EXPERIMENTAL MOLECULAR-DYNAMICS OF AN ALANINE-BASED HELICAL PEPTIDE DETERMINED BY SPIN-LABEL ELECTRON-SPIN-RESONANCE

The alanine-based 3K(I) peptide is reported to be very helical in aque ous solution. We have prepared a series of six nitroxide spin labeled analogs of the 3K(I) sequence and measured the variable-temperature ES R spectra for each in order to reveal the position-dependent peptide d ynamics. From analysis of these local dynamics under helix-forming con ditions at 1-degrees-C, we find that the helix termini show greater lo cal dynamics than the peptide center. Further, the C-terminus is more mobile than the N-terminus. Even in the helix-promoting solvent triflu oroethanol, the results indicate that there is still substantially gre ater dynamics at the helix termini than at the peptide center. The unf olded state is also investigated, and we find that the peptide unfolde d by guanidine hydrochloride is somewhat different than that found for high-temperature aqueous solution. Recently it was suggested that sho rt 16-mer peptides may adopt a 3(10)-helix structure instead of the ex pected alpha-helix. The data presented here at 1-degrees-C show that t here is sufficient disorder with in the peptide to accommodate the 3(1 0) structure. Also calculated are the backbone torsional fluctuations, and the results compare well to those from computer molecular dynamic s studies. A proposal is outlined that explains how the enhanced dynam ics found at the C-terminus results from the exposure of the helix hyd rogen bonds to aqueous solvent in this region of the peptide.