We previously applied x-ray reflectivity to study monolayers formed at the air/water interface by a dihelical peptide (palm2-alpha'SSalpha', also denoted BBC16) with a palmitoyl chain added at the N-terminus of each alpha-helix to enhance its amphiphilicity. Alone and in mixtures with a phospholipid (DLPE), the helices orient with their long axis nearly perpendicular to the interface under high surface pressure. Thus the residues of the peptide extend along the same direction that reflectivity probes. We produce the peptide synthetically, so we may choose any site in the sequence and replace the residue there with a deuterated version of the same amino acid, altering its neutron scattering properties. We synthesized a series of deuterium-labelled peptides with one protonated (H1) leucine replaced by leucine-H210 at different locations (9, 14, 28) within the sequence of the alpha-helical 31-mer. Neutron reflectivity from mixed DLPE/H2-labelled peptide monolayers exhibit sufficient sensitivity to distinguish among the various label positions, even in the limited q-range (qz < 0.2 A-1 = 11qcD2O) accessible at the air/water interface. Neutron interferometry from monolayers LB-deposited onto multilayer solid supports will expand the accessible q-range, making H2-labelling effects even more pronounced and permitting resolution suffiicient for studying structural perturbations due to prosthetic groups.
Supported by NIH GM33525, NSF MRSEC Program DMR96-32598.