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Weiche Materie Poster: Do., 13:00–15:30 D-P358
Structural investigation on the mode of action of the antimicrobial peptide
gramicidin S - a monolayer study
Lydia Woiterski 1 , Ann Falk 1 , Florian Rückerl 1 , Stefan Surber 1 , Josef
Käs 1 , Carsten Selle 1
1 Institut für Experimentelle Physik I, Linnestr. 5, 04103 Leipzig
The antimicrobial function of the amphiphilic peptide gramicidin S (GS) is based on
unspecific attack on the bacterial membrane integrity. The GS structure is a cyclic
beta-sheet, cyclo(Val-Orn-Leu-DPhe-Pro)2.
We present a study which aims to elucidate the fundamental non-specific interactions
governing the phase behavior of both pure GS and mixtures from GS with phospholipids
within Langmuir monolayers. In order to modulate electrostatic interactions
between the basic peptide molecules, we investigated monolayers on buffers with pH
values varied between 2 and 12 at room temperature and ionic strength of 0.19 M. The
pressure-area isotherms recorded for GS monolayers demonstrated marked differences.
The phase transition between two-dimensional liquid and solid phases was observed to
appear at higher lateral pressures at decreased pH. Striking differences were found for
the sizes and shapes of solid domains on the various subphases as observed by Brewster
angle microscopy. The latter even occur in a pH region which is far below from the
ornithin pK value indicating that a large pK shift of the basic ornithine (orn) residues
compared to the bulk appears at the monolayer surface. This pH sensitive behavior is
probably due to the high surface concentration of negative counterions present.
Furthermore, the lateral pressure of GS monolayers at pH 7.5 and constant ionic
strength was found to be dependent on the phosphate concentration. A possible qualtitative
explanation is that divalent phosphate ions present at this pH might bind tightly
to the cationic peptide inducing electrostatic repulsion. This could lead to an additional
contribution to the lateral pressure which increases at higher phosphate concentrations.
Tentatively, one could conclude that the negative surface charge of bacterial
membranes can be enhanced by cationic peptides in the presence of divalent phosphate
anions, resulting in membrane-destabilizing increased lateral pressure.
To investigate structural properties of the antimicrobial peptide, synchrotron X-ray
reflection (XR) and grazing incidence diffraction (GID) experiments were performed
on GS monolayers at HASYLAB, DESY, Hamburg. By use of the latter technique,
the lateral order within the film was studied while the reflectivity measurements provide
information about the electron density profiles in the vertical direction of the
monolayer.
First results are be presented for GS monolayers on different buffers and at varied lateral
surface pressures with and without phospholipids mimicking bacterial membranes.