Bilayer fragments and bilayered micelles (bicelles) of dimyristoylphosphatidylglycerol (DMPG) are induced by storage in distilled water at 4 degrees C

Hydration of the anionic lipid DMPG with distilled water and storage at sim ilar to4 degreesC results in a disintegration of bilayers into fragments of different sizes and bicelles with a diameter of similar to 50 nm. Tightly packed multilamellar aggregates are not observed under these conditions, wh ich are otherwise formed during cold-storage after hydration at a certain i onic strength and in the presence of Na+, with a characteristic phase trans ition at similar to 40 degreesC. The bilayer fragments are able to form rip ples after heating to a temperature between similar to 17 and 25 degreesC, and the larger ones form vesicles after heating to 30 degreesC. Differentia l scanning calorimetry of samples containing predominantly fragments reveal s pretransition and main transition peaks in agreement with the morphologic al observations, plus an additional subtransition peak at similar to 12 deg reesC. No low-temperature subtransition peak exists if the sample contains predominantly bicelles. The bicelles form neither ripples nor vesicles upon heating to higher temperatures, and consequently the DSC scan and rescan a re largely identical with transitions at similar to 21, 27 and 28.4 degrees C. To explain these three melting transitions, a partial chain interdigitat ion at the rim of the bicelles is discussed. Bicelles and fragments are als o formed by mixtures of DMPG with DMPC. (C) 2001 Elsevier Science B.V. All rights reserved.