Infrared reflection absorption spectroscopy of amphipathic model peptides at the air/water interface

Abstract

The linear sequence KLAL (KLALKLALKALKAALKLA-NH2) and its corresponding D,L-isomers k9a10-KLAL (KLALKLALkaLKAALKLA- NH2) and I11k12-KLAL (KLALKLALKAlkAALKLA- NH2) are model compounds for potentially amphipathic α-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 α-helical secondary structure, whereas at higher surface pressures or smaller areas it converts into a β-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 I11k12-KLAL, form only β-sheet structures at all surface pressures. The observed results are interpreted using a comparison of hydrophobic moments calculated for α-helices and β-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 β-sheet even has the larger hydrophobic moment.