Molecular dynamics and spatial distribution of TOAC spin-labelled peptaibols studied in glassy liquid by echo-detected EPR spectroscopy

2,2,6,6-tetramethylpiperidine-1-oxyl-4-carboxylic acid (TOAC) spin-labelled analogues of the Aib-rich peptide (peptaibol) Trichogin GA IV are investig ated in a glassy methanol/glycerol (70/30 v/v%) system, using conventional CW EPR and electron spin echo spectroscopy. Echo-detected (ED) EPR spectra indicate that the labels undergo restricted orientational motion (libration ). Comparison with the small molecular spin probe Tempone shows that the dy namics of peptide molecules is determined by their structure and not by the surrounding medium. At the terminal positions (position 1 and 8) TOAC resi dues were found to be more flexible than at the central 4th position. Insta ntaneous diffusion mechanism in electron spin echo, which also contributes to the ED EPR spectra, was employed for deriving the local concentration of peptaibols. This approach may serve as a tool for investigation of peptide aggregation. In the studied model glassy solution the peptaibols were foun d to be randomly distributed.