The insertion of charged amino acid residues into the hydrophobic part of lipid bilayers is energetically unfavorable yet found in many cationic membrane peptides and protein domains. To understand the mechanism of this translocation, we measured the (13)C-(31)P distances for an Arg-rich beta-hairpin antimicrobial peptide, PG-1, in the lipid membrane using solid-state NMR. Four residues, including two Arg's, scattered through the peptide were chosen for the distance measurements. Surprisingly, all residues show short distances to the lipid (31)P: 4.0-6.5 A in anionic POPE/POPG membranes and 6.5-8.0 A in zwitterionic POPC membranes. The shortest distance of 4.0 A, found for a guanidinium Czeta at the beta-turn, suggests N-H...O-P hydrogen bond formation. Torsion angle measurements of the two Arg's quantitatively confirm that the peptide adopts a beta-hairpin conformation in the lipid bilayer, and gel-phase 1H spin diffusion from water to the peptide indicates that PG-1 remains transmembrane in the gel phase of the membrane. For this transmembrane beta-hairpin peptide to have short (13)C-(31)P distances for multiple residues in the molecule, some phosphate groups must be embedded in the hydrophobic part of the membrane, with the local (31)P plane parallel to the beta-strand. This provides direct evidence for toroidal pores, where some lipid molecules change their orientation to merge the two monolayers. We propose that the driving force for this toroidal pore formation is guanidinium-phosphate complexation, where the cationic Arg residues drag the anionic phosphate groups along as they insert into the hydrophobic part of the membrane. This phosphate-mediated translocation of guanidinium ions may underlie the activity of other Arg-rich antimocrobial peptides and may be common among cationic membrane proteins.

Aggregation or oligomerization is important for the function of many membrane peptides such as ion channels and antimicrobial peptides. However, direct proof of aggregation and the determination of the number of molecules in the aggregate have been difficult due to the lack of suitable high-resolution methods for membrane peptides. We propose a 19F spin diffusion magic-angle-spinning NMR technique to determine the oligomeric state of peptides bound to the lipid bilayer. Magnetization transfer between chemically equivalent but orientationally different 19F spins on different molecules reduces the 19F magnetization in an exchange experiment. At long mixing times, the equilibrium 19F magnetization is 1/M, where M is the number of orientationally different molecules in the aggregate. The use of the 19F spin increases the homonuclear dipolar coupling and thus the distance reach. We demonstrate this technique on crystalline model compounds with known numbers of molecules in the asymmetric unit cell, and show that 19F spin diffusion is more efficient than that of 13C by a factor of approximately 500. Application to a beta-hairpin antimicrobial peptide, protegrin-1, shows that the peptide is almost completely dimerized in POPC bilayers at a concentration of 7.4 mol %. Decreasing the peptide concentration reduced the dimer fraction. Using a monomer-dimer equilibrium model, we estimate the DeltaG for dimer formation to be -10.2 +/- 2.3 kJ/mol. This is in good agreement with the previously measured free energy reduction for partitioning and aggregating beta-sheet peptides into phospholipid membranes. This 19F spin diffusion technique opens the possibility of determining the oligomeric structures of membrane peptides.

Pressure-area isotherms, Brewster angle microscopy, and grazing incidence X-ray diffraction measurements reveal that human lung surfactant protein SP-B1-78 and the dimer of the amino terminus dSP-B1-25 modify the phase behavior of lipid mixtures consisting of dipalmitoylphosphatidylcholine/palmitoyl-oleylphosphatidylglycerol/palmitic acid (DPPC/POPG/PA). The addition of SP-B increases the fraction of fluid phase in the liquid-expanded/liquid-condensed two-phase region. Brewster angle microscopy enabled the visualization of a fluid network, which separates the condensed phase domains. This network is stabilized by SP-B adsorption. GIXD measurements show that SP-B also alters the structure of the condensed chain lattice leading to higher tilt and increased area per hydrocarbon chain. The comparison of SP-B1-78 with the shorter peptide dSP-B1-25 exhibits, that the dimer alters the lipid order more drastically. The larger effects found for dSP-B1-25 were explained using a model that assumes a partial incorporation of the peptide into the layer. The specific behavior of the dimer could enhance the activity of the peptide as found in recent animal model studies. This is the first investigation showing a systematic influence of SP-B on the condensed chain lattice of phospholipids, thus verifying that SP-B not only interacts with the expanded phase, but also interactions with the condensed phase lipids have to be taken into account which might be essential for proper peptide function.