Naturally occurring oxidation-reduction protein complexes have a proven ability to insert vectorially into membranes and generate potential differences across these membranes, but also have complicated architectures and a fragility that make them difficult to adapt to materials applications. Given the ease of solid phase peptide synthesis and the robustness of simple structural motifs, synthetic peptides show promise as components of novel biomolecular materials, provided that the functions of these individual molecules can be translated into materials properties by organizing the peptides into macroscopically ordered ensembles. We have developed amphiphilic alpha-helical bundle peptides, analogous to membrane proteins, with the exterior of one end of the bundle hydrophilic and the exterior of the other end hydrophobic, in order to promote the organization of these peptides at interfaces. Just as in previously developed water-soluble helical bundle peptides, the inclusion of histidines in the sequence of these amphiphilic peptides provides sites for bis-His ligation of metallo-porphyrin prosthetic groups (e.g. heme) in the core of the bundle, letting them serve as a scaffold for the arrangement of donors and acceptors in an electron transfer chain. However, in amphiphilic peptides the chain may bridge the interface between polar and non-polar media and so generate a potential difference across a host lipid monolayer or bilayer.
We have designed and characterized a series of amphiphilic four-alpha-helical bundle peptides with the hydrophilic end based on the earlier water-soluble peptides, with the helices extended by sequences derived from a proton channel analog or from part of the transmembrane domain of the cytochrome bc1 complex. The purified peptides are highly alpha-helical, assemble into 4-helix bundles when detergent solubilized, and can bind heme groups in either the hydrophobic or hydrophilic end of the bundle, with binding affinities in the 10-100 nM range. The hemes, whether bound in the hydrophilic or hydrophobic end of the bundle, exhibit oxidation-reduction midpoint potentials in the range of -140 to -100 mV. X-ray reflectivity and grazing incidence diffraction studies of Langmuir monolayers show that as surface pressure is applied at the air/water interface, the peptides assemble into four-helix bundles oriented with the helical axes perpendicular to the interface. X-ray studies of the inclusion of these peptides into phospholipid vesicles are also underway.
This work supported by the NIH (GM55876) and the MRSEC program of the NSF (DMR00-79909). Synchrotron x-ray sources at Brookhaven and Argonne National Laboratories supported by the Department of Energy.