Effect of tryptophan derivatives on the phase properties of bilayers

The Antimicrobial Peptide 1018-K6 Interacts Distinctly with Eukaryotic and Bacterial Membranes, the Basis of Its Specificity and Bactericidal Activity

Since penicillin was discovered, antibiotics have been critical in the fight against infections. However, antibiotic misuse has led to drug resistance, which now constitutes a serious health problem. In this context, antimicrobial peptides (AMPs) constitute a natural group of short proteins, varying in structure and length, that act against certain types of bacterial pathogens. The antimicrobial peptide 1018-K6 (VRLIVKVRIWRR- NH2) has significant bactericidal and antibiofilm activity against Listeria monocytogenes isolates, and against different strains and serotypes of Salmonella. Here, the mechanism of action of 1018-K6 was explored further to understand the peptide-membrane interactions relevant to its activity, and to define their determinants. We combined studies with model synthetic membranes (liposomes) and model biological membranes, assessing the absorption maximum and the quenching of 1018-K6 fluorescence in aqueous and lipid environments, the self-quenching of carboxyfluorescein, as well as performing lipid sedimentation assays. The data obtained reflect the differential interactions of the 1018-K6 peptide with eukaryotic and prokaryotic membranes, and the specific interactions and mechanisms of action in the three prokaryotic species studied: Salmonella Typhimurium^2GN, Escherichia coli^3GN, and Staphylococcus aureus^3GP. The AMP 1018-K6 is a candidate to prevent (food preservation) or treat (antibiotic use) infections caused by certain pathogenic bacteria, especially some that are resistant to current antibiotics.

Correlation of charge, hydrophobicity, and structure with antimicrobial activity of S1 and MIRIAM peptides

Antimicrobial peptides are key elements of the innate immune system. Many of them interact with membranes of bacteria leading to perturbation of the lipid bilayer and eventually to inactivation of the pathogen. The emergence of multidrug-resistant bacteria has necessitated innovations of new and more powerful classes of antimicrobials. Here we present the in-depth study of an antimicrobial peptide, MIRIAM, derived from Sushi1 (S1), a well-characterized peptide from the horseshoe crab. MIRIAM interacts strongly with negatively charged lipids, forming an α-helical structure. MIRIAM was found to neutralize LPS and kill Gram-negative bacteria with high efficiency, while not releasing LPS. The promising therapeutic potential of MIRIAM is shown by hemolytic assays, which demonstrate that eukaryotic membranes are unaffected at bactericidal concentrations. Nanoparticle-conjugated MIRIAM used in single-molecule fluorescence and electron microscopy experiments showed that MIRIAM targets bacterial membranes to kill bacteria similarly to parental S1. Furthermore, fragments derived from MIRIAM and S1 provided insights on their molecular mechanisms of action, in particular, the relationships of functional motifs comprised by charge, hydrophobicity, and structure within each peptide. We conclude that the combination of charge, hydrophobicity, and length of the peptide is important. A close interaction of amino acids in a single molecule in a carefully balanced ensemble of sequence position and secondary structure is crucial.

Folding is not required for bilayer insertion: replica exchange simulations of an alpha-helical peptide with an explicit lipid bilayer

We implement the replica exchange molecular dynamics algorithm to study the interactions of a model peptide (WALP-16) with an explicitly represented DPPC membrane bilayer. We observe the spontaneous, unbiased insertion of WALP-16 into the DPPC bilayer and its folding into an alpha-helix with a transbilayer orientation. The free energy surface suggests that the insertion of the peptide into the DPPC bilayer precedes secondary structure formation. Although the peptide has some propensity to form a partially helical structure in the interfacial region of the DPPC/water system, this state is not a productive intermediate but rather an off-pathway trap for WALP-16 insertion. Equilibrium simulations show that the observed insertion/folding pathway mirrors the potential of mean force (PMF). Calculation of the enthalpic and entropic contributions to this PMF show that the surface bound conformation of WALP-16 is significantly lower in energy than other conformations, and that the insertion of WALP-16 into the bilayer without regular secondary structure is enthalpically unfavorable by 5-10 kcal/mol/residue. The observed insertion/folding pathway disagrees with the dominant conceptual model, which is that a surface-bound helix is an obligatory intermediate for the insertion of alpha-helical peptides into lipid bilayers. In our simulations, the observed insertion/folding pathway is favored because of a large (>100 kcal/mol) increase in system entropy that occurs when the unstructured WALP-16 peptide enters the lipid bilayer interior. The insertion/folding pathway that is lowest in free energy depends sensitively on the near cancellation of large enthalpic and entropic terms. This suggests the possibility that intrinsic membrane peptides may have a diversity of insertion/folding behaviors depending on the exact system of peptide and lipid under consideration.

Specific interactions of tryptophan with phosphatidylcholine and digalactosyldiacylglycerol in pure and mixed bilayers in the dry and hydrated state

Amphiphilic solutes play an important role in the desiccation tolerance of plant cells, because they can reversibly partition into cellular membranes during dehydration. Their effects on membrane stability depend on their chemical structure, but also on the lipid composition of the host membrane. We have shown recently that tryptophan destabilizes liposomes during freezing. The degree of destabilization depends on the presence of glycolipids in the membranes, but not on the phase preference (bilayer or non-bilayer) of the lipids in mixtures with the bilayer lipid phosphatidylcholine. Here, we have investigated the influence of tryptophan on the phase behavior and intermolecular interactions in dry and hydrated bilayers made from the phospholipid egg phosphatidylcholine and the plant chloroplast glycolipid digalactosyldiacylglycerol, or from a mixture (1:1) of these lipids, using Fourier-transform infrared spectroscopy. To distinguish effects of the hydrophobic ring structure of tryptophan from those of the amino acid moiety, we also performed experiments with the hydrophilic amino acid glycine. Our data show that there are specific interactions between tryptophan and either phospholipid or glycolipid in the dry state, as well as H-bonding interactions between the lipids and both solutes. In the rehydrated state, the H-bonding interactions between amino acids and lipids are mostly replaced by interactions between water and lipids, while the hydrophobic interactions between lipids and tryptophan mostly persist.

Quantifying molecular partition into model systems of biomembranes: an emphasis on optical spectroscopic methods

Optical spectroscopies have been intensively used to determine partition coefficients by a plethora of methodologies. The present review is intended to give detailed and useful information for the determination of partition coefficients and addresses several relevant aspects, namely: (i) definition and calculation of the partition coefficient between aqueous and lipidic phases; (ii) partition coefficients vs. "binding" formalisms; (iii) advantages of spectroscopic methodologies over separation techniques; (iv) formalisms for various experimental approaches based on UV-Vis absorption or fluorescence parameters (fluorescence intensity, lifetime, anisotropy and quenching); (v) experimental hints, artifacts and model limitations; and (vi) a brief survey of nonoptical techniques.

Tryptophan fluorescence study of the interaction of penetratin peptides with model membranes

Penetratin is a 16-amino-acid peptide, derived from the homeodomain of antennapedia, a Drosophila transcription factor, which can be used as a vector for the intracellular delivery of peptides or oligonucleotides. To study the relative importance of the Trp residues in the wild-type penetratin peptide (RQIKIWFQNRRMKWKK) two analogues, the W48F (RQIKIFFQNRRMKWKK) and the W56F (RQI KIWFQNRRMKFKK) variant peptides were synthesized. Binding of the three peptide variants to different lipid vesicles was investigated by fluorescence. Intrinsic Trp fluorescence emission showed a decrease in quantum yield and a blue shift of the maximal emission wavelength upon interaction of the peptides with negatively charged phosphatidylserine, while no changes were recorded with neutral phosphatidylcholine vesicles. Upon binding to phosphatidylcholine vesicles containing 20% (w/w) phosphatidylserine the fluorescence blue shift induced by the W56F-penetratin variant was larger than for the W48F-penetratin. Incorporation of cholesterol into the negatively charged lipid bilayer significantly decreased the binding affinity of the peptides. The Trp mean lifetime of the three peptides decreased upon binding to negatively charged phospholipids, and the Trp residues were shielded from acrylamide and iodide quenching. CD measurements indicated that the peptides are random in buffer, and become alpha helical upon association with negatively charged mixed phosphatidylcholine/phosphatidylserine vesicles, but not with phosphatidylcholine vesicles. These data show that wild-type penetratin and the two analogues interact with negatively charged phospholipids, and that this is accompanied by a conformational change from random to alpha helical structure, and a deeper insertion of W48 compared to W56, into the lipid bilayer.

A Monte Carlo study of peptide insertion into lipid bilayers: equilibrium conformations and insertion mechanisms

The membrane insertion behavior of two peptides, Magainin2 and M2 delta, was investigated by applying the Monte Carlo simulation technique to a theoretical model. The model included many novel aspects, such as a new semi-empirical lipid bilayer model and a new set of semi-empirical transfer energies, which reproduced the experimental insertion behavior of Magainin2 and M2 delta without parameter fitting. Additionally, we have taken into account diminished internal (intramolecular) hydrogen bonding at the N- and C-termini of helical peptides. All simulations were carried out at 305 K, above the membrane thermal phase transition temperature, and at pH 7.0. The peptide equilibrium conformations are discussed for a range of bilayers with tail polarities varying from octanol-like to alkane-like. Probability distributions of the individual amino-acid-residue positions show the dynamic nature of these equilibrium conformations. Two different insertion mechanisms for M2 delta, and a translocation mechanism for Magainin2, are described. A study of the effect of bilayer thickness on M2 delta insertion suggests a critical thickness above which insertion is unfavorable. Additionally, we did not need to use an orientational potential or array of hard cylinders to persuade M2 delta to insert perpendicular to the membrane surface. Instead, we found that diminished internal hydrogen bonding in the helical conformation anchored the termini in the headgroups and resulted in a nearly perpendicular orientation.

Influence of reverse micellar environments on the fluorescence emission properties of tryptophan octyl ester

A number of recent studies have presented perspectives on the hydrophobic fluorescence probe tryptophan octyl ester (TOE). This molecule has attracted notable attention as a suitable model for the natural fluorophore tryptophan, in case of membrane proteins. We report here, for the first time, the fluorescence emission behaviour of TOE in reverse micelles of aerosol-OT (AOT) in n-heptane, containing different amounts of water. Relevant studies in representative homogeneous solvent media are also included for comparison. The fluorescence emission parameters (especially emission maximum, relative intensity, and anisotropy) of TOE are found to exhibit significant variation upon changes in the water/surfactant molar ratio (w(0)) of the reverse micelles. Fluorescence decay studies on TOE which we have also performed, indicate biexponential decay kinetics in reverse micelles as well as in homogeneous solvent media. The implications of these findings are examined in relation to the potentialities of TOE as a novel fluorescence probe for membrane proteins present in water restricted environments prevailing at the interfaces of biomembranes (for which reverse micelles serve as ideal model systems).

Comparison of dynamic and integrated light-scattering techniques in the study of the interaction of Candida rugosa lipase with DPPC liposomes

Dynamic light-scattering (DLS) and wide angle integrated light-scattering (WAILS) spectroscopies were evaluated in the study of binding of Candida rugosa lipase (CRL) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes. The use of cumulants analysis on DLS data allowed for the determination of general lipase-liposome-binding trends. Particle intensity distributions obtained from DLS data by a discrete inversion method revealed the different populations created upon lipase-liposome interactions. Using a discrete inversion technique on WAILS data, not only these populations could be differentiated but also accurate number distributions were obtained in short periods of time. Both DLS and WAILS are excellent tools for the study of lipase binding to lipid vesicles; however, care must be exercised in the analysis of the experimental data whenever particle size distributions are multimodal. The selection of the light scattering technique will depend on the information required.

Release of aqueous contents from phospholipid vesicles induced by cecropin A (1-8)-magainin 2 (1-12) hybrid and its analogues

The membrane-disrupting properties of cecropin A (1-8)-magainin 2 (1-12) hybrid peptide, which has higher antitumor with less hemolytic activities than cecropin A (1-8)-melittin (1-12), and its analogues were assessed by measuring the induced release of vesicle-entrapped fluorescence probes. A model membrane was made of zwitterionic phospholipid (phosphatidylcholine) or the mixture of negatively and zwitterionic phospholipids (phosphatidylcholine and phosphatidylserine). The extent of leakage of the aqueous content of the phospholipid vesicles was found to have remarkable discrepancies according to the amphipathic nature of each analogue peptide. The entrapped high molecular weight solute (fluorescein-labeled immunoglobulin G, 55 kDa) also was released by the analogue which had the largest hydrophobic region and the highest amphipathic score among peptides tested. As the result of the determination of the relationships between the membrane-disrupting properties and the hydrophobicity values of peptides, it was found that the membrane-disrupting activity increased according to increasing the hydrophobicity of the peptide. The tryptophan fluorescence emission spectra and CD spectra showed that on interaction with the phospholipid vesicle, the peptide acquired the ordered structure and alpha-helical conformation by moving a tryptophan residue into the nonpolar environment of the phospholipid vesicle. These results suggest that the breakdown of the lipid bilayer was mediated by the alpha-helical amphipathic structure of the peptide interacting with the lipid bilayers as well as the by the hydrophobicity of the peptide.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Ionization, partitioning, and dynamics of tryptophan octyl ester: implications for membrane-bound tryptophan residues

The presence of tryptophan residues as intrinsic fluorophores in most proteins makes them an obvious choice for fluorescence spectroscopic analyses of such proteins. Membrane proteins have been reported to have a significantly higher tryptophan content than soluble proteins. The role of tryptophan residues in the structure and function of membrane proteins has attracted a lot of attention. Tryptophan residues in membrane proteins and peptides are believed to be distributed asymmetrically toward the interfacial region. Tryptophan octyl ester (TOE) is an important model for membrane-bound tryptophan residues. We have characterized this molecule as a fluorescent membrane probe in terms of its ionization, partitioning, and motional characteristics in unilamellar vesicles of dioleoylphosphatidylcholine. The ionization property of this molecule in model membranes has been studied by utilizing its pH-dependent fluorescence characteristics. Analysis of pH-dependent fluorescence intensity and emission maximum shows that deprotonation of the alpha-amino group of TOE occurs with an apparent pKa of approximately 7.5 in the membrane. The fluorescence lifetime of membrane-bound TOE also shows pH dependence. The fluorescence lifetimes of TOE have been interpreted by using the rotamer model for the fluorescence decay of tryptophan. Membrane/water partition coefficients of TOE were measured in both its protonated and deprotonated forms. No appreciable difference was found in its partitioning behavior with ionization. Analysis of fluorescence polarization of TOE as a function of pH showed that there is a decrease in polarization with increasing pH, implying more rotational freedom on deprotonation. This is further supported by pH-dependent red edge excitation shift and the apparent rotational correlation time of membrane-bound TOE. TOE should prove useful in monitoring the organization and dynamics of tryptophan residues incorporated into membranes.

Estimation of average depth of penetration of melanotropins in dimyristoylphosphatidylglycerol vesicles

The interaction of alpha-melanocyte stimulating hormone (alpha-MSH) and its analogs [Nle4,D-Phe7]-alpha-MSH (MSH-I) and [Nle4,Asp5,D-Phe7,Lys10]-alpha-MSH(4-10) (MSH-II) with vesicles of dimyristoylphosphatidylglycerol (DMPG) was studied by steady-state fluorescence spectroscopy. The association constants for the interaction were obtained from binding isotherms. Electrostatic effects on the interaction were taken into account through calculation of Gouy-Chapman potentials. The quenching of fluorescence of the peptides by acrylamide and nitroxide labeled lipids demonstrated that insertion of the peptides into the lipid phase of the vesicles causes the changes in the hormone's fluorescence in the presence of DMPG. The parallax method was employed for the estimation of an average depth of penetration of the peptides in the DMPG vesicles. It was found that the Trp residue in alpha-MSH and in MSH-II is positioned around the carbons 6 and 8 of the aliphatic chain. The analog MSH-I goes deeper into the bilayer compared to the others peptides, and the Trp residue locates between carbons 10 and 11 of the acyl chain. The average depth of penetration shows correlation with the number of lipid molecules that interact with one molecule of peptide. There is no direct correlation between the association constants for the lipid-peptide interactions and the depth of penetration of the hormone.

Interaction of indole and tryptophan derivatives with sodium dodecyl sulfate micelles measured with ultraviolet absorption and fluorescence quenching

The distribution of indole and tryptophan derivatives between sodium dodecyl sulfate (SDS) micellar and aqueous phases was analyzed using conventional methods of ultraviolet (UV) absorption spectroscopy and measurement of fluorescence quenching by succinimide. On the assumption of a simple pseudo-phase equilibrium between both phases the distribution coefficient was easily obtained by the measurement of the ratio Rpv of the absorbance intensity in the peak to that in the valley of the UV spectra or the fluorescence quenching constant Ksv. The possibilities and limitations of utilizing the ratio of the collisional quenching constant estimating from the Ksv value in the micellar phase to that in the aqueous phase for a measure of polarity of the microenvironment around the tryptophan derivatives in the SDS micelle is discussed in comparison with the Rpv values for the UV spectra. The indole ring in the derivatives in the SDS micelle is localized near or on the micelle-water interface with its imino group directed toward the aqueous phase. Thus it can serve as a feasible model for interpreting the distribution coefficients and Rpv values obtained for the various indole and tryptophan derivatives.

Thermodynamics of interaction of the fusion-inhibiting peptide Z-D-Phe-L-Phe-Gly with dioleoylphosphatidylcholine vesicles: direct calorimetric determination

The binding of the fusion-inhibiting peptide Z-D-Phe-L-Phe-Gly to unilamellar lipid vesicles of dioleoylphosphatidylcholine (DOPC) was investigated by isothermal titration calorimetry (ITC). The peptide Z-D-Phe-L-Phe-Gly is known to inhibit fusion of myxo- and paramyxoviruses with cells as well as cell-cell and vesicle-vesicle fusion in model systems. Calorimetric titrations conducted over a range of temperatures permitted characterization of the thermodynamics of the interaction of Z-D-Phe-L-Phe-Gly with model DOPC lipid membranes. Simultaneous global analysis of 15 ITC binding curves acquired at four different temperatures allowed determination of the equilibrium site association constant (K), stoichiometry of binding (n), binding enthalpy change (delta H), and heat capacity change of binding (delta Cp) in a single set of experiments. The binding affinity and enthalpy change per mole of DOPC bound at 25 degrees C was log K = 2.463 +/- 0.075 and delta H = -1.07 +/- 0.12 kcal/mol DOPC while the binding heat capacity change per mole of DOPC bound was delta Cp = -20.3 +/- 2.8 cal/(K.mol DOPC) with a temperature dependence (from 10-45 degrees C) of d(delta Cp)/dT = 0.37 +/- 0.18 cal/(K2.mol DOPC). A temperature-independent binding stoichiometry was determined to be n = 5.56 +/- 0.33 DOPC molecules per Z-D-Phe-L-Phe-Gly. A comparison of these results with previous peptide-lipid binding studies is discussed as is their relevance to a current model of the interaction of fusion-inhibiting peptides with phospholipid membranes.

Effect of charged residue substitutions on the thermodynamics of signal peptide-lipid interactions for the Escherichia coli LamB signal sequence

We have used tryptophan fluorescence spectroscopy to characterize the binding affinities of an Escherichia coli LamB signal peptide family for lipid vesicles. These peptides harbor charged residue substitutions in the hydrophobic core region. Titrations of peptides with vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine and 1-palmitoyl-2-oleoyl-sn-3-phosphoglycerol (65:35 mol%), in conjunction with evaluation of peptide dissociation rates from these vesicles, were used to determine binding parameters quantitatively. We find that under low ionic strength conditions, point mutations introducing negatively charged aspartate residues substantially reduce peptide affinity relative to the wild-type peptide. However, the difference between wild-type and mutant peptide affinities was much lower under approximately physiological ionic strength. In addition, the lipid affinities of model surface-binding and transmembrane peptides were determined. These comparative studies with signal and model peptides permitted semi-quantitative deconvolution of signal peptide binding into electrostatic and hydrophobic components. We find that both interactions contribute significantly to binding, although the theoretically available hydrophobic free energy is largely offset by unfavorable polar-group effects. The implications of these results for understanding the potential roles of the signal sequence in protein translocation are discussed.

Thermodynamic characterization of the association of small basic peptides with membranes containing acidic lipids

We measured the binding of the peptide acetyl-Trp-Lys7-amide to membranes formed from mixtures of the zwitterionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC) and the acidic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (PG). Direct filtration and equilibrium dialysis measurements demonstrate that binding increases sigmoidally with the mole fraction of PG as predicted from a simple Gouy-Chapman/mass action theoretical model. We used these measurements to calibrate two binding assays, one based on the increase in Trp fluorescence that occurs when the peptide binds to the membrane, the other on the quenching of Trp fluorescence that occurs when the peptide binds to membranes containing fluorescent lipids. Both fluorescence assays demonstrate that binding does not depend strongly on temperature, which suggests the enthalpy change, delta H, is small. Calorimetric measurements demonstrate this directly for the analogous basic peptide Lys5: delta H congruent to +1 kcal/mol for the binding of Lys5 to sonicated phospholipid vesicles and delta H congruent to 0 kcal/mol for its binding to large unilamellar vesicles. Thus, the decrease in the free energy that occurs when these peptides bind to the membrane is due to a positive change in the entropy of the system. Fluorescence measurements demonstrate the binding of the Trp-containing peptide to 4:1 PC/PG membranes is independent of pressure up to 2 kbar, which suggests that binding occurs without a significant change in volume.

Indole-3-acetic acid-mediated transport of Mn2+ and other ions across phosphatidylinositol vesicular membranes as determined by 31P-NMR

The indolic plant hormone, indole-3-acetic acid (IAA), mediated the transport of Mn2+ and other ions into small unilamellar vesicles prepared from soybean phosphatidylinositol (PI) and this process has been studied using 31P nuclear magnetic resonance spectroscopy (NMR). The rate of Mn2+ movement into PI vesicles is dependent on IAA concentration and temperature with an IAA stoichiometry of 4.1 and an activation energy of 16.8 kcal mol-1 derived for the rate-determining process. These values are altered by low concentrations of endogenous ions (which can be removed by treatment with EDTA) present in the PI. With non-EDTA-treated PI, values of 2.3 and 23.0 kcal mol-1 were obtained for the stoichiometry and activation energy, respectively. These values indicate that (a) IAA interacts with PI membranes; (b) IAA-induced changes in membrane permeability can be substantially modulated by ions and (c) IAA very significantly influences the rate of movement of some (but possibly not all) cations across PI membranes. Such effects are also modified by the oxidation state of the PI.

Titration calorimetric and differential scanning calorimetric studies of the interactions of n-butanol with several phases of dipalmitoylphosphatidylcholine

The interactions of n-butanol with dipalmitoylphosphatidylcholine (DPPC) were studied using titration calorimetry and differential scanning calorimetry (DSC). DSC results indicated that n-butanol induces the interdigitated phase in DPPC above 10 mg/mL butanol. A new application of titration calorimetry for measuring partition coefficients of nonsaturating solutes into lipids was developed. The partition coefficients and the heat of binding of n-butanol into DPPC were measured for the L beta', P beta', L alpha, and L beta I phases of DPPC. The partition coefficients were temperature dependent and ranged from 70 to 110 for the L beta I phase, from 170 to 183 for the L alpha phase, and similar to that for the L beta I phase in the P beta' phase. The binding to the L beta' phase could not be detected, giving an upper limit for this partition coefficient of 23. The enthalpies for binding to the L beta I and L alpha phases were 1.0 and 1.5 kcal/mol, respectively. The van't Hoff enthalpy was in good agreement with the calorimetric enthalpy for the partitioning into the L alpha phase; however, it was greater than the calorimetric enthalpy for the L beta I phase, suggesting that the interaction of n-butanol with this phase is cooperative in some way.

Peptide binding to lipid bilayers. Binding isotherms and zeta-potential of a cyclic somatostatin analogue

The binding of the cyclic somatostatin analogue SMS 201-995, (+)-D-Phe1-Cys2-Phe3-D-Trp4-(+)-Lys5-Thr6- Cys7-Thr(ol)8, to neutral and negatively charged lipids was investigated with a centrifugation assay and with electrophoretic and monolayer methods. Monolayers and bilayers were composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), either in pure form or in a 75/25 (mol/mol) mixture. The expansion of monolayer films demonstrated the intercalation of the peptide between the lipid molecules with a surface area requirement of 135 A2 per peptide molecule, indicating a parallel alignment of the peptide long axis with the membrane surface. Above a limiting pressure of 32.5 mN/m for POPC and 38.5 mN/m for POPG, peptide penetration was no longer possible. The peptide binding isotherm could be measured for mixed POPC/POPG bilayers up to a peptide concentration of 0.5 mM. Due to electrostatic attraction, binding between the positively charged peptide and the negatively charged membrane surface was enhanced as compared to the binding to a neutral membrane. After correction for electrostatic effects by means of the Gouy-Chapman theory, the binding isotherm as well as the electrophoretic zeta-potential measurement could be described by the same partition equilibrium with a surface partition constant of Kp = 36 +/- 4 M-1 (at 0.1 M NaCl). About 60-70% of SMS 201-995 is probably embedded in the headgroup region with little penetration into the lipid core. The partition constant increases with increasing salt concentration or with decreasing lipid lateral pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of charge and hydrophobicity in peptide-lipid interaction: a comparative study based on tryptophan fluorescence measurements combined with the use of aqueous and hydrophobic quenchers

The interaction of interrelated model peptides with model membranes has been studied by techniques based on tryptophan fluorescence. The peptides used are derivatives of the sequence H-Ala-Met-Leu-Trp-Ala-OH, which was designed for this purpose. Several modifications yielded a set of 13 penta- and hexapeptides varying in net charge, hydrophobicity, charge distribution, and the intramolecular position of the tryptophan residue with respect to the charge(s). The affinity of these peptides for small unilamellar vesicles (SUV) consisting of zwitterionic egg phosphatidylcholine (eggPC) and negatively charged beef heart cardiolipin (bhCL) has been investigated in a comparative way. The criteria for affinity comprise (1) intrinsic fluorescence changes upon titration of the peptides with the lipid vesicles, (2) reduced accessibility of the peptides to aqueous quenchers of tryptophan fluorescence (I- and acrylamide) in the presence of lipid, and (3) exposure to membrane-incorporated fluorescence quenchers, brominated phosphatidylcholines (BrPC). Application of BrPC brominated at different positions along the acyl chains provided information on the membrane topology of the peptides. With respect to the extent of affinity for zwitterionic membranes, the overall hydrophobicity of the peptides is the main determinant. A comparison of the affinity for PC of equally hydrophobic peptides carrying either a single positive or negative charge reveals preferential interaction of the cationic peptide. Both hydrophobic and electrostatic interactions determine the affinity of positively charged mono- and divalent peptides for CL vesicles. The distribution of the charged moieties in divalent positively charged peptides, either both at one end of the molecule or one at each end, has little influence on the affinity of these peptides for CL but does affect the extent of exposure to BrPC. Upon decreasing the surface charge density of the vesicles by diluting CL with increasing amounts of PC, both types of peptides show different behavior. The position of the tryptophan relative to the charged moiety in the peptide molecule is shown to affect the fluorescent properties upon interaction with vesicles. Concerning the membrane topology, all peptides adopt a localization near the membrane surface, with the neutral peptides inserting slightly deeper into the bilayer than the charged peptides. The results allow a comparative analysis of the factors determining the extents and modes of lipid-model peptide interaction; in addition, the validity of the methods applied is discussed.

Segregation of anionic lipophiles in bilayers monitored by binding of cationic dye NK-529

Fluorescence emission properties of a cationic indodicarbocyanine dye, NK-529, bound to anionic and zwitterionic vesicles, are examined under a variety of conditions to monitor lateral distribution of anionic amphiphiles in bilayers as a function of their phase properties. The change in the fluorescence properties of NK-529 arises from the binding of the dye to the bilayer that is dominated by ionic interactions when possible, as well as from the self-quenching of the dye bound to bilayers when the surface density of the dye is high. The binding affinity of the dye to anionic interfaces is more than 100-fold higher compared to that in zwitterionic bilayers. The limiting phospholipid/dye ratio in anionic bilayers at low vesicle concentrations is about 3. Thus the density of the bound dye in anionic bilayers can be more than 40-fold higher than that in zwitterionic bilayers, and therefore under such conditions the bound dye is completely self-quenched in vesicles or micelles of anionic phospholipids. The change in the fluorescence emission intensity on incorporation of anionic amphiphiles in zwitterionic bilayers is used to monitor segregation of the anionic amphiphiles. The organizational features of bilayers that cause a change in the fluorescence properties of bound NK-529 show that the lateral distribution of anionic amphiphiles is appreciably influenced not only by the mole fraction of the amphiphile but also in the presence of other additives, and by the gel-fluid thermotropic transition. As shown in the following paper, the fluorescence changes related to self-quenching in anionic bilayers containing NK-529 can be used to understand the organizational changes that occur during the course of interfacial catalysis by phospholipase A2 on zwitterionic bilayers.

Tryptophan sidechain dynamics in hydrophobic oligopeptides determined by use of 13C nuclear magnetic resonance spectroscopy

Two oligopeptides, t-boc-LAWAL-OMe and t-boc-LALALW-OMe, were synthesized for the purpose of examining the sidechain dynamics of the tryptophan residue in hydrophobic environments by 13C nuclear magnetic resonance and fluorescence spectroscopy. In both peptides, the tryptophan sidechain was greater than 95% enriched with 13C at the C delta 1 position. Spin-lattice relaxation time (T1) and steady-state nuclear Overhauser effect (NOE) data were obtained at 50.3 and 75.4 MHz for both peptides in CD3OD, and at 75.4 MHz for t-boc-LALALW-OMe in lysolecithin-D2O micelles. We have adapted the model-free approach of G. Lipari and A. Szabo (1982, J. Am. Chem. Soc. 104:4546) to interpret the 13C-NMR data. Computer-generated curves based on experimental data obtained at a single frequency demonstrate relationships between an effective correlation time for tryptophan sidechain motion (tau e), a generalized order parameter (sigma) describing the extent of motional restriction, and an overall correlation time for the peptide (tau m). Assuming predominantly dipolar relaxation, least-squares fits of the dual frequency relaxation data provide values for these parameters for both peptides. The contribution of chemical shift anisotropy (CSA), however, is also explicitly assessed in the data analysis, and is shown to perturb the predicted sigma, tau e, and tau m values and to decrease chi(2) values observed in nonlinear least-squares analysis of the data. Because of uncertainty in the contribution of CSA to the relaxation of the indole ring 13C delta 1 atom, nonlinear least-squares analysis of the relaxation data were performed with and without inclusion of a CSA term in the appropriate relaxation equations. Neglecting CSA, an overall peptide correlation time of 0.69 ns is predicted for t-boc-LAWAL-OMe in CD3OD at 20 degrees C compared with 1.28 ns for t-boc-LALALW-OMe. Given these tau m values and taking into account the effect of measurement error in the T1 and NOE data, the internal dynamics of the tryptophan residue of t-boc-LAWAL-OMe in this isotropic environment are described by a range of tau e values from 70 to 112 ps and sigma values between 0.22 and 0.36. Similarly, for t-boc-LALALW-OMe, 68 less than or equal to tau e less than or equal to 93 ps and 0.09 less than or equal to sigma less than or equal to 0.17. The Ch-terminal position of the tryptophan residue in the hexapeptide may account for its lower order parameter. In lysolecithin micelles, the model-free approach applied tot-boc-LALALW-OMe predicts a Te between 0.87 and 1.08 ns, and an order parameter range of 0.72-0.80, assuming an average Tm of 14 ns (Saunders, L. 1966. Biochim. Biophys. Acta. 125:70) for a typical peptide-micelle complex. In this case, measurement of only two 13C relaxation parameters at a single frequency yields sufficient information, plotted in the form of a composite T1-NOE solution curve, to constrain the allowed values of the model-free motional parameters within a relatively narrow range. The predicted range of eV and Te values for the peptide-micelle complex demonstrate that both the rate and spatial mobility of the indole moiety are markedly restrained in the anisotropic micelle environment relative to free methanol solution. Steady-state fluorescence anisotropy measurements made on the peptides dissolved in methanol or with synthetic lysolecithins in water were used to calculate apparent order parameters for tryptophan motion; these values agree well with order parameters calculated from 13C NMR data. The reported results are relevant to the issue of protein dynamic events occurring on the picosecond time scale predicted by molecular dynamics simulations.

Phospholipid structure determines the effects of peptides on membranes. Differential scanning calorimetry studies with pentagastrin-related peptides

The effect of phospholipid structure on the interaction between small peptides and phospholipid membranes has been studied by high-sensitivity differential scanning calorimetry. The peptides used, N-Boc-beta-Ala-Trp-Met-Arg-Phe-NH2 and N-Boc-beta-Ala-Trp-Met-Lys-Phe-NH2, are basic analogs of the hormone pentagastrin. These peptides split the gel-to-liquid crystalline phase transition of synthetic phosphatidylcholines into two components. For dimyristoyl (DMPC), dipalmitoyl (DPPC) and 1-stearoyl-2-oleoyl (SOPC) phosphatidylcholines, one component remains at the temperature corresponding to that of pure lipid and the other one is shifted towards higher temperatures. With increasing peptide concentration there is a gradual increase in the enthalpy of the high-temperature component at the expense of the low-temperature one, and there is also an increase in the total enthalpy of the transition. A mixture of the peptide with distearoylphosphatidylcholine (DSPC) behaves differently, with the transition occurring at a temperature below that of the pure lipid increasing with peptide concentration. The susceptibility of various phosphatidylcholines to perturbation by the peptides increases in the order DMPC greater than SOPC greater than DPPC greater than DSPC. The effect of these peptides on the phase transitions of acidic phosphatidylglycerols is generally greater than with the corresponding phosphatidylcholines, but the dependence on the length of lipid hydrocarbon chains is similar. Perturbation of the thermotropic phase transition is strongest for dimyristoylphosphatidylglycerol, followed by the dipalmitoyl and the distearoyl analogs. The effect of the peptides on the phase transition of dimyristoylphosphatidylserine is significantly smaller compared to that observed with dimyristoylphosphatidylglycerol and it is further reduced for dimyristoylphosphatidic acid. The phase transition of this latter lipid remains virtually unchanged, even in the presence of high concentrations of the peptide. Similar resistance to the perturbation of the phase transitions by the peptides is observed for synthetic phosphatidylethanolamine. The different susceptibility of various phospholipids to perturbation by the peptides is suggested to be related to different degrees of intermolecular interaction between phospholipid molecules, and particularly to different abilities of phospholipids to form intermolecular hydrogen bonding.

Interaction of myelin basic protein with different ionization states of phosphatidic acid and phosphatidylserine

Myelin basic protein (BP) has a perturbing effect on some lipids, causing, among other effects, a decrease in the temperature and enthalpy of the phase transition. This is believed to be a result of penetration of some hydrophobic residues of the protein partway into the lipid bilayer. Variations in the perturbing effect of BP on different acidic lipids has been attributed to the ability of the lipids to participate in intermolecular hydrogen bonding which inhibits penetration of the protein. Participation in intermolecular hydrogen bonding depends on the ionization state of the lipid as well as the type of lipid. In order to further test the dependence of the degree of penetration of BP on the intermolecular hydrogen bonding properties of lipids, the effect of BP on the phase transition of lipids in different ionization states was studied using differential scanning calorimetry. Dipalmitoylphosphatidic acid (DPPA) and dimyristoylphosphatidylserine (DMPS) were studied at different pH-values from 4 to 9.5. The results were compared to data obtained earlier with phosphatidylglycerol (PG), which is in the same ionization state at pH-values above 4, in order to distinguish the effects of pH on the protein from effects on the lipids. The perturbing effect of BP on PG increases with increase in pH. This is probably a result of the increasing hydrophobicity of the protein as the histidines become deprotonated, which allows greater penetration of the protein into the bilayer. In contrast, the effect on DPPA was greatest at low pH, where the state of ionization of the lipid is less than 1 and protein binding utilizes all of the hydrogen bond accepting sites (P-O-) on the lipid. BP had no perturbing effect on DPPA at higher pH where the state of ionization is between 1 and 1.5, and hydrogen bond accepting and donating sites (P-OH) are still available even after binding of the protein. Thus hydrogen bonding occurs at high pH and penetration of hydrophobic residues of the protein into DPPA is inhibited. BP had a large perturbing effect on DMPS at all pH values above 4 suggesting that lipid intermolecular hydrogen bonding does not occur in the presence of the protein and its hydrophobic residues consequently can penetrate into the bilayer. The protein may inhibit hydrogen bonding by binding electrostatically to the anionic hydrogen bond accepting group of PS.

Solute-induced acceleration of transbilayer movement and its implications on models of blood-brain barrier

Hexylglycerol accelerates the transbilayer (flip-flop) movement of phospholipids, lysophospholipids and peptides. For example, lysophosphatidylcholine added to dimyristoylphosphatidylcholine vesicles activates the action of pig pancreatic phospholipase A2 (Jain and DeHaas (1983) Biochim. Biophys. Acta 736, 157-162) This activating effect is dissipated slowly after mixing, and no activation is observed when the lysophospholipid molecules are equally distributed on both sides of the bilayer. The half time for transbilayer movement of lysophosphatidylcholine is about 7 h, and it is accelerated over 100-fold in the presence of n-hexylglycerol, as well as by a variety of other amphipathic solutes including n-alkanols, ketamine, and flufenamic acid. Hexylglycerol also accelerates the rate of transbilayer movement of an amphipathic hexapeptide bocLALALW, as well as of the phosphatidylcholine molecules in erythrocyte membrane. These effects are observed without any change in the gross bilayer organization as judged by 31P-NMR. Biophysical significance of such solute induced acceleration of transbilayer movement of amphipathic solutes is discussed to account for the effect of alkylglycerols on blood brain barrier.