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Infrared Reflection Absorption Spectroscopy of Amphipathic Model Peptides at the Air/Water Interface

Andreas Kerth ^*, Andreas Erbe ^*, Margitta Dathe  and Alfred Blume ^*

^* Institut für Physikalische Chemie, Martin-Luther-Universität Halle-Wittenberg, Halle, Germany; and Forschungsinstitut für Molekulare Pharmakologie, Berlin, Germany

Correspondence: Address reprint requests to Alfred Blume, Institute of Physical Chemistry, Martin-Luther-University, Halle-Wittenberg, Muehlpforte 1, 06108 Halle, Germany. Tel.: 49-345-552-5850; Fax: 49-345-552-7157; E-mail: blume{at}chemie.uni-halle.de.

The linear sequence KLAL (KLALKLALKALKAALKLA-NH₂) and its corresponding D,L-isomers k₉a₁₀-KLAL (KLALKLALkaLKAALKLA-NH₂) and l₁₁k₁₂-KLAL (KLALKLALKAlkAALKLA-NH₂) are model compounds for potentially amphipathic -helical peptides which are able to bind to membranes and to increase the membrane permeability in a structure- and target-dependent manner (Dathe and Wieprecht, 1999) We first studied the secondary structure of KLAL and its analogs bound to the air/water using infrared reflection absorption spectroscopy. For the peptide films the shape and position of the amide I and amide II bands indicate that the KLAL adopts at large areas per molecule an -helical secondary structure, whereas at higher surface pressures or smaller areas it converts into a ß-sheet structure. This transition could be observed in the compression isotherm as well as during the adsorption at the air/water interface from the subphase as a function of time. The secondary structures are essentially orientated parallel to the air/water interface. The analogs with D-amino acids in two different positions of the sequence, k₉a₁₀-KLAL and l₁₁k₁₂-KLAL, form only ß-sheet structures at all surface pressures. The observed results are interpreted using a comparison of hydrophobic moments calculated for -helices and ß-sheets. The differences between the hydrophobic moments calculated using the consensus scale are not large. Using the optimal matching hydrophobicity scale or the whole-residue hydrophobicity scale the ß-sheet even has the larger hydrophobic moment.

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