THE PRETRANSITION OF DIPALMITOYLLECITHIN BILAYERS AS PROBED BY THE FLUORESCENT PYRROLOPYRIMIDINE, U-104067

The amphiphilic pyrrolopyrimidine, U-104067, is a fluorophore ideally suited to report on the relative hydrophobicities of different microen vironments. It forms stable monomolecular layers at the air/water inte rface with a limiting molecular area of 51.9 +/- 0.3 Angstrom(2)/molec ule and a collapse pressure of about 18 dyn/cm. Differential scanning calorimetry of its mixed liposomes with dipalmitoyllecithin shows full solubility of the compound in the liquid disordered phase and insolub ility in the solid ordered phase. In aqueous solutions, the compound b inds to phospholipid bilayers with a stoichiometry of 13.2 +/- 1.2 mol es of lipid per mole of U-104067, with K-d = 0.33 +/- 0.05 mu M toward egg lecithin/phosphatidylserine bilayers and K-d = 1.5 +/- 0.3 mu M t oward pure egg lecithin bilayers. In liquid crystalline phospholipid b ilayers the compound behaves as two independently emitting species, on e accessible to acrylamide and the other one not. Doxyl fatty acid met hyl esters quench both species and show that the average position of t he fluorophore is at a depth corresponding to that of the 7th carbon o f a fatty acyl chain. Dissolved in the liquid disordered (L-alpha) pha se of dipalmitoyllecithin at 45 degrees C, U-104067 shows a single ion izable group, pK(a) = 3.19 +/- 0.03 while in the solid ordered (L-beta ) phase it displays two ionizable groups, pK(a1) = 4.99 +/- 0.10 and p K(a2) = 6.96 +/- 0.13. The most unusual property of this molecule is t hat it is miscible with the tilted (L-beta) and liquid (L-alpha) phase s of dipalmitoyllecithin but totally immiscible with the rippled (P-be ta) phase. Because of this, U-104067 is a sensitive reporter for the t ilted/rippled phase transition as monitored by its fluorescence anisot ropy and its quantum yield changes. (C) 1997 Elsevier Science Ireland Ltd.