A Novel Heavy-Atom Label for Side-Specific Peptide Iodination: Synthesis, Membrane Incorporation and X-ray Reflectivity

Structural parameters, such as conformation, orientation and penetration depth of membrane-bound peptides and proteins that may function as channels, pores or biocatalysts, are of persistent interest and have to be probed in the native fluid state of a membrane. X-ray scattering in combination with heavy-atom labeling is a powerful and highly appropriate method to reveal the position of a certain amino acid residue within a lipid bilayer with respect to the membrane normal axis up to a resolution of several Angstrom. Herein, we report the synthesis of a new iodine-labeled amino acid building block. This building block is intended for peptide incorporation to provide high intensities for electron density difference analysis of X-ray reflectivity data and improve the labeling potential for the lipid bilayer head-group and water region. The novel building block as well as the commercially available non-iodinated analogue, required for X-ray scattering, was implemented in a transmembrane peptide motif via manual solid-phase peptide synthesis (SPPS) following the fluorenylmethyloxycarbonyl (Fmoc)-strategy. The derived peptides were reconstituted in lipid vesicles as well as in highly aligned multilamellar lipid stacks and investigated via circular dichroism (CD) and X-ray reflectivity. Thereby, it has been revealed that the bulky iodine probe neither causes conformational change of the peptide structure nor lamellar disordering of the membrane complexes.