Lipid Rafts have Different Sizes Depending on Membrane Composition: A Time-resolved Fluorescence Resonance Energy Transfer Study

The ternary lipid system palmitoylsphingomyelin (PSM)/palmitoyloleoylphosphatidylcholine (POPC)/cholesterol is a model for lipid rafts. Previously the phase diagram for that mixture was obtained, establishing the composition and boundaries for lipid rafts. In the present work, this system is further studied in order to characterize the size of the rafts. For this purpose, a time-resolved fluorescence resonance energy transfer (FRET) methodology, previously applied with success to a well-characterized phosphatidylcholine/cholesterol binary system, is used. It is concluded that: (1) the rafts on the low raft fraction of the raft region are small (below 20 nm), whereas on the other side the domains are larger; (2) on the large domain region, the domains reach larger sizes in the ternary system (> approximately 75-100 nm) than in binary systems phosphatidylcholine/cholesterol (between approximately 20 and approximately 75-100 nm); (3) the raft marker ganglioside G(M1) in small amounts (and excess cholera toxin subunit B) does not affect the general phase behaviour of the lipid system, but can increase the size of the rafts on the small to intermediate domain region. In summary, lipid-lipid interactions alone can originate lipid rafts on very different length scales. The conclusions presented here are consistent with the literature concerning both model systems and cell membrane studies.

Diphenylhexatriene membrane probes DPH and TMA-DPH: A comparative molecular dynamics simulation study

Fluorescence spectroscopy and microscopy have been utilized as tools in membrane biophysics for decades now. Because phospholipids are non-fluorescent, the use of extrinsic membrane probes in this context is commonplace. Among the latter, 1,6-diphenylhexatriene (DPH) and its trimethylammonium derivative (TMA-DPH) have been extensively used. It is widely believed that, owing to its additional charged group, TMA-DPH is anchored at the lipid/water interface and reports on a bilayer region that is distinct from that of the hydrophobic DPH. In this study, we employ atomistic MD simulations to characterize the behavior of DPH and TMA-DPH in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and POPC/cholesterol (4:1) bilayers. We show that although the dynamics of TMA-DPH in these membranes is noticeably more hindered than that of DPH, the location of the average fluorophore of TMA-DPH is only ~3-4Å more shallow than that of DPH. The hindrance observed in the translational and rotational motions of TMA-DPH compared to DPH is mainly not due to significant differences in depth, but to the favorable electrostatic interactions of the former with electronegative lipid atoms instead. By revealing detailed insights on the behavior of these two probes, our results are useful both in the interpretation of past work and in the planning of future experiments using them as membrane reporters.

FRET in Membrane Biophysics: An Overview

Förster resonance energy transfer (FRET), in most applications used as a “spectroscopic ruler,” allows an easy determination of the donor-acceptor intermolecular distance. However, the situation becomes complex in membranes, since around each donor there is an ensemble of acceptors at non-correlated distances. In this review, state-of-the-art methodologies for this situation are presented, usually involving time-resolved data and model fitting. This powerful approach can be used to study the occurrence of phase separation (“rafts” or other type of domains), allowing their detection as well as size evaluation. Formalisms for studying lipid–protein and protein–protein interactions according to specific topologies are also addressed. The advantages and added complexity of a specific type of FRET (energy homotransfer or energy migration) are described, as well as applications of FRET under the microscope.

Nonequilibrium phenomena in the phase separation of a two-component lipid bilayer

Lipid bilayers composed of two phospholipids with significant acyl-chain mismatch behave as nonideal mixtures. Although many of these systems are well characterized from the equilibrium point of view, studies concerning their nonequilibrium dynamics are still rare. The kinetics of lipid demixing (phase separation) was studied in model membranes (large unilamellar vesicles of 1:1 dilauroylphosphatidylcholine (C(12) acyl chain) and distearoylphosphatidylcholine (C(18) acyl chain)). For this purpose, photophysical techniques (fluorescence intensity, anisotropy, and fluorescence resonance energy transfer) were applied using suitable probes (gel phase probe trans-parinaric acid and fluid phase probe N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-dilauroylphosphatidylethanolamine). The nonequilibrium situation was induced by a sudden thermal quench from a one-fluid phase equilibrium situation (higher temperature) to the gel/fluid coexistence range (lower temperature). We verified that the attainment of equilibrium is a very slow process (occurs in a time scale of hours), leading to large domains at infinite time. The nonequilibrium structure stabilization is due essentially to temporarily rigidified C(12) chains in the interface between gel/fluid domains, which decrease the interfacial tension by acting as surfactants. The relaxation process becomes faster with the increase of the temperature drop. In addition, heterogeneity is already present in the supposed homogeneous fluid mixture at the higher temperature.

Location and dynamics of acyl chain NBD-labeled phosphatidylcholine (NBD-PC) in DPPC bilayers. A molecular dynamics and time-resolved fluorescence anisotropy study

100-ns molecular dynamics simulations of fluid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, both pure and containing 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) acyl-chain labeled fluorescent analogs (C6-NBD-PC and C12-NBD-PC), are described. These molecules are widely used as probes for lipid structure and dynamics. The results obtained here for pure DPPC agree with both experimental and theoretical published works. We verified that the NBD fluorophore of both derivatives loops to a transverse location closer to the interface than to the center of the bilayer. Whereas this was observed previously in experimental literature works, conflicting transverse locations were proposed for the NBD group. According to our results, the maximum of the transverse distribution of NBD is located around the glycerol backbone/carbonyl region, and the nitro group is the most external part of the fluorophore. Hydrogen bonds from the NH group of NBD (mostly to glycerol backbone lipid O atoms) and to the nitro O atoms of NBD (from water OH groups) are continuously observed. Rotation of NBD occurs with approximately 2.5-5 ns average correlation time for these probes, but very fast, unresolved reorientation motions occur in <20 ps, in agreement with time-resolved fluorescence anisotropy measurements. Finally, within the uncertainty of the analysis, both probes show lateral diffusion dynamics identical to DPPC.

HIV fusion inhibitor peptide T-1249 is able to insert or adsorb to lipidic bilayers. Putative correlation with improved efficiency

T-1249 is a HIV fusion inhibitor peptide under clinical trials. Its interaction with biological membrane models (large unilamellar vesicles) was studied using fluorescence spectroscopy. A gp41 peptide that includes one of the hydrophobic terminals of T-1249 was also studied. Both peptides partition extensively to liquid-crystalline POPC (1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine) (DeltaG = -7.0 kcal/mol and -8.7 kcal/mol, for T-1249 and terminal peptide, respectively) and are located at the interface of the membrane. T-1249 is essentially in a random coil conformation in this lipidic medium, although a small alpha-helix contribution is present. When other lipid compositions are used (DPPC, POPG + POPC, and POPC + cholesterol) (DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and POPG (1-palmitoyl-2-oleyl-sn-glycero-3-[phospho-rac-(1-glycerol)), partition decreases, the most severe effect being the presence of cholesterol. Partition experiments and fluorescence resonance energy transfer analysis show that T-1249 adsorbs to cholesterol-rich membranes. The improved clinical efficiency of T-1249 relative to enfuvirtide (T20) may be related to its bigger partition coefficient and ability to adsorb to rigid lipidic areas on the cell surface, where most receptors are inserted. Moreover, adsorption to the sterol-rich viral membrane helps to increase the local concentration of the inhibitor peptide at the fusion site.

How to tackle the issues in free energy simulations of long amphiphiles interacting with lipid membranes: convergence and local membrane deformations

One of the great challenges in membrane biophysics is to find a means to foster the transport of drugs across complex membrane structures. In this spirit, we elucidate methodological challenges associated with free energy computations of complex chainlike molecules across lipid membranes. As an appropriate standard molecule to this end, we consider 7-nitrobenz-2-oxa-1,3-diazol-4-yl-labeled fatty amine, NBD-Cn, which is here dealt with as a homologous series with varying chain lengths. We found the membrane-water interface region to be highly sensitive to details in free energy computations. Despite considerable simulation times, we observed substantial hysteresis, the cause being the small frequency of insertion/desorption events of the amphiphile's alkyl chain in the membrane interface. The hysteresis was most pronounced when the amphiphile was pulled from water to the membrane and compromised the data that were not in line with experiments. The subtleties in umbrella sampling for computing distance along the transition path were also observed to be potential causes of artifacts. With the PGD (pull geometry distance) scheme, in which the distance from the molecule was computed to a reference plane determined by an average over all lipids in the membrane, we found marked deformations in membrane structure when the amphiphile was close to the membrane. The deformations were weaker with the PGC (pull geometry cylinder) method, where the reference plane is chosen based on lipids that are within a cylinder of radius 1.7 nm from the amphiphile. Importantly, the free energy results given by PGC were found to be qualitatively consistent with experimental data, while the PGD results were not. We conclude that with long amphiphiles there is reason for concern with regard to computations of their free energy profiles. The membrane-water interface is the region where the greatest care is warranted.

Recent developments in molecular dynamics simulations of fluorescent membrane probes

Due to their sensitivity and versatility, the use of fluorescence techniques in membrane biophysics is widespread. Because membrane lipids are non-fluorescent, extrinsic membrane probes are widely used. However, the behaviour of these probes when inserted in the bilayer is often poorly understood, and it can be hard to distinguish between legitimate membrane properties and perturbation resulting from probe incorporation. Atomistic molecular dynamics simulations present a convenient way to address these issues and have been increasingly used in recent years in this context. This article reviews the application of molecular dynamics to the study of fluorescent membrane probes, focusing on recent work with complex design fluorophores and ordered bilayer systems.

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

Nitrobenzoxadiazole (NBD)-labeled lipids are popular fluorescent membrane probes. However, the understanding of important aspects of the photophysics of NBD remains incomplete, including the observed shift in the emission spectrum of NBD-lipids to longer wavelengths following excitation at the red edge of the absorption spectrum (red-edge excitation shift or REES). REES of NBD-lipids in membrane environments has been previously interpreted as reflecting restricted mobility of solvent surrounding the fluorophore. However, this requires a large change in the dipole moment (Δμ) of NBD upon excitation. Previous calculations of the value of Δμ of NBD in the literature have been carried out using outdated semi-empirical methods, leading to conflicting values. Using up-to-date density functional theory methods, we recalculated the value of Δμ and verified that it is rather small (∼2 D). Fluorescence measurements confirmed that the value of REES is ∼16 nm for 1,2-dioleoyl-sn-glycero-3-phospho-l-serine-N-(NBD) (NBD-PS) in dioleoylphosphatidylcholine vesicles. However, the observed shift is independent of both the temperature and the presence of cholesterol and is therefore insensitive to the mobility and hydration of the membrane. Moreover, red-edge excitation leads to an increased contribution of the decay component with a shorter lifetime, whereas time-resolved emission spectra of NBD-PS displayed an atypical blue shift following excitation. This excludes restrictions to solvent relaxation as the cause of the measured REES and TRES of NBD, pointing instead to the heterogeneous transverse location of probes as the origin of these effects. The latter hypothesis was confirmed by molecular dynamics simulations, from which the calculated heterogeneity of the hydration and location of NBD correlated with the measured fluorescence lifetimes/REES. Globally, our combination of theoretical and experiment-based techniques has led to a considerably improved understanding of the photophysics of NBD and a reinterpretation of its REES in particular.

Sifuvirtide screens rigid membrane surfaces. establishment of a correlation between efficacy and membrane domain selectivity among HIV fusion inhibitor peptides

Sifuvirtide, a 36 amino acid negatively charged peptide, is a novel and promising HIV fusion inhibitor, presently in clinical trials. Because of the aromatic amino acid residues of the peptide, its behavior in aqueous solution and the interaction with lipid-membrane model systems (large unilammelar vesicles) were studied by using mainly fluorescence spectroscopy techniques (both steady-state and time-resolved). No significant aggregation of the peptide was observed with aqueous solution. Various biological and nonbiological lipid-membrane compositions were analyzed, and atomic force microscopy was used to visualize phase separation in several of those mixtures. Results showed no significant interaction of the peptide, neither with zwitterionic fluid lipid membranes (liquid-disordered phase), nor with cholesterol-rich membranes (liquid-ordered phase). However, significant partitioning was observed with the positively charged lipid models (K(p) = (2.2 +/- 0.3) x 10(3)), serving as a positive control. Fluorescence quenching using Förster resonance acrylamide and lipophilic probes was carried out to study the location of the peptide in the membrane models. In the gel-phase DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) membrane model, an adsorption of the peptide at the surface of these membranes was observed and confirmed by using Förster resonance energy-transfer experiments. These results indicate a targeting of the peptide to gel-phase domains relatively to liquid-disordered or liquid-ordered phase domains. This larger affinity and selectivity toward the more rigid areas of the membranes, where most of the receptors are found, or to viral membrane, may help explain the improved clinical efficiency of sifuvirtide, by providing a local increased concentration of the peptide at the fusion site.

Quantification of Protein-Lipid Selectivity using FRET: Application to the M13 Major Coat Protein

Quantification of lipid selectivity by membrane proteins has been previously addressed mainly from electron spin resonance studies. We present here a new methodology for quantification of protein-lipid selectivity based on fluorescence resonance energy transfer. A mutant of M13 major coat protein was labeled with 7-diethylamino-3((4'iodoacetyl)amino)phenyl-4-methylcoumarin to be used as the donor in energy transfer studies. Phospholipids labeled with N-(7-nitro-2-1,3-benzoxadiazol-4-yl) were selected as the acceptors. The dependence of protein-lipid selectivity on both hydrophobic mismatch and headgroup family was determined. M13 major coat protein exhibited larger selectivity toward phospholipids which allow for a better hydrophobic matching. Increased selectivity was also observed for anionic phospholipids and the relative association constants agreed with the ones already presented in the literature and obtained through electron spin resonance studies. This result led us to conclude that fluorescence resonance energy transfer is a promising methodology in protein-lipid selectivity studies.

Dependence of M13 major coat protein oligomerization and lateral segregation on bilayer composition

M13 major coat protein was derivatized with BODIPY (n-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-yl)methyl iodoacetamide), and its aggregation was studied in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and DOPC/1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DOPG) or 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE)/DOPG (model systems of membranes with hydrophobic thickness matching that of the protein) using photophysical methodologies (time-resolved and steady-state self-quenching, absorption, and emission spectra). It was concluded that the protein is essentially monomeric, even in the absence of anionic phospholipids. The protein was also incorporated in pure bilayers of lipids with a strong mismatch with the protein transmembrane length, 1,2-dierucoyl-sn-glycero-3-phosphocholine (DEuPC, longer lipid) and 1,2-dimyristoleoyl-sn-glycero-3-phosphocholine (DMoPC, shorter lipid), and in lipidic mixtures containing DOPC and one of these lipids. The protein was aggregated in the pure vesicles of mismatching lipid but remained essentially monomeric in the mixtures as detected from BODIPY fluorescence emission self-quenching. From fluorescence resonance energy transfer (FRET) measurements (donor-n-(iodoacetyl)aminoethyl-1-sulfonaphthylamine (IAEDANS)-labeled protein; acceptor-BODIPY labeled protein), it was concluded that in the DEuPC/DOPC and DMoPC/DOPC lipid mixtures, domains enriched in the protein and the matching lipid (DOPC) are formed.

Absence of clustering of phosphatidylinositol-(4,5)-bisphosphate in fluid phosphatidylcholine

Phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P(2)] plays a key role in the modulation of actin polymerization and vesicle trafficking. These processes seem to depend on the enrichment of PI(4,5)P(2) in plasma membrane domains. Here, we show that PI(4,5)P(2) does not form domains when in a fluid phosphatidylcholine matrix in the pH range of 4.8-8.4. This finding is at variance with the spontaneous segregation of PI(4,5)P(2) to domains as a mechanism for the compartmentalization of PI(4,5)P(2) in the plasma membrane. Water/bilayer partition of PI(4,5)P(2) is also shown to be dependent on the protonation state of the lipid.

Characterization of DNA/lipid complexes by fluorescence resonance energy transfer

Fluorescence resonance energy transfer (FRET) is a potential method for the characterization of DNA-cationic lipid complexes (lipoplexes). In this work, we used FRET models assuming a multilamellar lipoplex arrangement. The application of these models allows the determination of the distance between the fluorescent intercalator on the DNA and a membrane dye on the lipid, and/or the evaluation of encapsulation efficiencies of this liposomal vehicle. The experiments were carried out in 1,2-dioleoyl-3-trimethylammonium-propane/pUC19 complexes with different charge ratios. We used 2-(3-(diphenylhexatrienyl)propanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine (DPH-PC) and 2-(4,4-difluoro-5-octyl-4-bora-3a,4a-diaza-s-indacene-3-pentanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine (BODIPY-PC) as membrane dyes, and ethidium bromide (EtBr) and BOBO-1 as DNA intercalators. In cationic complexes (charge ratios (+/-) >or= 2), we verified that BOBO-1 remains bound to DNA, and FRET occurs to the membrane dye. This was also confirmed by anisotropy and lifetime measurements. In complexes with all DNA bound to the lipid (charge ratio (+/-) = 4), we determined 27 A as the distance between the donor and acceptor planes (half the repeat distance for a multilamellar arrangement). In complexes with DNA unbound to the lipids (charge ratio (+/-) = 0.5 and 2), we calculated the encapsulation efficiencies. The presented FRET methodology is, to our knowledge, the first procedure allowing quantification of lipid-DNA contact.

Interaction of peptides with binary phospholipid membranes: application of fluorescence methodologies

The application of fluorescence methodologies to obtain information about the extent, dynamics and topology of peptide interaction with binary phospholipid (mainly zwitterionic/anionic) mixtures is reviewed. First, general approaches based on peptide (tryptophan residues) fluorescence properties that give information about its partition, location and dynamics will be presented. Then, methodologies based on membrane probes fluorescence that report the influence of peptide binding and/or incorporation on the lateral organization (phase separation) of membrane phospholipids will be described. Specific examples taken from the literature that illustrate both situations are presented as well as formalisms for data analysis. It is shown that steady-state and time-resolved fluorescence data (particularly important in the case of fluorescence resonance energy transfer studies) give complementary information, allowing a molecular picture of peptide interaction with biphasic systems to be drawn.

Quantification of protein-lipid selectivity using FRET

Membrane proteins exhibit different affinities for different lipid species, and protein-lipid selectivity regulates the membrane composition in close proximity to the protein, playing an important role in the formation of nanoscale membrane heterogeneities. The sensitivity of Förster resonance energy transfer (FRET) for distances of 10 A up to 100 A is particularly useful to retrieve information on the relative distribution of proteins and lipids in the range over which protein-lipid selectivity is expected to influence membrane composition. Several FRET-based methods applied to the quantification of protein-lipid selectivity are described herein, and different formalisms applied to the analysis of FRET data for particular geometries of donor-acceptor distribution are critically assessed.

Cholesterol modulates the organization of the gammaM4 transmembrane domain of the muscle nicotinic acetylcholine receptor

A 28-mer gammaM4 peptide, obtained by solid-state synthesis and corresponding to the fourth transmembrane segment of the nicotinic acetylcholine receptor gamma-subunit, possesses a single tryptophan residue (Trp453), making it an excellent model for studying peptide-lipid interactions in membranes by fluorescence spectroscopy. The gammaM4 peptide was reconstituted with synthetic lipids (vesicles of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, i.e., POPC) rich and poor in cholesterol and analyzed using steady-state and time-resolved fluorescence techniques. The decrease in gammaM4 intrinsic fluorescence lifetime observed upon incorporation into a cholesterol-rich lo phase could be rationalized on the basis of a dynamic self-quenching owing to the formation of peptide-rich patches in the membrane. This agrees with the low Förster type resonance energy transfer efficiency from the Trp453 residue to the fluorescent cholesterol analog, dehydroergosterol, in the lo phase. In the absence of cholesterol the gammaM4 nicotinic acetylcholine receptor peptide is randomly distributed in the POPC bilayer with its hydrophobic moiety matching the membrane thickness, whereas in the presence of cholesterol the increase in the membrane thickness and variation of the material properties favor the formation of peptide-enriched patches, i.e., interhelix interaction energy is essential for obtaining a stabilized structure. Thus, the presence of a cholesterol-rich, ordered POPC phase drives the organization of peptide-enriched patches, in which the gammaM4 peptide occupies approximately 30% of the patch area.

Ciprofloxacin interactions with bacterial protein OmpF: modelling of FRET from a multi-tryptophan protein trimer

The outer membrane protein F (OmpF) is known to play an important role in the uptake of fluoroquinolone antibiotics by bacteria. In this study, the degree of binding of the fluoroquinolone antibiotic ciprofloxacin to OmpF in a lipid membrane environment is quantified using a methodology based on Förster resonance energy transfer (FRET). Analysis of the fluorescence quenching of OmpF is complex as each OmpF monomer presents two tryptophans at different positions, thus sensing two different distributions of acceptors in the bilayer plane. Specific FRET formalisms were derived accounting for the different energy transfer contributions to quenching of each type of tryptophan of OmpF, allowing the recovery of upper and lower boundaries for the ciprofloxacin-OmpF binding constant (K(B)). log (K(B)) was found to lie in the range 3.15-3.62 or 3.58-4.00 depending on the location for the ciprofloxacin binding site assumed in the FRET modelling, closer to the centre or to the periphery of the OmpF trimer, respectively. This methodology is suitable for the analysis of FRET data obtained with similar protein systems and can be readily adapted to different geometries.

Is there a preferential interaction between cholesterol and tryptophan residues in membrane proteins?

Recently, several indications have been found that suggest a preferential interaction between cholesterol and tryptophan residues located near the membrane-water interface. The aim of this study was to investigate by direct methods how tryptophan and cholesterol interact with each other and what the possible consequences are for membrane organization. For this purpose, we used cholesterol-containing model membranes of dimyristoylphosphatidylcholine (DMPC) in which a transmembrane model peptide with flanking tryptophans [acetyl-GWW(LA)8LWWA-amide], called WALP23, was incorporated to mimic interfacial tryptophans of membrane proteins. These model systems were studied with two complementary methods. (1) Steady-state and time-resolved Förster resonance energy transfer (FRET) experiments employing the fluorescent cholesterol analogue dehydroergosterol (DHE) in combination with a competition experiment with cholesterol were used to obtain information about the distribution of cholesterol in the bilayer in the presence of WALP23. The results were consistent with a random distribution of cholesterol which indicates that cholesterol and interfacial tryptophans are not preferentially located next to each other in these bilayer systems. (2) Solid-state 2H NMR experiments employing either deuterated cholesterol or indole ring-deuterated WALP23 peptides were performed to study the orientation and dynamics of both molecules. The results showed that the quadrupolar splittings of labeled cholesterol were not affected by an interaction with tryptophan-flanked peptides and, vice versa, that the quadrupolar splittings of labeled indole rings in WALP23 are not significantly influenced by addition of cholesterol to the bilayer. Therefore, both NMR and fluorescence spectroscopy results independently show that, at least in the model systems studied here, there is no evidence for a preferential interaction between cholesterol and tryptophans located at the bilayer interface.

Interaction of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-labeled fatty amines with 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine bilayers: a molecular dynamics study

A complete homologous series of fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled fatty amines of varying alkyl chain length, NBD-C(n), inserted in 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers, was studied using atomistic molecular dynamics (MD) simulations. For all amphiphiles, the NBD fluorophore locates near the glycerol backbone/carbonyl region of POPC and establishes stable hydrogen bonding with POPC ester oxygen atoms. Small differences observed in the transverse location of the fluorophore correlate with other calculated parameters and with small discrepancies recently measured in the photophysical properties of the molecules. The longer-chained NBD-C(n) amphiphiles show significant mass density near the bilayer midplane, and the chains of these derivatives interdigitate to some extent the opposite bilayer leaflet. This phenomenon leads to a slower lateral diffusion for the longer-chained derivatives (n > 12). Effects of these amphiphiles on the structure and dynamics of the host lipid were found to be relatively mild, in comparison with acyl-chain-labeled NBD probes. The molecular details obtained by this work allow the rationalization of the nonmonotonic behavior, recently obtained experimentally, for the photophysical parameters of the amphiphiles and the kinetic and thermodynamic parameters for their interaction with the POPC membranes.