X -ray scattering studies of maquette peptide monolayers

We have applied x-ray scattering techniques to characterize the structure of Langmuir and Langmuir-Blodgett monolayers containing maquette peptides. These small, soluble, synthetic, de novo designed peptides were developed as test systems for studying electron transfer in proteins, but may also be useful as a new class of thin film materials for endowing surfaces with functionalities. In the family of maquettes under consideraton, the 31-mer unit synthesized forms an amphipathic α-helix with a flexible [special characters omitted] tether at the N-terminus. In solution, pairs of such units associate and dimerize covalently via a disulfide bond between the cysteine residues, and then further associate into 4-helix bundles. Each dihelix forms a pair of bis-His sites for binding prosthetic groups. We studied the prototype peptide of this family, H10H24, and a derivative, BBC16, palmitoylated at the N-terminus. Both peptides form monolayers at the air/water interface with a plateau in their isotherms. The more amphiphilic BBC16 forms more stable Langmuir monolayers that could be studied via synchrotron-based x-ray reflectivity at surface pressures above and below the plateau, while H10H24 could only be studied below the plateau. The data, analyzed by slab-model fitting and a model-independent refinement procedure, demonstrate that BBC16 undergoes an orientational transition as pressure increases. At low pressures, both peptides exist as dihelices parallel to the plane of the air/water interface, but at high pressure BBC16\u27s helices are approximately normal to the interface. The addition of free DLPE to BBC16 monolayers increased their stability sufficiently to allow collection of off-specular inplane scattering for long periods. The difference signal between a 2:1 DLPE:BBC16 and a pure DLPE monolayer indicates that the upright helices form a 2-dimensional glass. X-ray interferometry demonstrates that BBC16 monolayers LB deposited onto alkylated solid supports can maintain the perpendicular orientation that the peptide has in the precursor Langmuir film. H10H24 monolayers LB deposited at low pressure onto a thiol surface also have the peptide oriented perpendicular to the plane of the support, indicating that rearrangement of the molecules must occur during or after deposition. The suitability of these systems for future electrochemical and neutron scattering studies is discussed