Solvent effect on the distance distribution between spin labels in aggregated spin labeled trichogin GA IV dimer peptides as studied by pulsed electron-electron double resonance

Pulsed electron-electron double resonance (PELDOR) was used to study aggregate formation in frozen glassy solutions of mono- and double-spin labeled trichogin GA IV dimers in a toluene-methanol mixture. The modified method proposed and used for the distance distribution function calculation from the PELDOR data. Distance distribution functions between spin labels in the peptide molecules and their aggregates in solution were determined as a function of solvent composition. Double-labeled peptide molecules in aggregates in solutions with low methanol content display two types of structures, i.e. the alpha-helix with a 2.8 nm distance between labels and the 3(10)-helix with a 3.2 nm distance between labels. As the methanol content of the solvent increases, a part of conformations at 3.2 nm changes. An increase of the methanol content leads to disruption of the aggregates and a change to the peptide conformation as well. In pure methanol peptides fail to form aggregates and a wide distribution of distances between labels centered at 3 nm were observed.