Small unilamellar dipalmitoylphosphatidylcholine vesicles formed by sonication are shown to fuse spontaneously below the phase transition temperature. The ultimate fusion products are unilamellar vesicles about 700 A in diameter, which are stable and provide an intact ionic permeation barrier either above or below the phase transition. The fused vesicles have been characterized by gel chromatography, trapped volume, 31P nuclear magnetic resonance, and negative stain and freeze-fracture electron microscopy.

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Mixtures of dipalmitoylphosphatidylcholine (DPPC) with palmitic, stearic, and myristic acids and the sodium salts of these acids were analyzed by differential thermal analysis (DTA) over a wide range of lipid compositions, all in excess water. All three fatty acids raise the liquid-crystal phase transition temperature and form sharp-melting complexes, with 1:2 DPPC--fatty acid stoichiometry observed for palmitic and stearic acids and suggested for myristic acid. Phase diagrams of the peritectic type, indicating nonideal mixing, was fitted to the DPPC--palmitic acid and DPPC--stearic acid data. In contrast, DPPC forms nearly ideal mixtures with the putative DPPC--myristic acid complex. At levels of only a few mole percent, both sodium stearate and myristate remove the pretransition and main transition and produce new peaks at approximately 30 and approximately 48 degrees C; the relative areas of the new peaks were unreproducible for the DPPC--myristate system. Sodium palmitate is the least disruptive of any of the sodium soaps or fatty acids; up to 80 mol % palmitate, the transition is lowered 3 degrees C and approximately doubled in width. The pretransition is detectable up to 36 mol %, and the main transition persists up to 88 mol % palmitate. The apparent pK of palmitic acid (12 mol %) in DPPC bilayers was determined to be 10.2 by direct pH measurement of ternary DPPC mixtures with known palmitic acid/sodium palmitate ratios; the intrinsic pK is estimated to be less than or approximately 8.5.

BIOCHEMISTRY 3882 nature of solvent. Furthermore it seems that there is little influence on bond stability due to neighboring CH3 substituents, for if methyl group effects were present these should be modified by a transfer from an aqueous to an apolar solvent. The intrinsic interaction between hydrogen-donor and hydrogen-acceptor groups is thus not sensitive to its immediate (noncovalent) environment.

This study investigated the relative benefits to the student of working in homogeneous versus heterogeneous classroom groups. Correlation analysis of 18 desirable outcomes versus 8 personality-based heterogeneity variables reveals that heterogeneity associates with advantages as well as disadvantages. Ways in which group composition might be customized to benefit students with particular needs or to emphasize particular goals are described, along with their potential downsides.