Abstract
In order to examine the antimicrobial activity of n-fattyacyl mono-aminoacyl derivatives of colistin nonapeptide (CNP), twenty-one derivatives were synthesized and characterized on their chemical, physicochemical and antimicrobial properties. Five α-amino acids, glycine, L-alanine, DL-aminobutyric acid, DL-norvaline and DL-norleucine, and two ω-amino acids, β-alanine and γ-aminobutyric acid, were first acylated with each of three kinds of acid chloride, n-octanoyl, n-decanoyl and n-dodecanoyl chloride, and then esterified with p-nitrophenol prior to the coupling of those n-fattyacyl amino acids to the terminal threonine of CNP. The coupling reaction was carried out in an aqueous solvent buffered at pH 5.0 without any protection of the γ-amino groups of CNP, as reported previously.
All of the derivatives obtained in hydrochloride salts were hygroscopic white amorphous powder showing decomposing points at 180_??_230°C. Each purified preparation exhibited a ninhydrin positive spot on a paper chromatogram, and each spot coincided with respective inhibitory zone at Rf 0.54_??_0.72 on a bioautogram obtained with Escherichia coli NIHJ or Bacillus subtilis. The antimicrobial spectra of the fifteen n-fattyacyl α-aminoacyl CNPs against gram-negative bacteria were narrower and the activities are less than those of the corresponding n-fattyacyl derivatives of CNP or colistin. Of n-fattyacyl ω-aminoacyl derivatives, n-dodecanoyl β-alanyl-CNP showed unique antimicrobial spectrum against gramnegative bacteria, in which colistin insensitive Serratia marcescens and Proteus morganii were inhibited at the concentrations of 25 and 50mcg/ml, respectively. In conclusion, the elongation of the peptide chain in the CNP molecule by an mono-amino acid acylated with n-fatty acid resulted in the reduction of the antimicrobial activity except for a case of β-alanine; the activity seems to depend on the kind of amino acid introduced.
