Behaviour of NBD-head group labelled phosphatidylethanolamines in POPC bilayers: a molecular dynamics study

Abstract

A complete homologous series of fluorescent phosphatidylethanolamines (diC n PE), labelled at the head group with a 7-nitrobenz-2-oxa-1,3-diazo-4-yl(NBD) fluorophore and inserted in 1-palmitoyl, 2-oleoyl-snglycero-3-phosphocholine (POPC) bilayers, was studied using atomistic molecular dynamics simulations. The longer-chained derivatives of NBD-diC n PE, with n = 14, 16, and 18, are commercially available, and widely used as fluorescent membrane probes. Properties such as location of atomic groups and acyl chain order parameters of both POPC and NBD-diC n PE, fluorophore orientation and hydrogen bonding, membrane electrostatic potential and lateral diffusion were calculated for all derivatives in the series. Most of these probes induce local disordering of POPC acyl chains, which is on the whole counterbalanced by ordering resulting from binding of sodium ions to lipid carbonyl/glycerol oxygen atoms. An exception is found for NBD-diC 16 PE, which displays optimal matching with POPC acyl chain length and induces a slight local ordering of phospholipid acyl chains. Compared to previously studied fatty amines, acyl chain-labelled phosphatidylcholines, and sterols bearing the same fluorescent tag, the chromophore in NBD-diC n PE locates in a similar region of the membrane (near the glycerol backbone/carbonyl region) but adopts a different orientation (with the NO 2 group facing the interior of the bilayer). This modification leads to an inverted orientation of the P-N axis in the labelled lipid, which affects the interface properties, such as the membrane electrostatic potential and hydrogen bonding to lipid head group atoms. The implications of this study for the interpretation of the photophysical properties of NBD-diC n PE (complex fluorescence emission kinetics, differences with other NBD lipid probes) are discussed.

Influence of the sterol aliphatic side chain on membrane properties: a molecular dynamics study

“…As we 70 and others 65,67,76,77 have recently demonstrated, small cations such as Na + bind to the ester/carbonyl atoms of phospholipids, inducing a slight enhancement of the lipid order (whereas common counter ions such as Cl À remain firmly in the aqueous phase and do not affect the lipids). Although we have not included ions in these simulations, we expect that the addition of a salt such as NaCl would lead to a systematic ordering effect, identical for all sterols along the series, and therefore all comparative conclusions of this study would be unchanged.…”

“…This interdigitation of alkyl or acyl chains of long-tailed solutes has been observed previously in MD simulation studies. [68][69][70] Fig . 3 Average transverse position hzi of specific POPC and sterol atoms in the simulations of systems containing none or two sterol molecules (A) and 20 mol% sterol (B).…”

Fluorescence of nitrobenzoxadiazole (NBD)-labeled lipids in model membranes is connected not to lipid mobility but to probe location

“…This situation is reminiscent of another recently reported monoanionic head-labeled NBD probe, NBD-PE. 36,56 In NBD-PE, the fluorophore as a whole had an even more internal location (around or above that of the phospholipid C13 atom). This is also observed in other NBD probes such as acyl-chainlabelled NBD-PC, 55 fatty amine-NBD-C n , 57 and NBD-Chol.…”

“…36,55 The steep water gradient within this region creates substantial changes in the fluorescence properties of NBD as one goes deeper into the bilayer. Changing the excitation wavelength in a REES experiment selectively excites the shallower, more hydrated fluorophores, which in turn emit at longer wavelengths.…”

“…54 In addition, it has previously been found that NBD probes are located within a relatively wide range of depths within lipid bilayers. 36,[55][56][57] It is known that the depth profile of the water gradient is steep in lipid bilayers and changes dramatically in the interfacial region.…”