X-RAY REFLECTIVITY STUDIES OF MIXED LANGMUIR MONOLAYERS OF A LIPOPEPTIDE MAQUETTE AND A PHOSPHOLIPID

We characterized Langmuir monolayers of the apo-form of a synthetic peptide, BBC16, whose holo-form was designed de novo to be a model (maquette) for studying electron transfer. The synthetic unit is a mostly alpha-helical 31-mer with a palmitoyl (C₁₆) chain bonded to the N-terminal cysteine to enhance the peptide's amphiphilicity. The units dimerize via a disulfide bridge between the two cysteines (alpha-S-S-alpha) and form two bis-His heme-binding sites. Prior x-ray reflectivity measurements on monolayers of pure apoBBC16 and mixtures with palmitic acid (PA) showed that the peptide's orientation at the air/water interface changes with surface pressure: the alpha-helical axes are parallel to the interface at low pressures and nearly perpendicular at high pressures. To enhance the ordering of the alkyl chains and provide a more fluid matrix for the peptide, we made mixed monolayers of apoBBC16 and DLPE. The structural features and pressure-dependence of these monolayers are similar to those of pure BBC16 and BBC16/PA mixtures. So far we are unable to gain information about the packing of the alpha-helices by in-plane diffraction (GID). Nonetheless, the enhanced stability of the mixed monolayers of DLPE and BBC16 at high pressures should facilitate neutron reflectivity measurements on monolayers made with selectively deuterated BBC16. These can provide a set of key distance measurements probing the intra-molecular structure of not only apoBBC16, but also BBC16 with prosthetic groups bound.