CONFORMATIONAL STUDIES OF THE CYCLIC L,D-LIPOPEPTIDE SURFACTIN BY FOURIER-TRANSFORM-INFRARED-SPECTROSCOPY

The infrared spectra of acidic surfactin, its mono- and di-methylester derivatives were measured in dry state, (H2O)-H-2, dimethylsulfoxide and trifluoroethanol. The IR spectra of the lipopeptides show two comp onent bands at 1737-1742 and 1718 cm(-1), indicating the presence of a mixture of non hydrogen-bonded and hydrogen-bonded C=O groups of the lactone ring. These bands overlapped with the C=O groups of the Asp an d Glu residues of protonated surfactin. Changes in relative intensitie s of the 1737-1742 and 1718 cm(-1) bands were assigned to modification s of population of hydrogen-bonded carbonyl groups, involving solvent molecules and/or other groups of the lipopeptides. The IR spectra of s urfactin and its derivatives display in the amide I region, at least, three component bands located at 1672-1681, 1656-1659 and 1641-1649 cm (-1). The position and the magnitude of each component band depend on the natures of the solvent and of the lipopeptide. The 1641-1649 cm(-1 ) band was indicative of a hydrogen-bonded C=O group as probed by temp erature variation and was tentatively assigned to C=O group involved i n beta-turn. Slight spectral differences between surfactin and its der ivatives may indicate small structural distorsion of the peptide backb one. We propose that the small structural differences between surfacti n and its derivatives, observed on their respective IR spectra, could be related to their different biological properties. (C) 1997 Elsevier Science B.V.