Detection of the metastable rippled gel phase in hydrated phosphatidylcholine by fluorescence spectroscopy Original Research Article

Steady-state and time-resolved emission spectroscopy of 1-anilinonaphthalene-8-sulfonic acid (ANS) have been used for characterization of the metastable rippled gel phase, Pβ′(mst), formed in fully-hydrated dipalmitoylphosphatidylcholine (DPPC) upon cooling from the liquid crystalline phase Lα [Tenchov et al., Biophys. J. 56 (1989) 757]. The Pβ′(mst) phase of DPPC clearly differs from the stable Pβ′ phase by increased (≈27%) ANS emission intensity, by enhanced (≈23%) average radiative rate constant, and by reduced (≈18%) non-radiative quenching rate constant. The fluorescence intensity peak at the Pβ′→Lα transition temperature is replaced by a large, reversible stepwise intensity drop at the Pβ′(mst)→Lα transition. No such effects have been found for dimiristoylphosphatidylcholine (DMPC) dispersions confirming previous results that DMPC does not form a Pβ′(mst) phase. Since ANS is known to predominantly reside in the interfacial region, the observed effects indicate differences between the stable and metastable rippled phases in the organization and dynamics of their lipid/water interfaces. The data demonstrate that the metastable rippled phase manifests its appearance also through interactions with small molecules (ANS size ∼8 Å).