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

Abstract

The linear sequence KLAL (KLALKLALKALKAALKLA-NH(2)) and its corresponding d,l-isomers k(9)a(10)-KLAL (KLALKLALkaLKAALKLA-NH(2)) and l(11)k(12)-KLAL (KLALKLALKAlkAALKLA-NH(2)) are model compounds for potentially amphipathic alpha-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 alpha-helical secondary structure, whereas at higher surface pressures or smaller areas it converts into a beta-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, k(9)a(10)-KLAL and l(11)k(12)-KLAL, form only beta-sheet structures at all surface pressures. The observed results are interpreted using a comparison of hydrophobic moments calculated for alpha-helices and beta-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 beta-sheet even has the larger hydrophobic moment.

FIGURE 1

(A) Surface pressure versus time course of KLAL films with different peptide concentrations starting with the injection of the appropriate volume of the stock solution into the subphase: 500 nM (▾), 250 nM (▴), 100 nM (•), and 25 nM (▪). (B) IRRA spectra of the KLAL film with the 25 nM peptide concentration at the respective positions a–d of the surface pressure time curve in Fig. 1 A. All spectra have been recorded at an angle of incidence of 40° and with s-polarized light.

FIGURE 2

(A) Surface pressure versus time course of a l₁₁k₁₂-KLAL film with a peptide concentration of 500 nM starting with the injection of the appropriate volume of the stock solution into the subphase. (B) IRRA spectra of the l₁₁k₁₂-KLAL film at the respective positions a–d of the surface pressure time curve in Fig. 2 A. All spectra have been recorded at an angle of incidence of 40° and with p-polarized light.

FIGURE 3

Simulation of IRRA spectra of a β-sheet secondary structure on H₂O. The calculation has been performed for p-polarized light and an angle of incidence of 40° for the amide I bands at 1625 and 1690 cm⁻¹ and the amide II band at 1535 cm⁻¹. The respective β-sheet is lying flat at the air/water interface (solid line), oriented perpendicular to the interface with the peptide chains parallel to the interface (dashed line), or oriented perpendicular to the interface with the direction of the peptide chains perpendicular to the interface (dotted line).

FIGURE 4

(A) Surface pressure versus area per amino acid residue of a KLAL film spread from an aqueous buffer solution. (B) IRRA spectra of the KLAL film during the compression at the respective areas per amino acid residues a–f of the surface pressure area curve in Fig. 3 A. The intensity of the spectra taken at positions e and f has been reduced by a factor of 2 for clarity. All spectra have been recorded at an angle of incidence of 40° and with p-polarized light.

FIGURE 5

(A) Surface pressure versus area per amino acid residue of a k₉a₁₀-KLAL film spread from an aqueous buffer solution. (B) IRRA spectra of the k₉a₁₀-KLAL film during the compression at the respective areas per amino acid residues a–d of the surface pressure area curve in Fig. 4 A. All spectra have been recorded at an angle of incidence of 40° and with p-polarized light.

FIGURE 6

(A) Surface pressure versus area per amino acid residue of a l₁₁k₁₂-KLAL film spread from an aqueous buffer solution. (B) IRRA spectra of the l₁₁k₁₂-KLAL film during the compression at the respective areas per amino acid residues a–d of the surface pressure area curve in Fig. 5 A. All spectra have been recorded at an angle of incidence of 40° and with p-polarized light.

FIGURE 7

CPK model representation of KLAL in the extended twisted β-sheet conformation (single chain). Top: side view of the molecule to visualize the orientation of the lysine side chains (black). Bottom: view onto the end of the molecule with the lysine side chains pointing to the bottom.