Isolation and structural characterization of colistin components

Preparative-scale separation of colistin sulphate bulk sample was carried out on a preparative poly(styrene-divinylbenzene) stationary phase. Isocratic elution with acetonitrile-sodium sulphate solution (0.7%.m/v; pH adjusted to 2.5 with TFA) - water (16 : 50 : 34, % v/v/v) was carried out at a flow rate of 4.0ml min(-1). Six colistin components were isolated and characterized using H-1 and C-13 NMR. The molecular weights were confirmed by mass spectrometry. The structures of 2 components were determined for the first time. Polymyxin E-7 was identified as having the same composition as polymyxin E-1, except that the fatty acid moiety was 7-methyloctanoic acid. Isoleucine polymyxin E-8 was characterized as having the same composition as isoleucine polymyxin E-1 with 7-methylnonanoic acid as the fatty acid moiety.