Molecular interactions between magainin 2 and model membranes in situ

Abstract

In this paper, we investigated the molecular interactions of magainin 2 with model cell membranes using sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Symmetric 1-palmitoyl-2-oleoyl-sn-glycero-3-[Phospho-rac-(1-glycerol)] (POPG) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, which model the bacterial and mammalian cell membranes, respectively, were used in the studies. It was observed by SFG that magainin 2 orients relatively parallel to the POPG lipid bilayer surface at low solution concentrations, around 200 nM. When increasing the magainin 2 concentration to 800 nM, both SFG and ATR-FTIR results indicate that magainin 2 molecules insert into the POPG bilayer and adopt a transmembrane orientation with an angle of about 20 degrees from the POPG bilayer normal. For the POPC bilayer, even at a much higher peptide concentration of 2.0 microM, no ATR-FTIR signal was detected. For this concentration on POPC, SFG studies indicated that magainin 2 molecules adopt an orientation nearly parallel to the bilayer surface, with an orientation angle of about 75 degrees from the surface normal. This shows that SFG has a much better detection limit than ATR-FTIR and can therefore be applied to study interfacial molecules with a much lower surface coverage. This magainin 2 orientation study and further investigation of the lipid bilayer SFG signals support the proposed toroidal pore model for the antimicrobial activity of magainin 2.

Figure 1

Schematic of the near total reflection experimental geometry in the SFG setup.

Figure 2

SFG ppp and ssp spectra of the C=O stretching frequency region collected from a POPG/POPG bilayer in contact with a 800 nM Magainin 2 solution.

Figure 3

Relationship between the χ_ppp/ χ_ssp ratio and the α-helix orientation angle of Magainin 2.

Figure 4

Polarized ATR-FTIR spectra of the C=O stretching frequency region collected from a POPG/POPG bilayer in contact with a 800 nM Magainin 2 solution.

Figure 5

SFG ppp and ssp spectra of the C=O stretching frequency region collected from a POPC/POPC bilayer in contact with a 2.0 µM Magainin 2 solution.

Figure 6

SFG spectra of the C-H stretching frequency region collected from a. POPC/POPC bilayer (top) and b. POPG/POPG bilayer (bottom) before interaction with Magainin 2.

Figure 7

SFG spectra of the C-H stretching frequency region collected from a. POPC/POPC bilayer in contact with 2.0 µM Magainin 2 solution (top); b. POPG/POPG bilayer in contact with 200 nM Magainin 2 solution (middle); c. POPG/POPG bilayer in contact with 800 nM Magainin 2 solution.