Guanidylfungin A

The structures of guanidylfungins A and B were elucidated from the physico-chemical properties of these compounds and the structures of the degradation products by ozonolysis and periodate oxidation. The guanidylfungins consist of a 36-membered polyhydroxyl lactone ring, a guanidine and a monoester of malonic acid. The labelling experiments with sodium [1-13C]acetate and sodium [1-13C]propionate revealed that twelve units of acetate and nine of propionate were incorporated into the molecule of guanidylfungin A.

In a previous study, we identified a Streptomyces sp., A3265, as exhibiting potent antifungal activity against various plant pathogenic fungi, including Botrytis cinerea, Colletotrichum gloeosporioides, and Rhizoctonia solani. This strain also exhibited a biocontrolling effect against ginseng root rot and damping-off disease, common diseases of ginseng and other crops. In this study, we isolated two antifungal substances responsible for this biocontrolling effect via Diaion HP-20 and Sephadex LH-20 column chromatography, medium pressure liquid chromatography, and high-performance liquid chromatography. These compounds were identified as guanidylfungin A and methyl guanidylfungin A by spectroscopic methods. These compounds exhibited potent antimicrobial activity against various plant pathogenic fungi as well as against bacteria.

Guanidylfungin A was chemically modified by alkylation, reduction and/or demalonylation. Demalonylmethylguanidylfungin A became soluble in water and showed approximately eight-fold higher activity against fungi and Gram-positive bacteria than guanidylfungin A along with strongly fungicidal effect. Similarly, copiamycin was converted to demalonylmethylcopiamycin, which also showed higher antifungal activity than copiamycin itself.