title: Depth-dependent fluorescent quenching of a tryptophan residue located at defined positions on a rigid 21-peptide helix in liposomes.
abstract: Five lipophilic 21-peptide analogs of the potential-dependent pore-former, alamethicin, were synthesized bearing tryptophan residues at the position 1, 6, 11, 16 and 21 on a long, conformationally rigid, alpha-helix. The alpha-helical conformation was induced and stabilized using the sequential oligomers (Ala-Aib-Ala-Aib-Ala)n as analyzed by CD and NMR. The partitioning of the N-t-butoxycarbonyl 21-peptide methyl esters and the N-terminally deprotected alpha-helices was followed by fluorescence enhancement in phospholipid bilayer vesicles. Quenching experiments were performed by titrating with n-doxyl stearic acids bearing the nitroxide label at positions 5, 7, 10, 12 and 16. This well-defined system revealed that the N- and C-terminal tryptophan residues become situated in the hydrophilic region. Tryptophan at position 11 was found in the lipophilic core, whereas the tryptophan at positions 6 and 16 were localized at intermediate depths of the lipid membrane. Therefore, the helices span the lipid bilayer with their long axis normal to the membrane surface.
mesh_terms: D000408:Alamethicin / Q000031:analogs & derivatives; D002942:Circular Dichroism; D007202:Indicators and Reagents; D008081:Liposomes; D008954:Models, Biological; D010455:Peptides / Q000138:chemical synthesis*; D010713:Phosphatidylcholines; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D013329:Structure-Activity Relationship; D014364:Tryptophan
chemical_list: D007202:Indicators and Reagents; D008081:Liposomes; D010455:Peptides; D010713:Phosphatidylcholines; D000408:Alamethicin; D014364:Tryptophan