Saturation transfer esr studies of molecular motion in phosphatidylglycerol bilayers in the gel phase Effects of pretransitions and pH titration

Abstract

The molecular motions of a phosphatidylglycerol spin label have been studied in dimyristoyl and dipalmitoyl phosphatidylglycerol bilayers below their ordered-fluid phase transition, both in the charged state at pH 8.0 and in the protonated state at pH 1.5. The saturation transfer ESR spectra, which are sensitive to motions in the correlation time range 10 −7—10 −3 s, show clear distinctions in the molecular motion both between the charged and the protonated states and between the states above and below the pretransition in the charged bilayers. At low temperatures, below the pH 8.0 pretransition, the saturation transfer ESR spectra indicate rather similar motion at pH 8.0 and 1.5, with the rates being approx. 2-times faster at pH 8.0. The effective rotational correlation times are approx. 0.5—1 · 10 −4 and approx. 1—2 · 10 −4 s, deduced from the outer lineheight ratios which are sensitive only to motion of the long molecular axis, and approx. 0.2—1 · 10 −4 and 0.2—0.5 · 10 −4 s, deduced from the central lineheight ratio which is sensitive also to rotation around the long molecular axis, where the first and second values refer to dimyristoyl and dipalmitoyl phosphatidylglycerol, respectively. As the temperature is increased the outer lineheight ratios at both pH values remain constant until the main transition, indicating little or no increase in motion of the long axis. In contrast, the central lineheight ratio at pH 8.0 shows a sharp decrease before the main transition, corresponding to cooperative onset of rapid rotation around the long molecular axis, at or immediately below the pretransition. The effective correlation time of this long axis rotation is approx. 10 −7 s, two orders of magnitude faster than that below the pretransition. The bilayers at pH 1.5, for which no pretransition is detected, show only a slow, non-cooperative decrease in the central lineheight ratio with increasing temperature, rapid long axis rotation in dimyristoyl phosphatidylglycerol occurring only at the main transition. The onset of long axis rotation at pH 8.0 is also detected in the conventional spectra of a steroid spin label, which begins to rotate around its long molecular axis with a correlation time of approx. 10 −9 s whilst the bilayers are still in the gel phase. These observations further strengthen the homology between phosphatidylglycerols in the charged state and the corresponding phosphatidylcholines, and provide the dynamic counterpart of the pH-induced structural change previously observed in phosphatidylglycerol bilayers in the gel phase (Watts, A., Harlos, K., Maschke, W. and Marsh, D. (1978) Biochim. Biophys. Acta 510, 63–74).