MOLECULAR-PARAMETERS OF SEMISYNTHETIC DERIVATIVES OF GANGLIOSIDES ANDSPHINGOSINE IN MONOLAYERS AT THE AIR-WATER-INTERFACE

The molecular parameters (molecular area, surface potential, collapse pressure, dipole moment contributions) of semisynthetic derivatives of ganglioside GM1 and of sphingosine were studied in lipid monolayers a t the air-NaCl (145 mM, pH 5.6) interface at 22 +/- 0.3-degrees-C. The chemical modifications included alterations of the fatty acyl chain m oiety linked to the 2-amino position of the sphingosine (Sph) base. Th e compounds studied were PKS-1 (N-acetyl Sph), PKS-2 (N-chloroacetyl S ph), PKS-3 (N-dichloroacetyl Sph), PKS-4 (N-trichloroacetyl Sph), Lyso -GM1 (ganglioside GM1 lacking the N-linked fatty acyl chain and the N- acetyl group on the sialic acid), Lip-4 (N-acetyl, lyso[NeuAc]GM1) and Liga-20 (N-dichloroacetyl, lyso[NeuAc]GM1). Relatively small modifica tions of the chemical structure of sphingolipids introduce dramatic co nsequences on their surface molecular properties. The absence of the l ong chain fatty acyl moiety and of the N-acetyl group on the neuramini c acid in Lyso-GM1 leads to a more condensed behavior and to an increa se of the collapse pressure compared with GM1. The acetylation or chlo roacetylation at the 2-amino position in Liga-4 and Lip-20 induce an e xpansion of the surface pressure-area isotherm and a decrease of the c ollapse pressure. The limiting molecular areas of GM1 derivatives, tak en at the collapse pressure point, are consistent with the oligosaccha ride chain being oriented approximately perpendicularly to the interfa ce. Sphingosine shows a liquid expanded isotherm. The acetylation and successive chlorination of the acetyl residue at the 2-amino position of Sph cause a progressive increase in the limiting molecular area. Th e variation of the resultant dipole moment under compression, calculat ed from the surface potential values, suggests the reorientation of se lective groups within these molecules that depend on the degree of int ermolecular packing. Thermodynamic-geometric correlations on the basis of the molecular parameters of these derivatives suggest that small a lterations of the substituent group at the 2-amino position of Sph cou ld have large and amplified consequences on the type, curvature and st ability of the possible self-aggregated structure that these lipids ma y form in aqueous medium.