Orientational distribution of DPH in lipid membranes: a comparison of molecular dynamics calculations and experimental time-resolved anisotropy experiments

Abstract

Characterization of the membrane phases is a crucial task in cell biology. Cells differ in composition of the lipids and consequently in adopted phases. The phases can be discriminated based upon lipid ordering and molecular diffusion and their identification could be used for characterization of cell membranes. Here we used molecular dynamics (MD) simulations to study the behavior of the fluorescent reporter molecule diphenylhexatriene (DPH) in different lipid phases – liquid disordered (L_d), liquid ordered (L_o), and solid ordered (S_o) composed of phosphatidylcholines (L_d and S_o) or a sphingomyelin/cholesterol (SM/Chol) mixture (L_o). To the best of our knowledge, this is the first simulation of DPH in L_o SM/Chol and S_o DPPC membranes. For the considered membrane compositions DPH is mostly oriented parallel to lipid tails. In the L_o phase we observed a significant fraction of DPH positioned in between membrane leaflets, which agrees with experimental findings, but which has not been observed in previous MD simulations of DPH in phosphatidylcholine membranes. Further, we calculated rotational autocorrelation functions (ROTACF) from our MD simulations in order to model the time-resolved fluorescence anisotropy decay. We observed that order parameters 〈P₂〉 and 〈P₄〉 are sufficient to fully describe the orientation distribution of DPH. We analyzed the ROTACFs by a so-called general model for the time-resolved fluorescence anisotropy [W. van der Meer et al., Biophys. J., 1984, 46, 515] and observed an overestimation of 〈P₄〉. We suggest a rescaling of the recovered 〈P₄〉 yielding an orientation distribution of DPH close to the one observed in our MD simulations.