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

Hydration of the anionic lipid DMPG with distilled water and storage at ∼4°C results in a disintegration of bilayers into fragments of different sizes and bicelles with a diameter of ∼50 nm. Tightly packed multilamellar aggregates are not observed under these conditions, which are otherwise formed during cold-storage after hydration at a certain ionic strength and in the presence of Na+, with a characteristic phase transition at ∼40°C. The bilayer fragments are able to form ripples after heating to a temperature between ∼17 and 25°C, and the larger ones form vesicles after heating to 30oC. Differential scanning calorimetry of samples containing predominantly fragments reveals pretransition and main transition peaks in agreement with the morphological observations, plus an additional subtransition peak at ∼12°C. 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 are largely identical with transitions at ∼21, 27 and 28.4°C. To explain these three melting transitions, a partial chain interdigitation at the rim of the bicelles is discussed. Bicelles and fragments are also formed by mixtures of DMPG with DMPC.