Thermodynamic Properties

Nisinic acid exists as a colorless to pale yellow viscous liquid at room temperature, with a melting point below 0°C due to its high degree of unsaturation. The compound demonstrates limited crystalline behavior and may form glassy states upon cooling. The boiling point under reduced pressure (1 mmHg) is approximately 210-220°C, though thermal decomposition typically occurs before vaporization at atmospheric pressure. The density measures approximately 0.92 g/cm³ at 20°C. Thermodynamic parameters include an estimated heat of combustion of -3500 kcal/mol, reflecting the high energy content characteristic of unsaturated hydrocarbons. The specific heat capacity ranges from 0.45-0.50 cal/g·°C in the liquid phase. The refractive index measures approximately 1.48 at 20°C and 589 nm wavelength, consistent with highly unsaturated long-chain compounds.

Spectroscopic Characteristics

Infrared spectroscopy of nisinic acid shows characteristic absorption bands at 1710 cm⁻¹ for the carbonyl stretch, 3010 cm⁻¹ for =C-H stretches, 2930 cm⁻¹ and 2850 cm⁻¹ for alkyl C-H stretches, and 1280-1320 cm⁻¹ for C-O stretches. Proton NMR spectroscopy reveals distinctive signals: a triplet at δ 0.98 ppm for the terminal methyl group, complex multiplet signals between δ 5.30-5.45 ppm for the olefinic protons, a triplet at δ 2.34 ppm for the α-carbonyl methylene group, and multiple signals between δ 2.70-2.90 ppm for bis-allylic methylene protons. Carbon-13 NMR shows signals at δ 180.5 ppm for the carbonyl carbon, signals between δ 127-132 ppm for olefinic carbons, and signals at δ 34.1 ppm for the α-carbonyl methylene carbon. UV-Vis spectroscopy demonstrates weak absorption maxima around 210-230 nm due to isolated double bonds, with molar absorptivity coefficients of approximately 10,000 L·mol⁻¹·cm⁻¹.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Nisinic acid exhibits characteristic reactivity of polyunsaturated carboxylic acids. The compound undergoes electrophilic addition reactions at carbon-carbon double bonds with second-order rate constants typically ranging from 10⁻³ to 10⁻² L·mol⁻¹·s⁻¹ for halogens and hydrogen halides. Catalytic hydrogenation proceeds completely to tetracosanoic acid with hydrogenation enthalpies of approximately -30 kcal/mol per double bond. Oxidation reactions represent particularly important pathways, with autoxidation rate constants of approximately 10⁻² M⁻¹·s⁻¹ for peroxyl radical formation. The carboxylic acid group demonstrates typical acid-base behavior, participating in esterification reactions with alcohols with rate constants of 10⁻⁴ to 10⁻³ L·mol⁻¹·s⁻¹ under acid catalysis. Decarboxylation requires elevated temperatures above 200°C with activation energies of approximately 45 kcal/mol.

Acid-Base and Redox Properties

The carboxylic acid group of nisinic acid exhibits a pKₐ value of approximately 4.8 in aqueous solution, consistent with typical aliphatic carboxylic acids. The compound functions as a weak organic acid, forming carboxylate salts with bases. Buffer capacity ranges from 0.01-0.02 mol/pH unit near its pKₐ value. Redox properties include standard reduction potentials of -0.3 to -0.4 V for one-electron reduction processes. The polyunsaturated system demonstrates susceptibility to oxidative degradation, with peroxide formation initiation rates of 10⁻⁶ to 10⁻⁵ M·s⁻¹ under atmospheric oxygen. The compound maintains stability in neutral and acidic conditions but undergoes decomposition under strongly basic conditions at elevated temperatures through saponification and double bond isomerization pathways.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of nisinic acid typically employs sequential Wittig reactions or Horner-Wadsworth-Emmons olefinations to construct the polyunsaturated chain. One established route begins with appropriate C6 and C18 synthons, utilizing phosphonium salts with controlled stereochemistry to ensure all cis configuration. Reaction conditions typically involve strong bases such as sodium hydride or n-butyllithium in anhydrous tetrahydrofuran at -78°C to 0°C, yielding intermediate alkenes with Z-selectivity exceeding 95%. Final hydrolysis of protective groups yields the free acid with overall yields of 15-25% after purification by column chromatography and recrystallization. Alternative approaches include partial hydrogenation of fully conjugated systems or enzymatic elongation from shorter-chain ω-3 fatty acids using specialized desaturase and elongase enzymes.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography-mass spectrometry represents the primary analytical method for nisinic acid identification and quantification. Derivatization to methyl esters using boron trifluoride-methanol reagent precedes analysis, with separation achieved using polar stationary phases such as cyanopropyl polysiloxane. Characteristic mass spectral fragments include m/z 79, 105, and 119 for diene fragments, with molecular ion peak at m/z 356 under electron impact ionization. High-performance liquid chromatography with UV detection at 205 nm provides alternative quantification methods with detection limits of approximately 0.1 μg/mL. Silver ion chromatography effectively separates nisinic acid from geometric isomers and saturation analogs based on complexation with silver ions.

Purity Assessment and Quality Control

Purity assessment of nisinic acid employs complementary chromatographic and spectroscopic techniques. Gas chromatography with flame ionization detection typically shows purity levels exceeding 98% for synthetic material, with major impurities including saturated analogs and geometric isomers. Nuclear magnetic resonance spectroscopy provides structural confirmation and quantifies isomeric purity through integration of olefinic proton signals. Peroxide value determination measures oxidative degradation, with acceptable limits below 5 meq/kg for stable samples. Acid value titration confirms carboxylic acid functionality, with theoretical value of 157 mg KOH/g for pure compound. Storage under nitrogen atmosphere at -20°C maintains stability for extended periods, with recommended shelf life of 6-12 months.

Applications and Uses

Research Applications and Emerging Uses

Nisinic acid serves primarily as a research chemical in lipid biochemistry and organic synthesis. The compound functions as a model system for studying oxidation kinetics in highly unsaturated fatty acids, with applications in understanding lipid peroxidation mechanisms. Materials science investigations utilize nisinic acid as a building block for liquid crystalline compounds, taking advantage of its extended rigid structure and functional groups for supramolecular assembly. Synthetic organic chemistry employs nisinic acid as a starting material for preparing specialized lipids with tailored physical properties. Research applications include investigation of chain-length effects on membrane properties and studies of intermolecular interactions in polyunsaturated systems.

Conclusion

Nisinic acid represents a structurally distinctive polyunsaturated fatty acid with six cis-configured double bonds in a 24-carbon chain. The compound exhibits physical and chemical properties characteristic of highly unsaturated carboxylic acids, including low melting point, susceptibility to oxidation, and complex spectroscopic signatures. Synthetic accessibility enables research applications in lipid chemistry and materials science, while its natural occurrence contributes to understanding metabolic pathways in marine organisms. The extended conjugated system presents opportunities for investigating structure-property relationships in polyunsaturated compounds and developing new materials with tailored characteristics. Further research directions include exploration of catalytic applications, development of stabilization methods against oxidative degradation, and investigation of supramolecular assembly behavior.