Synthesis Of Novel KOJIC ACID Derivatives And Their Application In Prawn Preservation

Prawn is a kind of healthy food containing high protein, low fat and low caloric. It is indispensable in dietary structure due to its delicious meat and unique flavor. However, fresh shrimp is easily prone to become black because of melanin formation during the storage, seriously affecting the appearance and the commercial value. Browning of prawn is mainly caused by the melanosis, which is initiated by the tyrosinase-catalyzed oxidation of tyrosine or its derivatives.Kojic acid posssesses strong tyrosinase inhibitory activity and has been used in the food industry. Therefore, synthesis of novel kojic acid derivatives by modifying the kojic acid molecule is an effective way to search for better tyrosinase inhibitors. The aim of this study were to develop a new type of kojic acid derivatives, to assess their tyrosinase inhibitory activity and antibacterial activity, and to explore the potential application in prawn preservation.In this thesis, two series of kojic acid derivatives were synthesized starting from kojic acid, namely condensation products of kojic acid with aldehydes, and amino acid derivatives of kojic acid. In this study, the inhibitory effects on tyrosinase and inhibition mechanism, antibacterial activity, and application in prawn preservation of kojic acid derivatives were investigated.The key results are shown as below:1. A range of kojic acid derivatives were synthesized using kojic acid as a starting material. The structures of these compounds were confirmed by1H NMR,13C NMR, ESI-MS and HRMS.2. L-tyrosine was used as the substrate to study the inhibitory abilities of the two series of kojic acid derivatives and IC50values were determined. The results showed that the inhibitory activity of compound7is stronger than that of compound3. The inhibitory mechanism of two compounds (7d,7e) was investigated using L-DOPA as the substrate. The mechanism was defined as a reversible mixed-type by using Lineweaver-Burk plot method.3. The antimicrobial assay was carried out by employing Oxford cup method. Escherichia coli, Staphylococus aureus, Salmonella, Bibrio Parahemolyticus and Bacillus subtilis were the tested strains. The antimicrobial activity was evaluated by measuring the inhibition zone diameter. The results indicated that the compound7e and7d exhibited stronger antimicrobial activity than kojic acid. The minimum batericidal concentrations (MBCs) of kojic acid against Escherichia coli, Staphylococus aureus, Salmonella, Bibrio Parahemolyticus and Bacillus subtilis were determined to be0.178.0.355.1.42.1.42and0.355mg/mL respectively. Whereas, the MBCs of compound7e against above5bacterial strains were determined as0.037.0.0185.0.148.0.037and0.295mg/mL, respectively. Cytotoxicity of two antibacterial agents (kojic acid and7e) on3cell lines was evaluated using MTT assay, indicating that the cell inhibitory rates were below5%.4. The sensory evaluation, bacterial counts, pH value and TVB-N were evaluated. The preliminary study on preservation effect of kojic acid derivative7e on Penaeus vannmei indicated that it appreciably slowed down the browning and putrescent velocity of prawn in comparison with kojic acid.