Conformational Changes as Driving Force for Phase Recognition: The Case of Laurdan

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 dipalmitoyl-phosphatidylcholine (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 beta-position of the naphthalene core (Conf-I) or to the alpha-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.S.O. acknowledges the National Science Centre, Poland, grant UMO-2015/19/P/ST4/03636, for the funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 665778. The Swedish Infrastructure Committee (SNIC) is acknowledged for the computational time granted through the medium allocations 1-87 and 1-415 (2016); 1-16, 1-102 (2017); and 3-397, 3-156, 3-396, 3-23 (2018). The Flemish Supercomputer Centre (VSC) (Flanders, Belgium) and the Herculesstichting (Flanders, Belgium) are acknowledged for the generously allocated computational time on the Tier-1 cluster Breniac as well as the Tier-2 cluster Thinking