On the use of the phase memory time T2 for the quantitative characterization of the rotational motions of proteins in lipid bilayer systems.

Numerical simulations of the echo responses from a nitroxide label rigidly attached to a large protein undergoing ultraslow rotational motions in a lipid bilayer are presented. The echoes are formed by the application of Hahn, COSY, and 2D-ELDOR sequences utilizing both soft and hard microwave pulses. The simulations address the question of whether the echo responses elicited by these sequences are affected by restricted angular excursions of the long axis of the protein relative to the normal to the bilayer plane. The results indicate that all three pulse sequences yield the same quantitative motional information regardless of the nature of the microwave pulses and there is no theoretical reason for preferring one sequence above the others.

Influence of High Orientational Order on the Shape of the Echo Response from a Hahn Pulse Sequence

The amplitude modulations in the simulations of the Hahn echo responses from cholestane spin labels in samples characterized by a high degree of orientational order are shown to arise from the use of "soft" pulses. Soft pulses have a limited spectral range and cover only a small portion of the CW-ESR spectra, so that not all the spins are on-resonance. The magnetization vectors of the off-resonance spins only partially tilted away from the laboratory z axis, the direction of the applied static magnetic field. They thus contribute oscillating components to the magnetization in the xy plane. The contribution from the off-resonance spins to the Hahn echo formation is significant in highly oriented samples, but cancels out in samples exhibiting a small degree of order. Experimental echo responses obtained from CSL molecules embedded in rigid matrices of eggPC bilayers and the liquid crystalline materials ZLI and MBBA confirm the theoretical predictions. Copyright 1998 Academic Press.

Rotational motion of Ca-ATPase monitored by electron spin echoes.

Electron spin echoes are used to study the dynamics of different aggregational forms of spin-labeled Ca-ATPase in the sarcoplasmic reticulum membrane. The 2D-ESE measurements are sensitive to motions on the microsecond time scale. The motional information is extracted from the variation 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 membrane surface and only undergoes rotational motion about its long axis. The echo-amplitude decays have been evaluated in the time domain by solving the Bloch equations for the stochastic spin Hamiltonian on making use of stochastic trajectories for the orientational behavior of the spin-labeled protein. This approach provides a useful insight into the information provided by the 2D-ESE measurements and affords a direct comparison of the results obtained with different experimental techniques. It is shown that the 2D-ESE technique monitors the orientational motions of dimers or larger aggregates of Ca-ATPase molecules whose rotational correlation times vary between 200 microseconds and 1 ms for the temperature range between 37 and 4 degrees C.

Determination of the directions of the transition dipoles in tetrabutylperylene in stretched polymers.

The directions of the transition dipole moments of 2,5,8,11,-tetra-butylperylene were determined from angle-resolved fluorescence depolarization experiments on molecules embedded in a stretched anhydrous nitrocellulose matrix. The absorption transition moments lies almost parallel to the elongated axis of the molecule, but the emission transition moment makes an angle of 20° with the axis. The orientational distribution of the molecules in the polymer indicates significant deviations from a circular form.