The indanedione series of vinyl ketone spin-labelling reagents has been extended in two ways: by increasing the length of the rigid spacer between the reactive centre and the nitroxide ring, or by introducing an electrophilic substituent (that could also hinder its rotation) at the bridge head position of the nitroxide ring. Three reagents of this new series have been used to spin label the Class II thiol groups of membranous Na,K-ATPase from Squalus acanthias. With a conjugated diene spacer, the majority of spin labels are strongly held but a minor population is relatively mobile at 37 degrees C. With a conjugated triene spacer, the nitroxide is still strongly held but a portion of the label is non-covalently bound. The 4-bromo-pyrroline derivative (with short vinyl spacer) is tightly held at the attachment site, and the conventional electron paramagnetic resonance (EPR) spectra distinguish between the two enantiomeric structures which differ in their mobility at 37 degrees C. Saturation transfer EPR (ST-EPR) spectra of this label at 4 degrees C have been used to determine the dependence of the protein rotational mobility on ionic strength. Electrostatic repulsion contributes to the lateral interactions between Na,K-ATPase molecules.