Structure–activity Relationship Analysis for Antimicrobial Activities of Tryptanthrin Derivatives Using Quantum Chemical Calculations

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Tryptanthrin (T) and eight of its monosubstituted derivatives (T2NH₂, T2Cl, T2Br, T2NO₂, T8OMe, T8Me, T8F, and T8Br) were synthesized, and their antimicrobial activities were investigated against a fungus (Malassezia furfur) and a gram-positive bacterium (methicillin-resistant staphylococcus aureus, MRSA). Antimicrobial activities of these derivatives were influenced by the substituents on tryptanthrin, with the halogen-substituted tryptanthrin derivatives (T2Cl, T2Br, T8F, and T8Br) showing the highest potency against M. furfur and MRSA. Therefore, semiempirical molecular orbital calculations (PM3) were performed on T and its eight derivatives to investigate the cause of the differences in their antimicrobial activities. The results of the calculations showed that antimicrobial activities could be related to the electrophilicity of the carbonyl carbon of the five-membered ring.