Non-peptide linkages in gramicidin

Abstract

The absence of basic or acidic groups in gramicidin was confirmed by titrations in aqueous ethanol and in glacial acetic acid, by its failure to form a colored dinitrophenol derivative and by its failure to move as a charged molecule in electrophoresis. The presence of hydroxyl groups in the molecule has been confirmed by methylation with CH3I. Hydrolysis of such methylated gramicidin fails to liberate beta-methoxyethylamine as would be expected if the methylated hydroxyl groups were those of ethanolamine residues. The methoxyl groups of methylated gramicidin are shown to be acid-labile. Residues of amino-acetaldehyde probably do not serve as precursors of the ethanolamine found in acid hydrolysates of gramicidin. The relative rate of release of basic and acidic groups in the partial acid hydrolysis of gramicidin was followed by means of titration curves. Gramicidin is not readily hydrolyzed by alkali in the cold. The free amino groups of the peptides produced on acid hydrolysis were recognized as those of trypto-phan residues. Dinitrophenyl-tryptophan is not stable to the acid hydrolysis conditions used. Evidence is presented for the existence of amino-acid and peptide esters of ethanolamine in partial acid hydrolysates of gramicidin. A partial structure for gramicidin is proposed.