Saturation transfer, continuous wave saturation, and saturation recovery electron spin resonance studies of chain-spin labeled phosphatidylcholines in the low temperature phases of dipalmitoyl phosphatidylcholine bilayers. Effects of rotational dynamics and spin-spin interactions

The saturation transfer electron spin resonance (STESR) spectra of 10 different positional isomers of phosphatidylcholine spin-labeled in the sn-2 chain have been investigated in the low temperature phases of dipalmitoyl phosphatidylcholine (DPPC) bilayers. The results of continuous wave saturation and of saturation recovery measurements on the conventional ESR spectra were used to define the saturation properties necessary for interpreting the STESR results in terms of the chain dynamics. Spin labels with the nitroxide group located in the center of the chain tended to segregate preferentially from the DPPC host lipids in the more ordered phases, causing spin-spin interactions which produced spectral broadening and had a very pronounced effect on the saturation characteristics of the labels. This was accompanied by a large decrease in the STESR spectral intensities and diagnostic line height ratios relative to those of spin labels that exhibited a higher degree of saturation at the same microwave power. The temperature dependence of the STESR spectra of the different spin label isomers revealed a sharp increase in the rate of rotation about the long axis of the lipid chains at approximately 25 degrees C, correlating with the pretransition of gel phase DPPC bilayers, and a progressive increase in the segmental motion towards the terminal methyl end of the chains in all phases. Prolonged incubation at low temperatures led to an increase in the diagnostic STESR line height ratios in all regions of the spectrum, reflecting the decrease in chain mobility accompanying formation of the subgel phase. Continuous recording of the central diagnostic peak height of the STESR spectra while scanning the temperature revealed a discontinuity at approximately 14-17 degrees C, corresponding to the DPPC subtransition which occurred only on the initial upward temperature scan, in addition to the discontinuity at 29-31 degrees C corresponding to the pretransition which displayed hysteresis on the downward temperature scan.

Rotational motion of yeast cytochrome oxidase in phosphatidylcholine complexes studied by saturation-transfer electron spin resonance

Cytochrome oxidase from yeast has been covalently labeled with a nitroxide derivative of maleimide and reconstituted in lipid-substituted complexes with dimyristoyl-, dioleoyl-, or dielaidoyl-phosphatidylcholine. The rotational mobility of the enzyme in the complexes has been studied as a function of temperature and time, and of lipid/protein ratio, using saturation-transfer electron spin resonance spectroscopy. For complexes with dimyristoylphosphatidylcholine, the rotational mobility of the protein decreases abruptly below the gel-to-fluid-phase transition. This change is accompanied by a lateral segregation of the protein, as seen by freeze-fracture electron microscopy, and by an increase in the activation energy for the enzymatic activity. A time-dependent decrease in the rotational motion of the protein is observed on incubating at temperatures in the fluid phase of the lipid. This corresponds with a time-dependent loss of enzyme activity observed on incubation at temperatures in the fluid phase, but not at temperatures in the gel phase, over a period of 3 h. The rotational mobility decreases with increasing protein concentration in the complexes, both in the fluid and in the gel phases. The dependence of the protein mobility on lipid/protein ratio can be interpreted quantitatively in terms of the effect of increased random protein-protein contacts in the fluid phase. The maximum limiting rotational correlation time for the protein diffusion at high lipid/protein ratios in the fluid phase is tau R[[ approximately equal to 25 microseconds, suggesting that the protein is present as either a monomer or more probably a dimer in the reconstituted membrane.

Measurement of rotational molecular motion by time-resolved saturation transfer electron paramagnetic resonance

We have used saturation-recovery electron paramagnetic resonance (SR-EPR), a time-resolved saturation transfer EPR technique, to measure directly the microsecond rotational diffusion of spin-labeled proteins. SR-EPR uses an intense microwave pulse to saturate a spin population having narrow distribution of orientations with respect to the magnetic field. The time evolution of the signal is then observed. The signal increases in time as saturation is relieved by spin-lattice relaxation (Tl) as well as by saturation transfer due to spectral diffusion (Tsd), which is a function of rotational diffusion (Tr) and spectral position. In the presence of both events, the recovery is biphasic, with the initial phase related to both Tr and Tl, and the second phase determined only by Tl. We have measured the saturation recoveries of spin-labeled hemoglobin tumbling in media of known viscosities as a function of rotational correlation time (Tr) and pulse duration (tp). The Tr values estimated from the initial phase of recovery were in good agreement with theory. Variation of the pulse time can also be used to determine Tr. For tp less than Tsd, the recoveries were observed to be biphasic, for tp greater than Tsd a single-exponential. T1 values were determined from the recoveries after pulses quenching spectral diffusion or from the second phase of recovery after shorter pulses. These results demonstrate that SR-EPR is applicable to the study of motion of spin-labeled proteins. Its time resolution should provide a significant advantage over steady state techniques, particularly in the case of motional anisotropy or system heterogeneity.