Thermal behavior of fractionated and unfractionated bovine brain cerebrosides

Abstract

Bovine brain cerebrosides were fractionated into 2-hydroxy fatty acid containing cerebrosides (HFA-CER) and nonhydroxy fatty acid containing cerebrosides (NFA-CER). The thermal behavior of NFA-CER, HFA-CER, and unfractionated cerebroside model membranes was studied by differential scanning calorimetry. When NFA-CER is cooled at rates .gtoreq. 2.5 .degree. C/min, subsequent heating runs exhibit metastable behavior: a low enthalpy exotherm is observed at .apprx. 50.degree. C (.DELTA.H = -(1-3) cal/g), followed by a high enthalpy endotherm at 72.degree. C (.DELTA.H = 16-17 cal/g). Systematic variation of cooling/heating protocols indicates that NFA-CER possesses 2 low-temperature states, 1 metastable and the other stable. Cooling from liquid-crystalline state results in formation of the metastable low-temperature polymorph I, which must transform into the stable low-temperature polymorph II before the liquid-crystalline state can be reached again. By analogy with recent X-ray studies of synthetic N-palmitoylgalactosylsphingosine, it is proposed that metastable polymorph I is dehydrated relative to stable polymorph II. HFA-CER displays no metastability and exhibits a reversible thermal transition at .apprx. 68.degree. C (.DELTA.H = 7.3 cal/g). The thermal behavior of unfractionated cerebrosides is similar to that of HFA-CER, exhibiting a single reversible transition at .apprx. 67.degree. C (UDH = 6.9 cal/g). A function of hydroxy fatty acids in brain cerebrosides may be to prevent metastable dehydration in the cerebroside-rich myelin membrane.