Supramolecular structure of self-assembling alamethicin analog studied by ESR and PELDOR.
Three analogs of alamethicin F50/5, labelled with the TOAC (='2,2,6,6-tetramethylpiperidin-1-oxyl-4-amino-4-carboxylic acid') spin label at positions 1 (Alm1), 8 (Alm8), and 16 (Alm16), resp., were studied by Electron-Spin-Resonance (ESR) and Pulsed Electron-Electron Double-Resonance (PELDOR) techniques in solvents of different polarity to investigate the self-assembly of amphipathic helical peptides in membrane-mimicking environments. In polar solvents, alamethicin forms homogeneous solutions. In the weakly polar chloroform/toluene 1 : 1 mixture, however, this peptide forms aggregates that are detectable at 293 K by ESR in liquid solution, as well as by PELDOR in frozen, glassy solution at 77 K. In liquid solution, free alamethicin molecules and their aggregates show rotational-mobility correlation times tau(r) of 0.87 and 5.9 ns, resp. Based on these values and analysis of dipole-dipole interactions of the TOAC labels in the aggregates, as determined by PELDOR, the average number N of alamethicin molecules in the aggregates is estimated to be less than nine. A distance-distribution function between spin labels in the supramolecular aggregate was obtained. This function exhibits two maxima: a broad one at a distance of 3.0 nm, and a wide one at a distance of ca. 7 nm. A molecular-dynamics (MD)-based model of the aggregate, consisting of two parallel tetramers, each composed of four molecules arranged in a 'head-to-tail' fashion, is proposed, accounting for the observed distances and their distribution.
Milov
Alexander D
AD
Institute of Chemical Kinetics and Combustion, Novosibirsk, Russian Federation.
Samoilova
Marina I
MI
Tsvetkov
Yuri D
YD
Jost
Micha
M
Peggion
Cristina
C
Formaggio
Fernando
F
Crisma
Marco
M
Toniolo
Claudio
C
Handgraaf
Jan-Willem
JW
Raap
Jan
J
D000408:Alamethicin / Q000737:chemistry*; D000900:Anti-Bacterial Agents / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D011487:Protein Conformation
D000900:Anti-Bacterial Agents; D000408:Alamethicin