The development of a universal probe to assess the phase of a lipid membrane is one of the most ambitious goals for fluorescence spectroscopy. The ability of a well-known molecule as Laurdan to reach this aim is here exploited as the behavior of the probe is fully characterized in a dipalmitoylphosphatidylcholine (DPPC) solid gel (So) phase by means of molecular dynamics simulations. Laurdan can take two conformations, depending on whether the carbonyl oxygen points toward the β-position of the naphthalene core (Conf-I) or to the α-position (Conf-II). We observe that Conf-I has an elongated form in this environment, whereas Conf-II takes an L-shape. Interestingly, our theoretical calculations show that these two conformations behave in an opposite way from what is reported in the literature for a DPPC membrane in a liquid disordered (Ld) phase, where Conf-I assumes an L-shape and Conf-II is elongated. Moreover, our results show that in DPPC (So) no intermixing between the conformations is present, whereas it has been seen in a fluid environment such as DOPC (Ld). Through a careful analysis of angle distributions and by means of the rotational autocorrelation function, we predict that the two conformers of Laurdan behave differently in different membrane environments.

Biomedical Research Institute Hasselt University Agoralaan Building C 3590 Diepenbeek Belgium

Chemical and Biological Systems Simulation Lab Centre of New Technologies University of Warsaw Banacha 2C 02 097 Warsaw Poland

Department of Theoretical Chemistry and Biology School of Biotechnology Royal Institute of Technology SE 10691 Stockholm Sweden

RCPTM Department of Physical Chemistry Fac Sciences Palacký University 771 46 Olomouc Czech Republic

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