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

Steady-state and time-resolved emission spectroscopy of 1-anilinonaphthalen e-8-sulfonic acid (ANS) have been used for characterization of the metastab le rippled gel phase, P-beta'(mst), formed in fully-hydrated diparmitoylpho sphatidylcholine (DPPC) upon cooling from the liquid crystalline phase L-al pha [Tenchov et al., Biophys. J. 56 (3989) 757]. The P-beta'(mst) phase of DPPC clearly differs from the stable P-beta' phase by increased (approximat e to 27%) ANS emission intensity, by enhanced (approximate to 23%) average radiative rate constant, and by reduced (approximate to 18%) non-radiative quenching rate constant. The fluorescence intensity peak at the P-beta' --> L-alpha transition temperature is replaced by a large, reversible stepwise intensity drop at the P-beta'(mst) --> L-alpha transition. No such effects have been found for dimiristoylphosphatidylcholine (DMPC) dispersions conf irming previous results that DMPC does not form a P-beta'(mst) phase. Since ANS is known to predominantly reside in the interfacial region, the observ ed effects indicate differences between the stable and metastable rippled p hases in the organization and dynamics of their lipid/water interfaces. The data demonstrate that the metastable rippled phase manifests its appearanc e also through interactions with small molecules (ANS size similar to 8 Ang strom). (C) 2000 Elsevier Science B.V. All rights reserved.