Sensing hydration and behavior of pyrene in POPC and POPC/cholesterol bilayers: A molecular dynamics study

Abstract

Molecular dynamics (MD) simulations of bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with varying amounts of cholesterol (0, 5, 20, and 40mol%) were carried out in the absence and presence of inserted pyrene molecules. Both fluorophore and bilayer parameters were computed, for characterization of probe location and dynamics, as well as its effects on the host bilayer. In agreement with previous studies in fluid disordered bilayers, pyrene prefers to be located in the hydrophobic acyl chain region of POPC bilayers, close to the glycerol group of lipid molecules and causes ordering of the lipid acyl chains. However, incorporation of pyrene in binary POPC/cholesterol bilayers decreases the acyl chain order parameter (especially near the end of the chains), opposing the ordering effect of cholesterol. These effects are modest and mainly felt locally. Significantly, as the bilayer is enriched with cholesterol, the relative position of pyrene and the POPC carbonyl and phosphocholine groups is invariant, and the local water density around the probe decreases. This work clarifies and supports the cautious use of pyrene Ham effect to effectively measure equivalent polarity in lipid bilayers. Within the time scale of the MD simulations, which is of the magnitude of the fluorescence lifetime of pyrene, the thermally averaged polarity of lipid bilayers is nearly out of influence of spurious uncertainty in the transverse location of pyrene in the bilayers. This renders the values of equivalent polarity measurements through the pyrene Ham effect more reliable and reproducible than previously expected.

Behavior of pyrene as a polarity probe in palmitoylsphingomyelin and palmitoylsphingomyelin/cholesterol bilayers: A molecular dynamics simulation study

“…In accordance, the average tilt angles of the distributions shown in Fig. 9 (35-36º for 0-5 mol% Chol) are considerably smaller than those we obtained in POPC (48º) and even POPC/20 mol% Chol (41º), and almost comparable to that in POPC/40 mol% Chol (32º) [16]. For higher Chol content, the distributions shift progressively to lower tilt angles, albeit to a small extent (averages are 30º, 29º, 27º and 26º for 20, 40, 45 and 50 mol% Chol, respectively).…”

“…For example, for 50 mol% Chol, (0.535  0.013) nm 2 , (0.539  0.013) nm 2 and (0.545  0.013) nm 2 are obtained from the simulations with 0, 2 and 4 pyrene molecules, respectively. This is a first indication that pyrene may actually reduce the order of these systems, similarly to what we observed for POPC/Chol [16]. Although these variations are still within the statistical uncertainty associated with the calculations, it should be stressed that these values are averaged over all PSM molecules (not only those near the probe molecules), and local probe disordering effects are most probably more significant.…”

“…For this purpose, molecular dynamics (MD) simulations have been useful to characterize simultaneously membrane probe behavior and probe-induced perturbation with atomic detail, for several classes of fluorescent lipophilic probes (see [12,13] for reviews), including free pyrene [14,15]. Recently, we carried out MD simulations of pyrene in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, with varying amounts of Chol [16]. This work was valuable in the clarification of the use of the 1 Abbreviations: ACF, autocorrelation function; Chol, cholesterol; DPPC, dipalmitoylphosphatidylcholine; MD, molecular dynamics; MSD, mean squared displacement; PC, phosphatidylcholine; POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine; PSM, N-palmitoylsphingomyelin; SM, sphingomyelin pyrene Ham Effect to effectively measure equivalent polarity in mixed lipid bilayers containing unsaturated PC and Chol.…”

Molecular Dynamics Simulations of Depth Distribution of Spin-Labeled Phospholipids within Lipid Bilayer

“…23,24,25,26 Fluorescence techniques combined with MD simulations have been used successfully for studying the effect of hydration 27,28 and solvent penetration 29,30 on membrane properties. 31,32,33 …”

Interactions of Lipid Membranes with Fibrillar Protein Aggregates

“…Next, to monitor lipid bilayer modifications occurring at the level of acyl chains, we employed a classical fluorescent probe pyrene, a polycyclic aromatic compound that is primarily distributed at the level of carbons 4-13 in the hydrocarbon region of the bilayer (Loura et al 2013). Emission spectrum of this probe has characteristic vibronic structure in the wavelength range 370-400 nm, with relative intensities of vibronic transitions depending on the polarity of the fluorophore microenvironment via the so-called "Ham effect" (Nakajima 1971).…”