β-Glutamine-mediated self-association of transmembrane β-peptides within lipid bilayers

Transmembrane β-peptide helices and their association in lipid membranes are still widely unexplored. We designed and synthesized transmembrane β-peptides harboring different numbers of d-β³-glutamine residues (^hGln) by solid phase peptide synthesis. By means of circular dichroism spectroscopic measurements, the secondary structure of the β-peptides reconstituted into unilamellar vesicles was determined to be similar to a right-handed 3₁₄-helix. Fluorescence spectroscopy using d-β³-tryptophan residues strongly suggested a transmembrane orientation. Two or three ^hGln served as recognition units between the helices to allow helix-helix assembly driven by hydrogen bond formation. The association state of the transmembrane β-peptides as a function of the number of ^hGln residues was investigated by fluorescence resonance energy transfer (FRET). Therefore, two fluorescence probes (NBD, TAMRA) were covalently attached to the side chains of the transmembrane β-peptide helices. The results clearly demonstrate that only β-peptides with ^hGln as recognition units assemble into oligomers, presumably trimers. Temperature dependent FRET experiments further show that the strength of the helix-helix association is a function of the number of ^hGln residues in the helix.