High sensitivity differential scanning calorimetry is employed in the study of the gel to liquid crystal phase transition of a highly purified sample of dipalmitoylphosphatidylchohne imuilaear bilayer suspension. It is concluded from the calorimetric data that the purity of the sample is better than 99.94 mol % and that the transition closely approximates an isothermal first-order transition. The gel to liquid crystal transitions shown by synthetic phosphatidylcholines in multilamellar aqueous suspension have been found to possess a finite width, even when examined by a nonperturbing technique such as differential scanning calorimetry. Because such behavior has been observed with materials that by the usual criteria (such as thin-layer chromatography) appear to be quite pure, the question arises (1) as to whether these transitions deviate for some fundamental reason from the very nearly isothermal first-order melting transitions observed with pure crystalline substances. To investigate this question further we have studied by means of high sensitivity differential scanning calorimetry the transition shown by a highly purified sample of dipalmitoylphosphatidylcholine (DPPC). MATERIALS AND METHODSPreparation of DPPC. Very pure DPPC was prepared by synthesis from carefully purified components and subsequent recrystallization of the product (2).Fatty Acids. Many commercial samples of palmitic acid are adequately pure and sufficiently crystalline to use as the starting material for zone refining. A critical part of the process is selecting the most efficient rate of zone movement, which is determined by the rate at which a planar crystalline interface of the material will advance. This was estimated from crystal growth measurements (3) to be 5 cm per day. The use of multiple zones with heaters and coolers and a rotating horizontal tube (4) enabled a purification cycle of 20 zone passes to be completed in 10 days. Analysis of the acid by gas-liquid chromatography during refining showed that the major impurities are other fatty acids, and that these can be reduced to less than 1:1000.The purified acid was converted to the acid anhydride by reaction with dicyclohexyl carbodiimide (5). An acyl urea is formed as a byproduct; it was removed by zone refining, using a procedure similar to that employed with the acid.Glycerophosphocholine. The glycerophosphocholine part of the lecithin molecule having the naturally occurring configuration was prepared by modification of a published procedure (6). Changes in the procedure included dehydration of the crude lecithin solution by passage through a molecular sieve column. The cadmium chloride complex was crystallized slowly in a controlled manner from aqueous ethanol solution after seeding the slightly undersaturated solution. This procedure significantly affected the purity of the final product. After isolation the glycerophosphocholine readily crystallized from an aqueous solution on evaporation; reports in the literature emphasize the reluctance of this compound to cryst...

Quantitative experiments to investigate the effect of added impurity on the growth of sucrose crystals are described, l~ates of advance of growth steps were measured on crystals growing from pure solutions and from solutions to which raffinose was added. The temperature of the crystals was controlled at 30 °C. and the saturation ratio of the solutions was known. From the experimental results the diameter of the critical two dimensional nucleus at each saturation ratio was calculated and from this edge energy of a growth step.

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