Broken helix in vesicle and micelle-bound α-synuclein:: Insights from site-directed spin labeling-EPR experiments and MD simulations

The region 35-43 of human alpha-Synuclein bound to small unilamellar lipid vesicles and to sodium dodecyl sulfate micelles has been investigated by site-directed spin labeling and electron paramagnetic resonance spectroscopy. The distance distributions obtained from spectral fitting have been analyzed on the basis of the allowed rotamers of the spin-label side-chain. Very similar results have been obtained in the two environments: an unbroken helical structure of the investigated region can be ruled out. The distance distributions are rather compatible with the presence of conformational disorder, in agreement with previous findings for micelle-bound a-Synuclein. The propensity for helix breaking is confirmed by molecular dynamics simulations.