Langmuir monolayers as model membranes possess several advantages: control over thermodynamic variables of surface pressure, area/molecule, temperature, etc.; an effectively infinite aqueous subphase that does not constrain the hydrophilic domains of associated proteins; possible transfer to solid supports. The foremost means for structural characterization of these monolayers, x-ray and neutron reflectivity, determine the profile structure, the mean scattering length density distribution of the monolayer projected onto the coordinate normal to the liquid/vapor interface. For asymmetric molecules, this can be sufficient to determine the mean orientation of the monolayer constituents, but affords little information to locate sub-molecular moieties. For lipid/peptide mixtures, distinguishing the contribution from different components requires systematic study of varying mole ratios, unless the contrast between different components or sub-components can be varied. Solid phase synthesis of peptides permits selective labeling of individual residues with heavy atoms (Br, Se) whose scattering factors change in the vicinity of their x-ray absorption K edges. The comparison of x-ray reflectivity collected with incident photon energies below the edge and near the edge determines the profile structure of the monolayer and also the location of the resonant atom label within that profile structure with sub-Angstrom-level precision. This technique, especially when coupled with molecular dynamics simulations, allows us to dissect the profile structure of the monolayer and provides an alternative to neutron reflectivity from H2-labeled peptides, as demonstrated by data from Br-labeled di-alpha-helical peptides.
This work supported by the NIH (GM55876). The Advanced Photon Source is operated by the DOE under contract W-31-109-Eng-38.