Association of polyene antibiotics with sterol-free lipid membranes. II. Hydrophobic binding of nystatin to dilauroylphosphatidylcholine bilayers.

Interaction of nystatin A1 with multilamellar vesicles (MLV) of dilauroylphosphatidylcholine (DLPC), observed either by adding nystatin to preformed MLV (mixtures I) or by incorporating it during the formation of vesicles (mixtures II, inner lamellas of MLV in contact with nystatin) was investigated for 0.002 < or = nystatin/DLPC = R(A) < or = 0.20, by four complementary methods. The main results were: (i) Ultraviolet absorption and circular dichroism (CD) spectra of mixtures I revealed the occurrence of a saturable association with a stoichiometry (R(A) = 0.007 +/- 0.002) constant between 3 and 33 degrees C. (ii) By differential scanning calorimetry, thermograms of the two types of mixtures were similar only when water was in great excess. In the opposite (e.g., (H2O)/(DLPC) = R(W) < or = 300), mixture II thermograms displayed two features, upshifted by about 6.5 degrees C with respect to the sharp peak observed with mixture I, resembling those obtained for pure DLPC when the low-temperature phase was the subgel phase. For this R(W), the nystatin absolute concentrations were those for which nystatin form superaggregates as revealed by the nystatin CD spectra. It is proposed that these superaggregates are excluded from the interlamellar spacings of MLV and exert a pumping action on the interlamellar water. The subsequent dehydration of the inner lamellas is thought to convert them into the subgel state. (iii) 2H-NMR spectra of sn-2-perdeuterated DLPC MLV + nystatin mixtures II, confirmed such a temperature shift of the main transition. They showed, in addition, an ordering of the aliphatic chains immediately above the transition temperature, equivalent to a bilayer thickening of 2 A.