Electron spin resonance and electron-spin-echo study of oriented multilayers of L alpha-dipalmitoylphosphatidylcholine water systems.

A detailed electron spin resonance (ESR) study of spin-labeled-oriented multilayers of L alpha-dipalmitoylphosphatidylcholine (DPPC) water systems for low water content (2-10% by weight) is reported with the purpose of characterizing the dynamical and structural properties of model membrane systems. Emphasis is placed on the value of combining such experiments with detailed simulations based on current slow-motional theories. Information is obtained regarding ordering and anisotropic rotational diffusion rates via ESR lineshape analysis over the entire motional range, from the fast motional region through the moderately slow and slow to the rigid limit. This includes the low-temperature gel phase, the liquid crystalline L alpha (1) phase and what appears to be a third high-temperature phase above the L alpha phase. Cholestane (CSL) and spin-labeled DPPC (5-PC, 8-PC, and 16-PC) have been used to probe different depths of the bilayer. While CSL and 5-PC both reflect the high ordering of the bilayer close to the lipid-water interface, CSL appears to be located close enough to the water for the nitroxide to be involved in hydrogen bonding with water molecules. 16-PC reflects the relatively low ordering near the tail of the hydrocarbon chain in the bilayer. Quantitative estimates of ordering and motion are obtained for these cases. The results from CSL indicate that close to the lipid-water interface the DPPC molecule is oriented approximately perpendicular to the bilayer in these low water-content systems. However, all three labeled lipid probes indicate that the hydrocarbon chain of DPPC may be bent away from the bilayer normal by as much as 30 degrees and this evidence is stronger at low temperatures. When cholesterol is added to the DPPC-water system at a concentration greater than or equal to 2.5 mol %, the ordering is greatly increased although the rotational diffusion rate remains almost unaffected in the gel phase. Electron spin echoes (ESE) are observed for the first time from oriented lipid-water multilayers. Results obtained from cw ESR lineshape analysis are correlated with data from ESE experiments, which give a more direct measurement of relaxation times. These results indicate that for detection of very slow motions (close to the rigid limit) ESE experiments are more sensitive to dynamics than continuous wave ESR for which inhomogeneous broadening becomes a major problem

SPIN-ALIGNED HYDROGEN : SOME CONSIDERATIONS FOR ESR VS. NMR EXPERIMENTS AND PRELIMINARY OBSERVATIONS OF H↑ AT LOW TEMPERATURES

Nous faisons quelques comparaisons entre les méthodes d'études de l'hydrogène polarisé (H↑) par résonance magnétique électronique et par résonance magnétique nucléaire dans des expériences de basse température qui ont été proposées ; en particulier nous considérons les sensibilités théoriques des deux méthodes, la relaxation de spin et l'amortissement radiatif. Nous envisageons également les caractéristiques des montages expérimentaux. Nous décrivons les résultats préliminaires d'expériences sur H↑ à 0,1-0,5 K et à 60 kG utilisant une détection par bolomètre. Ils sont cohérents avec ceux de l'expérience de Silvera-Walraven : à savoir qu'ils impliquent que H↑ peut être stabilisé dans de telles conditions lorsqu'on utilise un enduit de 4He sur la paroi du récipient.We consider aspects of theoretical sensitivities, spin-relaxation, and radiation damping in proposed low temperature ESR vs. NMR studies on spin-aligned hydrogen (H↑). Also considered are experimental design features. Results are described in a preliminary report of experiments on H↑ at 0.1-0.5°K at 60 kG using bolometer detection. They are consistent with the Silvera-Walraven experiment : viz they imply that H↑ may be stabilized under these conditions when a surface coating of 4He is utilized