ORIENTED 1,2-DIMYRISTOYL-SN-GLYCERO-3-PHOSPHORYLCHOLINE GANGLIOSIDE MEMBRANES - A FOURIER-TRANSFORM INFRARED ATTENUATED TOTAL-REFLECTION SPECTROSCOPIC STUDY - BAND ASSIGNMENTS - ORIENTATIONAL, HYDRATIONAL, ANDPHASE-BEHAVIOR - AND EFFECTS OF CA2+ BINDING/

Fourier transform infrared (FTIR) attenuated total reflection (ATR) sp ectroscopy was used to elucidate the hydration behavior and molecular order of phospholipid/ganglioside bilayers, We examined dry and hydrat ed films of the gangliosides GM1, deacetyl-GM1, lyso-GM1, deacetyllyso -GM1, and GM3 and oriented mixed films of these gangliosides with 1,2- dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) using polarized ligh t. Analysis of the amide I frequencies reveals that the amide groups a re involved in intermolecular interactions via hydrogen bonds of varyi ng strengths, The tilt angle of the acyl chains of the lipids in mixed films was determined as a function of ganglioside structure. Deacetyl ation of the sialic acid in the headgroup has a stronger influence on the tilt angle than the removal of the ganglioside fatty acid. The pha se behavior was examined by FTIR ATR spectroscopy and by differential scanning calorimetry (DSC) measurements on lipid suspensions, At the s ame molar concentration, lyso-gangliosides have less effect on changes of transition temperature compared to the double-chain analogs. Disti nct differences in the amide band shapes were observed between mixture s with lyso-gangliosides and normal double-chain gangliosides. Determi ned from the dicroic ratio R(ATR), the orientation of the COO- group i n all DMPC/ganglioside mixtures was found to be relatively fixed with respect to the membrane normal. In 3:1 mixtures of DMPC with GM1 and d eacetyl-GM1, the binding of Ca2+ leads to a slight decrease in chain t ilt in the gel phase, probably caused by a dehydration of the membrane -water interface. In mixtures of DMPC with GM3 and deacetyl-lyso-GM1 a slight increase in chain tilt is observed. The chain tilt in DMPC/lys o-GM1 mixtures is unchanged. Analysis of the COO- band reveals that Ca 2+ does not bind to the carboxylate group of the sialic acid of GM1 an d deacetyl-GM1, the mixtures in which a decrease in chain tilt was obs erved. Binding to the sialic acid was only observed for mixtures of DM PC with GM3, lyso-GM1, and deacetyl-lyso-GM1. Ca2+ obviously accumulat es at the bilayer-water interface and leads to partial dehydration of the headgroup region in the gel as well as in the liquid-crystalline p hase. This can be concluded from the changes in the amide I band shape s, With the exception of DMPC/deacetyl-GM1, the effects on the ester C =O bands are small, The addition of Ca2+ has minor effects on the phas e behavior, with the exception of the DMPC/GM1 mixture.