STRUCTURE AND PROPERTIES OF N-PALMITOLEOYLGALACTOSYLSPHINGOSINE (CEREBROSIDE)

Differential scanning calorimetry (DSC) and X-ray diffraction have bee n used to study the structure and properties of N-palmitole oylgalacto sylsphingosine (NPoGS; 16.1 galactocerebroside). DSC of fully hydrated NPoGS shows a complex pattern of three endothermic transitions at 35, 39 and 53 degrees C. Using a combination of thermal protocols (varyin g heating/cooling rates, incubation at different temperatures, etc.), the three ordered chain (gel) phases responsible for the transitions h ave been isolated; transition I (T-m = 35 degrees C; Delta H-I = 6.3 k cal/mol), transition II (T-m = 39 degrees C; Delta H-II = 8.6 kcal/mol ), and transition III (T-m = 53 degrees C; Delta H-III, = 12.8 kcal/mo l). The gel phases do not interconvert but rather form independently f ollowing cooling from the melted chain phase. X-ray diffraction data o f the three isolated phases confirm that they all are bilayer structur es with different bilayer periodicities (L(I), 50.7 Angstrom; L(II) 51 .7 Angstrom; L(III) = 49.2 Angstrom) and different chain packing modes . The L(I), L(II), and L(III) bilayer phases each melt independently t o the melted chain L(a)lpha phase. Comparisons with other cerebrosides make it clear that alterations in chain length and chain unsaturation markedly affect the thermotropic behavior of cerebrosides and the met astable and stable phases they are able to form. As with phospholipids , introduction of cis-unsaturation into the N-acyl chain reduces both the chain melting temperature and enthalpy.