A passage saturation transfer paramagnetic resonance study of the rotational diffusion of the sarcoplasmic reticulum calcium-ATPase.

A study was made of the dependence on temperature of the saturation transfer ESR spectra obtained from sarcoplasmic reticulum vesicles labeled with N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)-maleimide. When the spin-labeled preparation was heated, some change occurred that was accompanied by an increase in the spectral parameters L"/L and H"/H (Thomas et al. (1976). J. Chem. Phys. 65, 3006-3024). This heat-induced increase in L"/L and H"/H competed with the reduction in these parameters that would normally accompany a reduction in tau2, with the result that a biphasic response to increasing temperature was observed. The heat-induced perturbation was partially irreversible. Consequently, although the preparation also exhibited a biphasic response to cooling, the heating and cooling curves did not coincide. It is suggested that a heat-induced re-orientation of the nitroxide probe with respect to the membrane normal, together with the anisotropic nature of the rotational motion, could be the cause of the departure from the expected monotonic relationship between the spectral parameters and temperature.