75748-25-3 Colistin heptapeptide Polymyxin E1, 1-de(N2-(6-methyl-1-oxooctyl)-L-2,4-diaminobutanoic acid)-2-de-L-threonine-3-de(L-2,4-diaminobutanoic acid)-, (1S-(1alpha(R*),3alpha,7beta,8beta(2S*,4S*),8abeta))-