A dye-leakage trace is a fluorescence intensity rising toward a plateau, and almost everything you might want to conclude from it — pore size, stoichiometry, whether a pore exists at all — sits underneath that curve rather than in it. The measurement reports one quantity: how much encapsulated fluorophore has stopped being quenched. Converting that into a statement about membrane mechanism means deciding what changed the quenching, and the assay by itself does not decide.
The dequenching readout
Two encapsulation designs dominate. In the self-quenched design a single fluorophore — carboxyfluorescein or calcein — is trapped at a concentration high enough that its own emission is largely quenched; release into the far larger external volume dilutes it and the signal rises. Rex and Schwarz's melittin work is a clean example of this format, following carboxyfluorescein efflux and treating the observed signal explicitly as dequenching of effluent dye [1]. In the quencher-pair design, reporter and quencher are separate molecules co-encapsulated together: 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS) with p-xylylenebis(pyridinium) bromide (DPX). Ellens and colleagues measured release kinetics continuously with that pair, and showed that release from phosphatidylethanolamine/cholesteryl hemisuccinate liposomes was a function of pH, of the ratio of the two components, and of the acyl chain composition of the phosphatidylethanolamine — composition dependence already present before any peptide enters the system [2].
The designs are not interchangeable. Separating dye from quencher is what makes the requenching experiment possible, and it is also what introduces a failure mode the self-quenched format does not have: a trisulfonated anion and a bis-pyridinium cation need not leave the vesicle at the same rate.
Graded, all-or-none, and why 50% is ambiguous
Half of maximum signal is consistent with two incompatible pictures: half the vesicles emptied completely while the rest were untouched, or every vesicle lost half its contents. Wimley, Selsted and White introduced the requenching method to separate the two without physically separating vesicles from released contents, establishing the mode of release from the fluorescence changes that occur when DPX is added back to the external solution of a partially released sample [3]. Applied to human neutrophil defensin HNP-2 on POPG vesicles, it showed native HNP-2 releasing contents in an all-or-none manner while the reduced peptide produced graded leakage — one sequence, two modes, depending on its disulfide state [3].
Ladokhin, Wimley and White then showed the original treatment was incomplete because it ignored preferential release of dye or quencher, and extended the theory to cover both preferential and graded release, with the ratio of release rates of the cationic quencher DPX and the anionic dye ANTS recoverable from the data [4]. Two consequences land directly on the bench. Under graded leakage the release of markers does not coincide with the fluorescence changes recorded in the standard leakage assay — and this is true for self-quenching dyes as well — so the amount of released material is overestimated and the kinetics come out non-exponential with artificially high apparent rates [4]. They also reported experimental evidence for peptide-induced preferential graded leakage of DPX from both neutral and anionic vesicles [4]. A standard leakage curve on a graded system is therefore not merely uninterpretable as to mechanism; its amplitude and its fitted rate constant are both biased.
Both modes are real and well documented. Rex and Schwarz found melittin efflux of carboxyfluorescein from POPC and DOPC vesicles of several sizes to be graded, characterised by a single-pore retention factor and pore lifetimes in the millisecond range; the pore formation rate rose in proportion to the amount of membrane-bound peptide, and every curve showed a pronounced slowing with two relaxation steps near 20 and 200 s that were largely independent of vesicle type and of peptide-to-lipid ratio [1]. Cecropin A goes the other way: the release rate increases with the number of peptides bound per vesicle and, at constant concentration, with the phosphatidylglycerol fraction of a phosphatidylcholine membrane, and an exact kinetic model fits in which a vesicle enters an unstable state and releases all of its contents [5].
Bulk requenching infers a mode from a population average; single-vesicle imaging observes it. Apellániz and colleagues ran both on two peptides derived from the membrane-proximal external region of a viral fusion glycoprotein, which bulk requenching classified as all-or-none and graded respectively, and found in giant unilamellar vesicles that the two differed in kinetics, in degree of dye filling and in the stability of the permeabilised state: the all-or-none peptide gave fast and total filling of individual vesicles, usually with stably open pores that still admitted a second dye added several hours later, while the graded peptide gave slower, transient, partial filling [6]. Quantitative analysis of the vesicle population distribution also identified mixed mechanisms, which no single bulk number can express [6]. Tamba and Yamazaki's single-GUV measurements show what all-or-none looks like as a process rather than a label: at 3–10 µM, magainin 2 triggered calcein leakage from individual 50% DOPG/50% DOPC vesicles stochastically, and once leakage began, complete release took 6–60 s, with the fraction of fully leaked vesicles rising with time and with peptide concentration [7].
The classification is not a fixed property of a sequence. Rice and colleagues showed that graded leakage by piscidin 1 converts to all-or-none simply by adding a defect-promoting lysophospholipid, and proposed a quantitative model treating the two as a continuum governed by local peptide concentration and the probability distribution of pore sizes, in which pores too small to pass dye supply the "none" component of all-or-none release [8]. Read graded versus all-or-none as a description of one system under one set of conditions.
Lipid composition is the dominant variable
Charge sets whether the peptide is on the membrane at all. HNP-2 carries net charge +3 and cannot bind vesicles formed from neutral lipids; binding to POPG/POPC mixtures is initiated through electrostatic interactions, and fractional leakage is a sigmoidal function of peptide concentration, with 50% of entrapped solute released by 0.7 µM HNP-2 against 300 µM POPG vesicles [3]. Cecropin A locates the charge effect kinetically: the on-rate onto vesicles is almost independent of composition, while the off-rate decreases by orders of magnitude as anionic lipid content rises [5]. What anionic lipid mainly buys, on that evidence, is residence time.
Curvature and elasticity carry the rest, and they are separable from binding energetics. Allende, Simon and McIntosh measured melittin transfer free energy and melittin-induced leakage against bilayer material properties: cholesterol, which raises the area compressibility modulus and the bending modulus, lowered both the transfer free energy and leakage; lysoPC (positive spontaneous curvature) and dioleoylglycerol (negative) had little effect on the transfer free energy but produced large changes in leakage, from 86% for 8:2 PC/lysoPC to 18% for 8:2 PC/dioleoylglycerol [9]. Leakage varied linearly with the area compressibility modulus and with the inverse square of the spontaneous curvature radius, and showed no correlation with bilayer hydrocarbon thickness — the observation that argues for defects lined in part by lipid rather than a peptide-only barrel [9].
Composition moves potency and mechanism at the same time. Fiedler and Heerklotz used a fluorescence-lifetime leakage assay on cyclic lipopeptides and found fengycin-induced leakage strongly inhibited by cholesterol as well as by phosphatidylethanolamine and phosphatidylglycerol, with moderate cholesterol raising the tolerable fengycin content in the membrane by an order of magnitude, to 0.5 fengycin per PC + cholesterol; ergosterol was only weakly antagonistic [10]. Surfactin, by contrast, was little affected by sterols and PE, but phosphatidylglycerol both increased its activity and changed its leakage mechanism toward all-or-none, which the authors read as more specific, larger and/or longer-lived defect structures [10]. A composition swap intended to make a mimetic more realistic can therefore relocate the mechanism you are trying to characterise. Specify composition in mol%, hold it fixed across every peptide in a comparison, and treat the companion guide on membrane mimetics as a prerequisite: the leakage assay inherits every limitation of the lipid mixture you chose.
Lipid-to-peptide ratio, not peptide concentration
Leakage tracks bound peptide per vesicle, not peptide in the cuvette. That is explicit in both kinetic treatments above: the pore formation rate rises in proportion to membrane-bound peptide [1], and the release rate scales with the ratio of peptide bound per vesicle [5]. At fixed peptide concentration, doubling total lipid halves the mole ratio in the membrane, so a peptide concentration alone does not identify the experiment. Published results are quoted on the ratio axis — melittin forms pores of 25–30 Å diameter in POPC vesicles at a lipid-to-peptide ratio of 50 [11], and the fengycin tolerance figure above is a mole ratio in the membrane rather than a bulk concentration [10].
The ratio also fixes how far a result can be carried. Wimley's comparison of the vesicle and microbial literatures notes that microbial experiments are done at peptide-to-lipid ratios at least four orders of magnitude higher than vesicle-based experiments, and proposes the interfacial activity model specifically to close that gap [12]. A vesicle assay and a cellular assay run on the same peptide are not sampling the same regime, which is the first thing to check when a leakage ranking and a cellular potency ranking disagree; the companion antimicrobial screening guide handles the cellular side.
Osmotic and dye-specific artefacts
Encapsulated dye and quencher are solutes, and if the external buffer is not iso-osmotic the vesicles are under tension before the peptide arrives. Tension is not a small correction. Polozov and colleagues found defect formation by the class L amphipathic helical peptide 18L strongly enhanced by membrane tension from osmotic swelling: even below the peptide/lipid ratios that normally induce leakage, membrane resistance to osmotic tension dropped from hundreds to tens of milliosmoles, with the size of the drop depending on peptide/lipid ratio and on lipid type, and the same tension modulation appeared for mastoparan and K18L [13]. Match osmolarity across the gel-filtration or dialysis step that removes untrapped dye, and record what you matched it to.
Dye chemistry is a second, system-specific error source. Firsov and colleagues showed that marker choice can decide the outcome: 5(6)-carboxyfluorescein proved highly susceptible to peroxidative destruction and unsuitable, while calcein was resistant to oxidative stress and suitable [14]. That comparison was made for cytochrome c/H2O2-mediated liposome permeabilisation rather than for a peptide, so the specific ranking does not transfer; the transferable point is that a fluorophore which can be chemically consumed or modified by anything in the assay generates intensity changes with no membrane event behind them. The ANTS/DPX analogue of this problem is electrostatic rather than redox, and is exactly the preferential release the requenching theory was extended to handle [4].
Baselines drift for reasons unconnected to the peptide. Grad and colleagues recorded significantly different spontaneous carboxyfluorescein leakage profiles from the same liposome batch depending on whether the cuvette was quartz, polystyrene or poly(methyl methacrylate), attributed the discrepancy, on quartz crystal microbalance with dissipation monitoring evidence, to liposomes likely attaching and spreading at the solution–cuvette interface, and showed that in some cases it can be reduced by supplementing liposomes with PEG-grafted lipids or gangliosides and by pre-adsorbing free PEG to the cuvette walls [15]. Run the peptide-free control in the same vessel material, at the same temperature, over the same duration as the sample.
Aggregation and fusion also masquerade as leakage, and both are peptide- and concentration-dependent. Wimley and colleagues mapped this by systematically varying vesicle and peptide concentrations and the POPG:POPC ratio: 300 µM POPG vesicles aggregated readily above 1 µM HNP-2, yet no mixing of vesicle contents was detectable up to 2 µM despite demonstrable intervesicular lipid mixing, indicating hemifusion rather than fusion [3]. A leakage number collected at a single vesicle concentration has not been shown to be free of these processes.
What the curve licenses
Properly controlled, a leakage curve licenses three statements: that this peptide permeabilised this composition at this lipid-to-peptide ratio on this timescale; a relative ranking against peptides measured under identical conditions; and, with requenching or single-vesicle imaging added, the mode of release [3, 4, 6]. It does not license a pore.
Size needs co-encapsulated markers that differ in size. In the HNP-2 study, at peptide concentrations releasing 100% of markers of roughly 400 Da, a 4,400 Da fluorescent dextran was partially retained and an 18,900 Da dextran mostly retained [3]; differential release of two co-encapsulated labelled dextrans is the generalised version of that experiment [11]. Stoichiometry needs more than kinetics: the cecropin A model fits with no peptide oligomerisation on the vesicle, the unstable pore state being a state of the whole vesicle that need not involve all the peptides bound to it, and the authors conclude that formation of well-defined, peptide-lined pores is unlikely [5]. Geometry likewise comes from lipid physics rather than release amplitude — it is the absence of any correlation between leakage and hydrocarbon thickness that argues against a barrel-stave arrangement for melittin [9].
Persistence is another measurement again, and it separates peptides that a fixed-timepoint endpoint would call equivalent. Krauson, He and Wimley assayed six permeabilising peptides in parallel and found the antimicrobial peptides LL37 and dermaseptin S1 are not potent pore formers, producing only transient permeabilisation pathways that are not detectable in bilayers at equilibrium, whereas melittin, alamethicin and the lentivirus lytic peptides LLP1 and LLP2 formed pores still detectable in vesicles after many hours [16]. Alamethicin was distinctive in being very potent, readily exchangeable between vesicles, and able to disturb local bilayer structure even at very low concentration, while the LLP peptides neither exchanged readily nor perturbed the bilayer at equilibrium [16].
So report the assay, not just the endpoint: lipid composition in mol%, extrusion pore size and vesicle preparation, total lipid, peptide concentration and the resulting lipid-to-peptide ratio, buffer identity and osmolarity, dye and quencher identity with encapsulated concentrations, how the 100% endpoint was defined, and the timepoint at which fractional leakage was read. Without composition and ratio attached, a leakage percentage is not a number anyone else can compare against [1, 5, 10].