THE EFFECT OF A POTENTIAL-SENSITIVE BARBITURIC-ACID MOLECULAR PROBE ON THE THERMAL PHASE-TRANSITION PROPERTIES OF MULTILAMELLAR 1,2-DIMYRISTOYL-SN-GLYCERO-3-PHOSPHOCHOLINE SUSPENSIONS

The interaction of the extrinsic, potential-sensitive probe diBa-C-2-( 5) with 1,2 dimyristoyl-sn-glycero-3-phosphocholine (DMPC) in the form of unilamellar vesicles or as multilamellar suspensions has been char acterized by probemembrane binding studies and differential scanning c alorimetry. The presence of diBa-C-2-(5) in DMPC suspensions lowers th e temperature at the midpoint of the gel-to-liquid-crystalline phase t ransition T-m by a maximal 0.15 degrees C. The enthalpy of the gel-to- liquid-crystalline phase transition is reduced by a maximal 4%, wherea s that of the pre-transition is unaffected by the presence of 5 mol% d iBa-C-2-(5). The cooperativity of the gel-to-liquid-crystaklline phase transition is reduced by a maximal 35%. When present at a concentrati on of 5 mol%, this indicator causes virtually no change in the pre-tra nsition cooperativity. The diBa-C-2-(S)/DMPC system can be described b y the ideal solution theory when the probe is present at a concentrati on of less than or equal to 0.2 mol%. Non-ideal behavior of the diBa-C -2-(5)/DMPC system is observed at concentrations >0.2 mol% probe. DiBa -C-2-(5) mildly perturbs the membrane in comparison to the effects of most other potential-sensitive probes, and is probably located at or n ear the surface of the bilayer.