PROPERTIES OF GANGLIOSIDE G(M1) IN PHOSPHATIDYLCHOLINE BILAYER-MEMBRANES

Gangliosides have been shown to function as cell surface receptors, as well as participating in cell growth, differentiation, and transforma tion. In spite of their multiple biological functions, relatively litt le is known about their structure and physical properties in membrane systems. The thermotropic and structural properties of ganglioside G(M 1) alone and in a binary system with 1,2-dipalmitoyl phosphatidylcholi ne (DPPC) have been investigated by differential scanning calorimetry (DSC) and x-ray diffraction. By DSC hydrated G(M1) undergoes a broad e ndothermic transition T-M = 26 degrees C (Delta H = 1.7 kcal/mol G(M1) ). X-ray diffraction below (-2 degrees C) and above (51 degrees C) thi s transition indicates a micellar structure with changes occurring onl y in the wide angle region of the diffraction pattern (relatively shar p reflection at 1/4.12 Angstrom(-1) at -2 degrees C; more diffuse refl ection at 1/4.41 Angstrom(-1) at 51 degrees C). In hydrated binary mix tures with DPPC, incorporation of G(M1) (0-30 mol %; zone 1) decreases the enthalpy of the DPPC pretransition at low molar compositions whil e increasing the T-M of both the pre- and main transitions (limiting v alues, 39 and 44 degrees C, respectively). X-ray diffraction studies i ndicate the presence of a single bilayer gel phase in zone 1 that can undergo chain melting to an L(alpha) bilayer phase. A detailed hydrati on study of G(M1) (5.7 mol %)/DPPC indicated a conversion of the DPPC bilayer gel phase to an infinite swelling system in zone 1 due to the presence of the negatively charged sialic acid moiety of G(M1). At 30- 61 mol % G(M1) (zone 2), two calorimetric transitions are observed at 44 and 47 degrees C, suggesting the presence of two phases. The lower transition reflects the bilayer gel --> L(alpha) transition (zone 1), whereas the upper transition appears to be a consequence of the format ion of a nonbilayer, micellar or hexagonal phase, although the structu re of this phase has not been defined by x-ray diffraction. At >61 mol % G(M1) (zone 3) the calorimetric and phase behavior is dominated by the micelle-forming properties of G(M1); the presence of mixed G(M1)/D PPC micellar phases is predicted.