Lipase-catalyzed Synthesis Of Long-chain Fatty Acid Esters Of Starch In Ionic Liquids

As one of the most important starch derivatives, high fatty acid starch ester has specialthermoplasticity, hydrophobicity and bio-degradability properties. This manuscript describedan exploratory study on lipase-catalyzed synthesis of high fatty acid starch esters in ionicliquids.A novel method based on the turbidity of the solution was investigated to determine thesolubility of starch in ionic liquids. Meanwhile, polarized light microscope was used toinvestigate the granular structure of starch heated in IL during the turbidimetric measurement.Quantitative solubility of three starches （normal maize starch, waxy maize starch and high-amylose maize starch） in two selected ionic liquids,1-butyl-3-methyl-imidazolium acetic（[BMIm]Ac） and1-butyl-3-methylimidazolium chloride （[BMIm]Cl）, was studied. Theinfluence of reaction temperature, starch concentration and water content on the realdissolution of starch in ILs was also investigated. The maximum theoretical solubility ofanhydrous starch dissolved in IL at different temperatures was also determined byextrapolation methodology.Different reaction media, lipases, starches were used in the lipase-catalyzed synthesisreactions of high fatty acid starch esters. According to the above experiments, the firstsynthesis method was proposed. Lipase-catalyzed synthesis of high fatty acid starch esterswas investigated in ionic liquids mixtures consisting of [BMIm]Ac and1-butyl-3-methyl-imidazolium tetrafouroborate （[BMIm][BF₄]）. Optimum modification conditions formaximum degree of substitute （DS0.144） of high fatty acid starch esters were shown asfollowing: high-amylose corn starch0.01mol, Candida rugosa （CRL） lipase dosage0.1g,reaction temperature60℃, reaction duration3h, total mass of ILs24g, mass ratio of[BMIm][BF₄]/[BMIm]Ac19:5and molar ratio of methyl palmitate/anhydroglucose units（AGU）3:1.Based on the results of first synthesis method, two-step modified methodology of starchwas investigated to make good starch solubility compatible with high enzymatic activity in ILsystem. Firstly, native starch was dissolved in the ionic liquid [BMIm]Cl to destroy granularstructure of starch. Then the dissolved starch was esterified in [BMIm][BF₄] by lauric acidusing lipase CRL as catalysis. The obtained maximum DS value of products was0.171underthe following conditions: high-amylose corn starch was0.01mol, starch concentration10%,lipase dosage0.3g, reaction temperature60℃, reaction time3h and molar ratio of lauricacid/AGU3:1. The formation of fatty acid starch esters was confirmed by the presence of the carbonylsignal in the FT-IR and NMR spectra. SEM and X-ray diffraction data showed that themorphology and crystallinity of native starch was largely disrupted during modification. TGanalysis suggested thermal stability of fatty acid starch esters decreased compared to nativestarch. Water contact-angle measurements revealed that the hydrophobicity of products wasimproved after modification.The polarity, potential of hydrogen （pH） and hydrophobicity of ionic liquids to those oforganic solvents were compared via solvatochromic, pH meter and octanol-water partitioncoefficient analysis, respectively. Compared to traditional organic solvents,[BMIm][BF₄] and[BMIm]Ac have high polarties. And they are hydrophilic and acid solvents. Moreover,fluorescence spectroscopy was used to follow the changes of enzyme structure in differentsolvents. The results showed that the lipase in ionic liquid [BMIm][BF₄] has high structurestability, while the lipase has low structure stability in [BMIm]Ac.At the end of each experiment, the ILs （[BMIm]Cl,[BMIm][BF₄]） was collected andrecycled, respectively. The purities of [BMIm]Cl and [BMIm][BF₄] determined by1H NMRspectroscopy were>99%. The DS values of fatty acid starch esters obtained in the recycledILs were in good accordance with that obtained in the fresh ILs.