Ca2+-induced changes of surfactin conformation: A FTIR and circular dichroism study

Previous NMR studies on surfactin proposed two gamma or beta -turn-containi ng conformers while recent CD studies described beta -sheets and alpha -hel ices in surfactin, Since these data were not obtained in the same condition s, the conformation of surfactin was reinvestigated by FTIR spectroscopy, a diagnostic method for beta -sheets, In trifluoroethanol, the FTIR spectra of surfactin and its diester are compatible with gamma and/or beta -turn(s) and the differences in their CD spectra show the importance of the Glu(1) and Asp(5) COOH groups in stabilizing the lipopeptide conformation. The cal cium-induced spectral changes of both lipopeptides suggest a first binding of the divalent ions to the surfactin COOH groups (until calcium-lipopeptid e mole ratio reached 1) followed by bulk conformational changes (at higher mole ratios). In Tris buffer at pH 8.5, the FTIR amide I band shape, withou t the typical 1610-1628 and 1675-1695 cm(-1) bands, ascertains the absence of beta -sheets. (C) 2001 Academic Press.