Heavy-Atom Labeled Transmembrane β-Peptides: Synthesis, CD-Spectroscopy, and X-ray Diffraction Studies in Model Lipid Multilayer.

Transmembrane β-peptides are promising candidates for the design of well-controlled membrane anchors in lipid membranes. Here, we present the synthesis of transmembrane β-peptides with and without tryptophan anchors, as well as a novel iodine-labeled d-β3-amino acid. By using one or more of the heavy-atom labeled amino acids as markers, the orientation of the helical peptide was inferred based on the electron-density profile determined by X-ray reflectivity. The β-peptides were synthesized through manual Fmoc-based solid-phase peptide synthesis (SPPS) and reconstituted in unilamellar vesicles forming a right-handed 314-helix secondary structure, as shown by circular dichroism spectroscopy. We then integrated the β-peptide into solid-supported membrane stacks and carried out X-ray reflectivity and grazing incidence small-angle X-ray scattering to determine the β-peptide orientation and its effect on the membrane bilayers. These β-peptides adopt a well-ordered transmembrane motif in the solid-supported model membrane, maintaining the basic structure of the original bilayer with some distinct alterations. Notably, the helical tilt angle, which accommodates the positive hydrophobic mismatch, induces a tilt of the acyl chains. The tilted chains, in turn, lead to a membrane thinning effect.