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Biophysical Journal 86:3722-3733 (2004)
© 2004 The Biophysical Society

Mesoscopic Structure in the Chain-Melting Regime of Anionic Phospholipid Vesicles: DMPG

K. A. Riske *, L. Q. Amaral {dagger}, H.-G. Döbereiner * {ddagger} and M. T. Lamy {dagger}

* Max Planck Institute of Colloids and Interfaces, Theory Department, Am Mühlenberg, Golm, Germany; {dagger} Instituto de Física da Universidade de São Paulo, São Paulo, Brazil; and {ddagger} Department of Biological Sciences, Columbia University, New York, New York

Correspondence: Address reprint requests to Lia Queiroz Amaral, Institute of Physics, University of São Paulo, IFUSP Caixa Postal 66318, São Paulo, SP CEP 05315-970, Brasil. Tel.: 55-11-3091-6706; E-mail: amaral{at}if.usp.br.

In a range of low ionic strength, aqueous dispersions of the anionic phospholipid DMPG (dimyristoylphosphatidylglycerol) have a transparent intermediate phase (IP, between and ) between the turbid gel and fluid membrane phases, evidenced in turbidity data. Small angle x-ray scattering results on DMPG dispersions show that, besides the bilayer peak present in all phases, a peak corresponding to a mesoscopic structure at ~400 Å is detected only in IP. The dependence of this peak position on DMPG concentration suggests a correlation in the bilayer plane, consistent with the stability of vesicles in IP. Moreover, observation of giant DMPG vesicles with phase contrast light microscopy show that vesicles "disappear" upon cooling below and "reappear" after reheating. This further proves that although vesicles cannot be visualized in IP, their overall structure is maintained. We propose that the IP in the melting regime corresponds to unilamellar vesicles with perforations, a model which is consistent with all described experimental observations. Furthermore, the opening of pores across the membrane tuned by ionic strength, temperature, and lipid composition is likely to have biological relevance and could be used in applications for controlled release from nanocompartments.




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