γ-Glutamyl and d- or l-peptide linkages in mycobacillin, a cyclic peptide antibiotic

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Abstract
Mycobacillin lacks amino groups but contains two free α-carboxyl groups, indicating the presence of two side-chain peptide linkages. The five aspartic acid residues of mycobacillin are all in α-peptide linkage whereas the two glutamic acid residues are in γ-linkage. Mycobacillin does not react with hydroxylamine to give hydroxamate, indicating the absence of anhydride, lactone and ester linkages. This is also confirmed by i.r. spectroscopy and titration of the molecule. Of the 15 peptides obtained from partial hydrolysates of mycobacillin, 12 contain aspartic acid. Results obtained by treatment of hydrolysates of aspartic acid-containing peptides with d-amino acid oxidase and l-glutamate decarboxylase (containing l-aspartate decarboxylase activity) indicate that residue 5 is l-aspartic acid and residues 2, 8, 11 and 13 are d-aspartic acid. The d- or l-peptide sequence and nature of peptide linkages in mycobacillin are proposed on the basis of these findings and the amino acid sequence reported earlier.