Synthesis of the sophoroselipid-copper complex and its bactericidal properties

The copper (II) ion is valuable as an antimicrobial reagent and is highly safe for humans, animals, plants, and the environment. In this study, the effect of sodium sophoroselipid (SL-Na) on the bactericidal activity of CuSO₄ was investigated. SL-Na enhanced the bactericidal activity of CuSO₄ when incubated with it for 10 min or more at 20℃. Its bactericidal activity is significant against gram-negative bacterial cells. CuSO₄ at a concertation of 100 µM, when mixed with 100 µM SL-Na, effectively killed Escherichia coli cells at a rate of more than one order of magnitude higher than that observed without SL-Na after 30 minutes at 20℃. The structural differences between SL-Na, CuSO₄ and their mixture were confirmed by ultraviolet visible absorption spectrum analyses, resulting in the absorption of ca. 800 nm derived from copper (II) ions shifting to ca. 680 nm in the mixture. This shift suggests that the acid-form sophoroselipids and copper (II) ions bind weakly to form a complex (SL-Cu complex）. The purified SL-Cu complex also revealed a unique absorption at approximately 1,600 cm^-1 using a Fourier transform infrared analysis and demonstrated a higher bactericidal activity than CuSO₄. Our results indicate that SL-Cu complex formation enhanced the bactericidal activity of copper (II) ions.