ROTATIONAL MOTION OF CA-ATPASE MONITORED BY ELECTRON-SPIN ECHOES

Electron spin echoes are used to study the dynamics of different aggre gational forms of spin-labeled Ca-ATPase in the sarcoplasmic reticulum membrane, The 2D-ESE measurements are sensitive to motions on the mic rosecond time scale, The motional information is extracted from the va riation of the echo decays across the CW-ESR absorption spectrum, The motional contribution to the decays is described by assuming that the Ca-ATPase molecule is perfectly oriented along the normal to the membr ane surface and only undergoes rotational motion about its long axis, The echo-amplitude decays have been evaluated in the time domain by so lving the Bloch equations for the stochastic spin Hamiltonian on makin g use of stochastic trajectories for the orientational behavior of the spin-labeled protein, This approach provides a useful insight into th e information provided by the 2D-ESE measurements and affords a direct comparison of the results obtained with different experimental techni ques, It is shown that the 2D-ESE technique monitors the orientational motions of dimers or larger aggregates of Ca-ATPase molecules whose r otational correlation times vary between 200 mu s and 1 ms for the tem perature range between 37 and 4 degrees C. (C) 1996 Academic Press, In c.