Saturation transfer ESR studies of molecular motion in phosphatidylglycerol bilayers in the gel phase: effects of pretransitions and pH titration

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).

Lipid-lipid and lipid-protein interactions in chromaffin granule membranes. A spin label ESR study

The ESR spectra of six different positional isomers of a stearic acid and three of a phosphatidylcholine spin label have been studied as a function of temperature in chromaffin granule membranes from the bovine adrenal medulla, and in bilayers formed by aqueous dispersion of the extracted membrane lipids. Only minor differences were found between the spectra of the membranes and the extracted lipid, indicating that the major portion of the membrane lipid is organized in a bilayer arrangement which is relatively unperturbed by the presence of the membrane protein. The order parameter profile of the spin label lipid chain motion is less steep over the first half of the chain than over the section toward the terminal methyl end of the chain. This 'stiffening' effect is attributed to the high proportion of cholesterol in the membrane and becomes less marked as the temperature is raised. The isotropic hyperfine splitting factors of the various positional isomers display a profile of decreasing polarity as one penetrates further into the interior of the membrane. No marked differences are observed between the effective polarities in the intact membranes and in bilayers of the extracted membrane lipids. The previously observed temperature-induced structural change occurring in the membranes at approx. 35 degrees C was found also in the extracted lipid bilayers, showing this to be a result of lipid-lipid interactions and not lipid-protein interactions in the membrane. A steroid spin label indicated a second temperature-dependent structural change occurring in the lipid bilayers at lower temperatures. This correspond to the onset of a more rapid rotation about the long axis of the lipid molecules at a temperature of approx. 10 degrees C. The lipid bilayer regions probed by the spin labels used in this study may be involved in the fusion of the chromaffin granule membrane leading to hormone release by exocytosis.

Simple metallic compounds as pulp-capping agents

A study is described in which six metallic compounds identified in a previous pilot experiment were tested as pulp-capping agents using the exposed rat molar. Barium sulfate, calcium carbonate, cupric oxide, ferric oxide, and heavy magnesium oxide gave poor results. Stannic oxide gave more favorable results and is being studied further.

Simple metallic compounds as pulp-capping agents. A pilot study

A pilot study is described in which the exposed pulps of molar teeth of normal laboratory rats were capped with various metallic compounds in general medical usage. There appeared to be no relationship between the response obtained and the pH of the compounds studied. Aluminum powder, aluminum hydroxide, Aludrox, aluminum oxide, light magnesium oxide, magnesium hydroxide, and titanium dioxide gave poor results. Barium sulfate, calcium carbonate, cupric oxide, ferric oxide, heavy magnesium oxide, and stannic oxide merited more detailed study.

Cooperativity of the phase transition in single- and multibilayer lipid vesicles

The effect of membrane morphology on the cooperativity of the ordered-fluid, lipid phase transition has been investigated by comparing the transition widths in extended, multibilayer dispersons of dimyristoyl phosphatidyl-choline, and also of dipalmitoyl phosphatidylcholine, with those in the small, single-bilayer vesicles obtained by sonication. The electron spin resonance spectra of three different spin-labelled probes, 2,2,6,6-tetramethylpiperdine-N-oxyl, phosphatidylcholine and stearic acid, and also 90 degrees light scattering and optical turbidity measurements were used as indicators of the phase transition. In all cases the transition was broader in the single-bilayer vesicles than in the multibilayer dispersions, corresponding to a decreased cooperativity on going to the small vesicles. Comparison of the light scattering properties of centrifuged and uncentrifuged, sonicated vesicles suggests that these are particularly sensitive to the presence of intermediate-size particles, and thus the spin label measurements are likely to give a more reliable measure of the degree of cooperativity of the small, single-bilayer vesicles. Application of the Zimm and Bragg theory ((1959) J. Chem. Phys. 31, 526-535) of cooperative transitions to the two-dimensional bilayer system shows that the size of the cooperative unit, 1/square root sigma, is a measure of the mean number of molecules per perimeter molecule, in a given region of ordered or fluid lipid at the centre of the transition. From this result it is found that it is the vesicle size which limits the cooperativity of the transition in the small, single-bilayer vesicles. The implications for the effect of membrane structure and morphology on the cooperativity of phase transitions in biological membranes, and for the possibility of achieving lateral communication in the plane of the membrane, are discussed.

Evidence for phase boundary lipid. Permeability of Tempo-choline into dimyristoylphosphatidylcholine vesicles at the phase transition

The existence of distinct regions of mismatch in molecular packing at the interfaces of the fluid and ordered domains during the phase transition of dimyristoylphosphatidylcholine vesicles has been demonstrated by measuring the temperature dependence of the permeability to a spin-label cation and comparing this with a statistical mechanical calculation of the fraction of interfacial lipid. The kinetics of uptake and release of the 2,2,6,6-tetramethylpiperidinyl-1-oxycholine (Tempo-choline) spin label by single-bilayer dimyristoylphosphatidylcholine vesicles were measured using electron spin resonance spectroscopy to quantitate the amount of spin label present within the vesicles after removal of the external spin-label by ascorbate at 0 degrees C. Both the uptake and release experiments show that the Tempo-choline permeability peaks to a sharp maximum at the lipid-phase transition, the vesicles being almost impermeable to Tempo-choline below the transition and having a much reduced permeability above. The temperature profile of the permeability is in reasonable quantitative agreement with calculations of the fraction of interfacial boundary lipid from the Zimm and Bragg theory of cooperative transitions, which use independent spin-label measurements of the degree of transition to determine the cooperativity parameter. The relatively high intrinsic permeability of the interfacial regions (P approximately 0.2-1.0 X 10(-8) cm/s) is attributed to the mismatch in molecular packing of the lipid molecules at the ordered-fluid boundaries, which could have important implications not only for permeability in natural membranes (e.g., in transmitter release), but also for the function of membrane-bound enzymes and transport proteins.