Susceptibility corrections in solid-state NMR experiments with oriented membrane samples. Part I: applications

Homo- and heteromeric interaction strengths of the synergistic antimicrobial peptides PGLa and magainin 2 in membranes

PGLa and magainin 2 (MAG2) are amphiphilic α-helical frog peptides with synergistic antimicrobial activity. In vesicle leakage assays we observed the strongest synergy for equimolar mixtures of PGLa and MAG2. This result was consistent with solid-state (15)N-NMR data on the helix alignment in model membranes. The Hill coefficients determined from the vesicle leakage data showed that the heterodimeric (PGLa-MAG2) interactions were stronger than the homodimeric (PGLa-PGLa and MAG2-MAG2) interactions. This result was also reflected in the free energy of dimerization determined from oriented circular dichroism and quantitative solid-state (19)F-NMR analysis.

Action of the multifunctional peptide BP100 on native biomembranes examined by solid-state NMR

Membrane composition is a key factor that regulates the destructive activity of antimicrobial peptides and the non-leaky permeation of cell penetrating peptides in vivo. Hence, the choice of model membrane is a crucial aspect in NMR studies and should reflect the biological situation as closely as possible. Here, we explore the structure and dynamics of the short multifunctional peptide BP100 using a multinuclear solid-state NMR approach. The membrane alignment and mobility of this 11 amino acid peptide was studied in various synthetic lipid bilayers with different net charge, fluidity, and thickness, as well as in native biomembranes harvested from prokaryotic and eukaryotic cells. (19)F-NMR provided the high sensitivity and lack of natural abundance background that are necessary to observe a labelled peptide even in protoplast membranes from Micrococcus luteus and in erythrocyte ghosts. Six selectively (19)F-labeled BP100 analogues gave remarkably similar spectra in all of the macroscopically oriented membrane systems, which were studied under quasi-native conditions of ambient temperature and full hydration. This similarity suggests that BP100 has the same surface-bound helical structure and high mobility in the different biomembranes and model membranes alike, independent of charge, thickness or cholesterol content of the system. (31)P-NMR spectra of the phospholipid components did not indicate any bilayer perturbation, so the formation of toroidal wormholes or micellarization can be excluded as a mechanism of its antimicrobial or cell penetrating action. However, (2)H-NMR analysis of the acyl chain order parameter profiles showed that BP100 leads to considerable membrane thinning and thereby local destabilization.

AMPs and OMPs: Is the folding and bilayer insertion of β-stranded outer membrane proteins governed by the same biophysical principles as for α-helical antimicrobial peptides?

The folding and function of membrane proteins is controlled not only by specific but also by unspecific interactions with the constituent lipids. In this review, we focus on the influence of the spontaneous lipid curvature on the folding and insertion of peptides and proteins in membranes. Amphiphilic α-helical peptides, as represented by various antimicrobial sequences, are compared with β-barrel proteins, which are found in the outer membrane of Gram-negative bacteria. It has been shown that cationic amphiphilic peptides are always surface-bound in lipids with a negative spontaneous curvature like POPC, i.e. they are oriented parallel to the membrane plane. On the other hand, in lipids like DMPC with a positive curvature, these peptides can get tilted or completely inserted in a transmembrane state. Remarkably, the folding and spontaneous membrane insertion of β-barrel outer membrane proteins also proceeds more easily in lipids with a positive intrinsic curvature, while it is hampered by negative curvature. We therefore propose that a positive spontaneous curvature of the lipids promotes the ability of a surface-bound molecule to insert more deeply into the bilayer core, irrespective of the conformation, size, or shape of the peptide, protein, or folding intermediate. This article is part of a Special Issue entitled: Lipid-protein interactions.

Structure analysis and conformational transitions of the cell penetrating peptide transportan 10 in the membrane-bound state

Structure analysis of the cell-penetrating peptide transportan 10 (TP10) revealed an exemplary range of different conformations in the membrane-bound state. The bipartite peptide (derived N-terminally from galanin and C-terminally from mastoparan) was found to exhibit prominent characteristics of (i) amphiphilic α-helices, (ii) intrinsically disordered peptides, as well as (iii) β-pleated amyloid fibrils, and these conformational states become interconverted as a function of concentration. We used a complementary approach of solid-state (19)F-NMR and circular dichroism in oriented membrane samples to characterize the structural and dynamical behaviour of TP10 in its monomeric and aggregated forms. Nine different positions in the peptide were selectively substituted with either the L- or D-enantiomer of 3-(trifluoromethyl)-bicyclopent-[1.1.1]-1-ylglycine (CF3-Bpg) as a reporter group for (19)F-NMR. Using the L-epimeric analogs, a comprehensive three-dimensional structure analysis was carried out in lipid bilayers at low peptide concentration, where TP10 is monomeric. While the N-terminal region is flexible and intrinsically unstructured within the plane of the lipid bilayer, the C-terminal α-helix is embedded in the membrane with an oblique tilt angle of ∼ 55° and in accordance with its amphiphilic profile. Incorporation of the sterically obstructive D-CF3-Bpg reporter group into the helical region leads to a local unfolding of the membrane-bound peptide. At high concentration, these helix-destabilizing C-terminal substitutions promote aggregation into immobile β-sheets, which resemble amyloid fibrils. On the other hand, the obstructive D-CF3-Bpg substitutions can be accommodated in the flexible N-terminus of TP10 where they do not promote aggregation at high concentration. The cross-talk between the two regions of TP10 thus exerts a delicate balance on its conformational switch, as the presence of the α-helix counteracts the tendency of the unfolded N-terminus to self-assemble into β-pleated fibrils.

(19)F NMR screening of unrelated antimicrobial peptides shows that membrane interactions are largely governed by lipids

Many amphiphilic antimicrobial peptides permeabilize bacterial membranes via successive steps of binding, re-alignment and/or oligomerization. Here, we have systematically compared the lipid interactions of two structurally unrelated peptides: the cyclic β-pleated gramicidin S (GS), and the α-helical PGLa. (19)F NMR was used to screen their molecular alignment in various model membranes over a wide range of temperatures. Both peptides were found to respond to the phase state and composition of these different samples in a similar way. In phosphatidylcholines, both peptides first bind to the bilayer surface. Above a certain threshold concentration they can re-align and immerse more deeply into the hydrophobic core, which presumably involves oligomerization. Re-alignment is most favorable around the lipid chain melting temperature, and also promoted by decreasing bilayer thickness. The presence of anionic lipids has no influence in fluid membranes, but in the gel phase the alignment states are more complex. Unsaturated acyl chains and other lipids with intrinsic negative curvature prevent re-alignment, hence GS and PGLa do not insert into mixtures resembling bacterial membranes, nor into bacterial lipid extracts. Cholesterol, which is present in high concentrations in animal membranes, even leads to an expulsion of the peptides from the bilayer and prevents their binding altogether. However, a very low cholesterol content of 10% was found to promote binding and re-alignment of both peptides. Overall, these findings show that the ability of amphiphilic peptides to re-align and immerse into a membrane is determined by the physico-chemical properties of the lipids, such as spontaneous curvature. This idea is reinforced by the remarkably similar behavior observed here for two structurally unrelated molecules (with different conformation, size, shape, charge), which further suggests that their activity at the membrane level is largely governed by the properties of the constituent lipids, while the selectivity towards different cell types is additionally ruled by electrostatic attraction between peptide and cell surface. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

In situ NMR spectroscopy of supercapacitors: insight into the charge storage mechanism

Electrochemical capacitors, commonly known as supercapacitors, are important energy storage devices with high power capabilities and long cycle lives. Here we report the development and application of in situ nuclear magnetic resonance (NMR) methodologies to study changes at the electrode-electrolyte interface in working devices as they charge and discharge. For a supercapacitor comprising activated carbon electrodes and an organic electrolyte, NMR experiments carried out at different charge states allow quantification of the number of charge storing species and show that there are at least two distinct charge storage regimes. At cell voltages below 0.75 V, electrolyte anions are increasingly desorbed from the carbon micropores at the negative electrode, while at the positive electrode there is little change in the number of anions that are adsorbed as the voltage is increased. However, above a cell voltage of 0.75 V, dramatic increases in the amount of adsorbed anions in the positive electrode are observed while anions continue to be desorbed at the negative electrode. NMR experiments with simultaneous cyclic voltammetry show that supercapacitor charging causes marked changes to the local environments of charge storing species, with periodic changes of their chemical shift observed. NMR calculations on a model carbon fragment show that the addition and removal of electrons from a delocalized system should lead to considerable increases in the nucleus-independent chemical shift of nearby species, in agreement with our experimental observations.

Self-assembly of flexible β-strands into immobile amyloid-like β-sheets in membranes as revealed by solid-state 19F NMR

The cationic peptide [KIGAKI](3) was designed as an amphiphilic β-strand and serves as a model for β-sheet aggregation in membranes. Here, we have characterized its molecular conformation, membrane alignment, and dynamic behavior using solid-state (19)F NMR. A detailed structure analysis of selectively (19)F-labeled peptides was carried out in oriented DMPC bilayers. It showed a concentration-dependent transition from monomeric β-strands to oligomeric β-sheets. In both states, the rigid (19)F-labeled side chains project straight into the lipid bilayer but they experience very different mobilities. At low peptide-to-lipid ratios ≤1:400, monomeric [KIGAKI](3) swims around freely on the membrane surface and undergoes considerable motional averaging, with essentially uncoupled φ/ψ torsion angles. The flexibility of the peptide backbone in this 2D plane is reminiscent of intrinsically unstructured proteins in 3D. At high concentrations, [KIGAKI](3) self-assembles into immobilized β-sheets, which are untwisted and lie flat on the membrane surface as amyloid-like fibrils. This is the first time the transition of monomeric β-strands into oligomeric β-sheets has been characterized by solid-state NMR in lipid bilayers. It promises to be a valuable approach for studying membrane-induced amyloid formation of many other, clinically relevant peptide systems.

Solid state 19F NMR parameters of fluorine-labeled amino acids. Part II: aliphatic substituents

A representative set of amino acids with aliphatic 19F-labels has been characterized here, following up our previous compilation of NMR parameters for single 19F-substituents on aromatic side chains. Their isotropic chemical shifts, chemical shift tensor parameters, intra-molecular 19F dipole-dipole couplings and temperature-dependent T1 and T2 relaxation times were determined by solid state NMR on twelve polycrystalline amino acid samples, and the corresponding isotropic 19F chemical shifts and scalar couplings were obtained in solution. Of particular interest are amino acids carrying a trifluoromethyl-group, because not only the 19F chemical shift but also the intra-CF3 homonuclear dipolar coupling can be used for structural studies of 19F-labeled peptides and proteins. The CF3-groups are further compared with CH2F-, CD2F-, and CD3-groups, using both 19F and 2H NMR to describe their motional behavior and to examine the respective linebroadening effects of the protonated and deuterated neighbors. We have also characterized two unnatural amino acids in which a CF3-label is rigidly connected to the backbone by a phenyl or bicyclopentyl moiety, and which are particularly well suited for structure analysis of membrane-bound polypeptides. The 19F NMR parameters of the polycrystalline amino acids are compared with data from the correspondingly labeled side chains in synthetic peptides.

Solid-state 19F NMR spectroscopy reveals that Trp41 participates in the gating mechanism of the M2 proton channel of influenza A virus

The integral membrane protein M2 of influenza A virus assembles as a tetrameric bundle to form a proton-conducting channel that is activated by low pH. The side chain of His37 in the transmembrane alpha-helix is known to play an important role in the pH activation of the proton channel. It has also been suggested that Trp41, which is located in an adjacent turn of the helix, forms part of the gating mechanism. Here, a synthetic 25-residue peptide containing the M2 transmembrane domain was labeled with 6F-Trp41 and studied in lipid membranes by solid-state 19F NMR. We monitored the pH-dependent differences in the 19F dipolar couplings and motionally narrowed chemical shift anisotropies of this 6F-Trp41 residue, and we discuss the pH activation mechanism of the H+ channel. At pH 8.0, the structural parameters implicate an inactivated state, while at pH 5.3 the tryptophan conformation represents the activated state. With the aid of COSMOS force field simulations, we have obtained new side-chain torsion angles for Trp41 in the inactivated state (chi1 = -100 degrees +/- 10 degrees , chi2 = +110 degrees +/- 10 degrees ), and we predict a most probable activated state with chi1 = -50 degrees +/- 10 degrees and chi2 = +115 degrees +/- 10 degrees . We have also validated the torsion angles of His37 in the inactivated state as chi1 = -175 degrees +/- 10 degrees and chi2 = -170 degrees +/- 10 degrees .

Evaluating the amino acid CF3-bicyclopentylglycine as a new label for solid-state19F-NMR structure analysis of membrane-bound peptides

The conformation, alignment and dynamic behavior of membrane-bound peptides is readily accessible by solid-state (19)F-NMR spectroscopy, but it has been difficult to incorporate suitable (19)F-labelled amino acids into synthetic peptides. To avoid the drawbacks of previously used labels, we have rationally designed and synthesized a novel amino acid that suits all theoretical and practical requirements for peptide synthesis and subsequent (19)F-NMR structure analysis [Mikhailiuk et. al, Angew. Chem. 2006, 118, 5787-5789]. The enantiomerically pure L-form of 3-(trifluoromethyl)bicyclopent-[1.1.1]-1-ylglycine (CF(3)-Bpg) carries a CF(3) group that is rigidly attached to the peptide backbone and does not racemize during peptide synthesis. It could be demonstrated for several different peptides that their biological activity is usually not affected by a single label, nor the conformation, as monitored by circular dichroism. Here, we carry out a more detailed structure analysis to evaluate the potential and reliability of CF(3)-Bpg for solid-state NMR, using the well-known alpha-helical antimicrobial peptide PGLa as a test case. We have collected several orientational constraints from the anisotropic (19)F--(19)F dipolar couplings of CF(3)-Bpg in various positions of PGLa embedded in lipid bilayers. These resulting structural parameters are then compared with those previously determined from 4-CF(3)-phenylglycine and 3,3,3-d(3)-alanine labels on the same peptide. The analysis confirms that CF(3)-Bpg does not perturb the alpha-helical conformation of PGLa. Likewise, the helix alignment is shown to follow the established concentration-dependent pattern in realigning from a surface-bound S-state to an obliquely tilted T-state. Hence, the advantages of CF(3)-Bpg over all previously used (19)F-labeled side chains are evident, as they combine ease of chemical incorporation and peptide purification with high NMR sensitivity and absent background signals, allowing a straightforward analysis of the dipolar splittings with no need for chemical shift referencing without any ambiguity in the sign of the couplings.

Assigning solid-state NMR spectra of aligned proteins using isotropic chemical shifts

A method for assigning solid-state NMR spectra of membrane proteins aligned in phospholipid bicelles that makes use of isotropic chemical shift frequencies and assignments is demonstrated. The resonance assignments are based on comparisons of 15N chemical shift differences in spectra obtained from samples with their bilayer normals aligned perpendicular and parallel to the direction of the applied magnetic field.

Solid state NMR analysis of the dipolar couplings within and between distant CF3-groups in a membrane-bound peptide

Dipolar couplings contain information on internuclear distances as well as orientational constraints. To characterize the structure of the antimicrobial peptide gramicidin S when bound to model membranes, two rigid 4-CF3-phenylglycine labels were attached to the cyclic backbone such that they reflect the behavior of the entire peptide. By solid state 19F NMR we measured the homonuclear dipolar couplings of the two trifluoromethyl-groups in oriented membrane samples. Using the CPMG experiment, both the strong couplings within each CF3-group as well as the weak coupling between the two CF3-groups could be detected. An intra-CF3-group dipolar coupling of 86 Hz and a weak inter-group coupling of 20 Hz were obtained by lineshape simulation of the complex dipolar spectrum. It is thus possible to explore the large distance range provided by 19F-labels and to resolve weak dipolar couplings even in the presence of strong intra-CF3 couplings. We applied this approach to distinguish and assign two epimers of the labeled gramicidin S peptide on the basis of their distinct 19F dipolar coupling patterns.

Synergistic transmembrane alignment of the antimicrobial heterodimer PGLa/magainin

The antimicrobial activity of amphipathic alpha-helical peptides is usually attributed to the formation of pores in bacterial membranes, but direct structural information about such a membrane-bound state is sparse. Solid state (2)H-NMR has previously shown that the antimicrobial peptide PGLa undergoes a concentration-dependent realignment from a surface-bound S-state to a tilted T-state. The corresponding change in helix tilt angle from 98 to 125 degrees was interpreted as the formation of PGLa/magainin heterodimers residing on the bilayer surface. Under no conditions so far, has an upright membrane-inserted I-state been observed in which a transmembrane helix alignment would be expected. Here, we have demonstrated that PGLa is able to assume such an I-state in a 1:1 mixture with magainin 2 at a peptide-to-lipid ratio as low as 1:100 in dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol model membranes. This (2)H-NMR analysis is based on seven orientational constraints from Ala-3,3,3-d(3) substituted in a non-perturbing manner for four native Ala residues as well as two Ile and one Gly. The observed helix tilt of 158 degrees is rationalized by the formation of heterodimers. This structurally synergistic effect between the two related peptides from the skin of Xenopus laevis correlates very well with their known functional synergistic mode of action. To our knowledge, this example of PGLa is the first case where an alpha-helical antimicrobial peptide is directly shown to assume a transmembrane state that is compatible with the postulated toroidal wormhole pore structure.

Conditions affecting the re-alignment of the antimicrobial peptide PGLa in membranes as monitored by solid state 2H-NMR

The cationic antimicrobial peptide PGLa is electrostatically attracted to bacterial membranes, binds as an amphiphilic alpha-helix, and is thus able to permeabilize the lipid bilayer. Using solid state (2)H-NMR of non-perturbing Ala-d(3) labels on the peptide, we have characterized the helix alignment under a range of different conditions. Even at a very high peptide-to-lipid ratio (1:20) and in the presence of negatively charged lipids, there was no indication of a toroidal wormhole structure. Instead, PGLa re-aligns from a surface-bound S-state to an obliquely tilted T-state, which is presumably dimeric. An intermediate structure half-way between the S- and T-state was observed in fully hydrated multilamellar DMPC vesicles at 1:50, suggesting a fast exchange between the two states on the time scale of >50 kHz. We demonstrate that this equilibrium is shifted from the S- towards the T-state either upon (i) increasing the peptide concentration, (ii) adding negatively charged DMPG, or (iii) decreasing the level of hydration. The threshold concentration for re-alignment in DMPC is found to be between 1:200 and 1:100 in oriented samples at 96% humidity. In fully hydrated multilamellar DMPC vesicles, it shifts to an effective peptide-to-lipid ratio of 1:50 as some peptides are able to escape into the bulk water phase.

Solid-state NMR analysis of the PGLa peptide orientation in DMPC bilayers: structural fidelity of 2H-labels versus high sensitivity of 19F-NMR

The structure and alignment of the amphipathic alpha-helical antimicrobial peptide PGLa in a lipid membrane is determined with high accuracy by solid-state 2H-NMR. Orientational constraints are derived from a series of eight alanine-3,3,3-d3-labeled peptides, in which either a native alanine is nonperturbingly labeled (4x), or a glycine (2x) or isoleucine (2x) is selectively replaced. The concentration dependent realignment of the alpha-helix from the surface-bound "S-state" to a tilted "T-state" by 30 degrees is precisely calculated using the quadrupole splittings of the four nonperturbing labels as constraints. The remaining, potentially perturbing alanine-3,3,3-d3 labels show only minor deviations from the unperturbed peptide structure and help to single out the unique solution. Comparison with previous 19F-NMR constraints from 4-CF3-phenylglycine labels shows that the structure and orientation of the PGLa peptide is not much disturbed even by these bulky nonnatural side chains, which contain CF3 groups that offer a 20-fold better NMR sensitivity than CD3 groups.

Water in tendon: orientational analysis of the free induction decay

The orientation dependence of the free induction decay (FID) of 1H NMR water signal in ex vivo bovine digital flexor tendon at the native level of hydration is reported. Residual dipolar coupling due to the overall tissue anisotropy produces a 6:1 change in the signal intensity as an angle between the long axis of a specimen and the external magnetic field is changed from the "magic angle" of 54.7 degrees to 0 degrees. The strength of residual dipolar interactions between water protons was estimated by orientational analysis of the signal intensity to be equal to 780 Hz. Apparent signal maxima are observed at orientations 8-13 degrees away from 54.7 degrees due to an inhomogeneous contribution to the decay. A small fraction of total water in tendon is detectable at all orientations and exhibits a shift in the precession frequency. It is hypothesized that this water fraction resides in the interconnecting gaps at the ends of collagen molecules. The gaps have a disordered environment that allows for a zero time average of dipolar interactions. Measured frequency and phase shifts are interpreted as signatures of the bulk magnetic susceptibility effect due to geometry of the cavity formed by adjacent gaps at the ends of the collagen molecules. The multiexponentiality of the FID decay is hypothesized to be due to the exchange between orientationally restricted water structured along the length of the collagen molecule and disordered water in the cavity.

15N and 31P solid-state NMR study of transmembrane domain alignment of M2 protein of influenza A virus in hydrated cylindrical lipid bilayers confined to anodic aluminum oxide nanopores

This communication reports the first example of a high resolution solid-state 15N 2D PISEMA NMR spectrum of a transmembrane peptide aligned using hydrated cylindrical lipid bilayers formed inside nanoporous anodic aluminum oxide (AAO) substrates. The transmembrane domain SSDPLVVA(A-15N)SIIGILHLILWILDRL of M2 protein from influenza A virus was reconstituted in hydrated 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine bilayers that were macroscopically aligned by a conventional micro slide glass support or by the AAO nanoporous substrate. 15N and 31P NMR spectra demonstrate that both the phospholipids and the protein transmembrane domain are uniformly aligned in the nanopores. Importantly, nanoporous AAO substrates may offer several advantages for membrane protein alignment in solid-state NMR studies compared to conventional methods. Specifically, higher thermal conductivity of aluminum oxide is expected to suppress thermal gradients associated with inhomogeneous radio frequency heating. Another important advantage of the nanoporous AAO substrate is its excellent accessibility to the bilayer surface for exposure to solute molecules. Such high accessibility achieved through the substrate nanochannel network could facilitate a wide range of structure-function studies of membrane proteins by solid-state NMR.

Concentration-dependent realignment of the antimicrobial peptide PGLa in lipid membranes observed by solid-state 19F-NMR

The membrane-disruptive antimicrobial peptide PGLa is found to change its orientation in a dimyristoyl-phosphatidylcholine bilayer when its concentration is increased to biologically active levels. The alignment of the alpha-helix was determined by highly sensitive solid-state NMR measurements of (19)F dipolar couplings on CF(3)-labeled side chains, and supported by a nonperturbing (15)N label. At a low peptide/lipid ratio of 1:200 the amphiphilic peptide resides on the membrane surface in the so-called S-state, as expected. However, at high peptide concentration (>/=1:50 molar ratio) the helix axis changes its tilt angle from approximately 90 degrees to approximately 120 degrees , with the C-terminus pointing toward the bilayer interior. This tilted "T-state" represents a novel feature of antimicrobial peptides, which is distinct from a membrane-inserted I-state. At intermediate concentration, PGLa is in exchange between the S- and T-state in the timescale of the NMR experiment. In both states the peptide molecules undergo fast rotation around the membrane normal in liquid crystalline bilayers; hence, large peptide aggregates do not form. Very likely the obliquely tilted T-state represents an antiparallel dimer of PGLa that is formed in the membrane at increasing concentration.

Solid-state NMR investigation of the selective disruption of lipid membranes by protegrin-1

The interaction of a beta-hairpin antimicrobial peptide, protegrin-1 (PG-1), with various lipid membranes is investigated by (31)P, (2)H, and (13)C solid-state NMR. Mixed lipid bilayers containing anionic lipids and cholesterol are used to mimic the bacterial and mammalian cell membranes, respectively. (31)P and (2)H spectra of macroscopically oriented samples show that PG-1 induces the formation of an isotropic phase in anionic bilayers containing phosphatidylglycerol. Two-dimensional (31)P exchange experiments indicate that these isotropic lipids are significantly separate from the residual oriented lamellar bilayers, ruling out toroidal pores as the cause for the isotropic signal. (1)H spin diffusion experiments show that PG-1 is not exclusively bound to the isotropic phase but is also present in the residual oriented lamellar bilayers. This dynamic and morphological heterogeneity of the anionic membranes induced by PG-1 is supported by the fact that (13)C T(2) relaxation times measured under cross polarization and direct polarization conditions differ significantly. In contrast to the anionic membrane, the zwitterionic phosphatidylcholine (PC) membrane does not form an isotropic phase in the presence of PG-1 but shows significant orientational disorder. The addition of cholesterol to the PC bilayer significantly reduces this orientational disorder. The (13)C T(2) relaxation times of the PC lipids in the presence of both cholesterol and PG-1 suggest that the peptide may decrease the dynamic heterogeneity of the cholesterol-containing membrane. The observed selective interaction of PG-1 with different lipid membranes is consistent with its biological function and may be caused by its strong cationic and amphipathic structure.

Orientation of the antimicrobial peptide PGLa in lipid membranes determined from 19F-NMR dipolar couplings of 4-CF3-phenylglycine labels

A highly sensitive solid state (19)F-NMR strategy is described to determine the orientation and dynamics of membrane-associated peptides from specific fluorine labels. Several analogues of the antimicrobial peptide PGLa were synthesized with the non-natural amino acid 4-trifluoromethyl-phenylglycine (CF(3)-Phg) at different positions throughout the alpha-helical peptide chain. A simple 1-pulse (19)F experiment allows the simultaneous measurement of both the anisotropic chemical shift and the homonuclear dipolar coupling within the rotating CF(3)-group in a macroscopically oriented membrane sample. The value and sign of the dipolar splitting determines the tilt of the CF(3)-rotational axis, which is rigidly attached to the peptide backbone, with respect to the external magnetic field direction. Using four CF(3)-labeled peptide analogues (with L-CF(3)-Phg at Ile9, Ala10, Ile13, and Ala14) we confirmed that PGLa is aligned at the surface of lipid membranes with its helix axis perpendicular to the bilayer normal at a peptide:lipid ratio of 1:200. We also determined the azimuthal rotation angle of the helix, which agrees well with the orientation expected from its amphiphilic character. Peptide analogues with a D-CF(3)-Phg label resulting from racemization of the amino acid during synthesis were separately collected by HPLC. Their spectra provide additional information about the PGLa structure and orientation but allow only to discriminate qualitatively between multiple solutions. The structural and functional characterization of the individual CF(3)-labeled peptides by circular dichroism and antimicrobial assays showed only small effects for our four substitutions on the hydrophobic face of the helix, but a significant disturbance was observed in a fifth analogue where Ala8 on the hydrophilic face had been replaced. Even though the hydrophobic CF(3)-Phg side chain cannot be utilized in all positions, it allows highly sensitive NMR measurements over a wide range of experimental conditions and dynamic regimes of the peptide.

4-fluorophenylglycine as a label for 19F NMR structure analysis of membrane-associated peptides

The non-natural amino acid 4-fluorophenylglycine (4F-Phg) was incorporated into several representative membrane-associated peptides for dual purpose. The (19)F-substituted ring is directly attached to the peptide backbone, so it not only provides a well-defined label for highly sensitive (19)F NMR studies but, in addition, the D and L enantiomers of the stiff side chain may serve as reporter groups on the transient peptide conformation during the biological function. Besides peptide synthesis, which is accompanied by racemisation of 4F-Phg, we also describe separation of the epimers by HPLC and removal of trifluoroacetic acid. As a first example, 18 different analogues of the fusogenic peptide "B18" were prepared and tested for induction of vesicle fusion; the results confirmed that hydrophobic sites tolerated 4F-Phg labelling. Similar fusion activities within each pair of epimers suggest that the peptide is less structured in the fusogenic transition state than in the helical ground state. In a second example, five doubly labelled analogues of the antimicrobial peptide gramicidin S were compared by using bacterial growth inhibition assays. This cyclic beta-sheet peptide could accommodate both L and D substituents on its hydrophobic face. As a third example, we tested six analogues of the antimicrobial peptide PGLa. The presence of d-4F-Phg reduced the biological activity of the peptide by interfering with its amphiphilic alpha-helical fold. Finally, to illustrate the numerous uses of l-4F-Phg in (19)F NMR spectroscopy, we characterised the interaction of labelled PGLa with uncharged and negatively charged membranes. Observing the signal of the free peptide in an aqueous suspension of unilamellar vesicles, we found a linear saturation behaviour that was dominated by electrostatic attraction of the cationic PGLa. Once the peptide is bound to the membrane, however, solid-state (19)F NMR spectroscopy of macroscopically oriented samples revealed that the charge density has virtually no further influence on the structure, alignment or mobility of the peptide.

'Boomerang'-like insertion of a fusogenic peptide in a lipid membrane revealed by solid-state 19F NMR

Solid state (19)F NMR revealed the conformation and alignment of the fusogenic peptide sequence B18 from the sea urchin fertilization protein bindin embedded in flat phospholipid bilayers. Single (19)F labels were introduced into nine distinct positions along the wild-type sequence by substituting each hydrophobic amino acid, one by one, with L-4-fluorophenylglycine. Their anisotropic chemical shifts were measured in uniaxially oriented membrane samples and used as orientational constraints to model the peptide structure in the membrane-bound state. Previous (1)H NMR studies of B18 in 30% TFE and in detergent micelles had shown that the peptide structure consists of two alpha-helical segments that are connected by a flexible hinge. This helix-break-helix motif was confirmed here by the solid-state (19)F NMR data, while no other secondary structure (beta-sheet, 3(10)-helix) was compatible with the set of orientational constraints. For both alpha-helical segments we found that the helical conformation extends all the way to the respective N- and C-termini of the peptide. Analysis of the corresponding tilt and azimuthal rotation angles showed that the N-terminal helix of B18 is immersed obliquely into the bilayer (at a tilt angle tau approximately 54 degrees), whereas the C-terminus is peripherally aligned (tau approximately 91 degrees). The azimuthal orientation of the two segments is consistent with the amphiphilic distribution of side-chains. The observed 'boomerang'-like mode of insertion into the membrane may thus explain how peptide binding leads to lipid dehydration and acyl chain perturbation as a prerequisite for bilayer fusion to occur.

Susceptibility corrections in solid state NMR experiments with oriented membrane samples. Part II: theory

In solid state NMR analysis of oriented biomembranes the samples typically have the shape of a rectangular block, formed by stacking a number of glass slides coated with the membranes under investigation. Reference material may be provided internally in the volume of the block or as an external layer on its surface, as described in the accompanying paper [J. Magn. Reson. 164 (2003) 104-114]. The demagnetizing field resulting in such non-spheroidal samples is inhomogeneous. It shifts and broadens the NMR lines of both the sample and of the reference, as compared to the ideal of a spherical sample. The magnitude of these effects is typically of the order of a few ppm. To determine the necessary corrections, a general analysis is presented here for the demagnetizing field of a layered sample of rectangular block geometry, with the normal of the layers parallel to the main field or tilted about an axis of the block. The correction to the line position of the block sample is found to be approximately equal to that of the spheroid which can be inscribed into the block, and for which the correction is well known. For an external reference layer, placed on top of the block, the correction can be found by the same approximation, invoking a simple mirror concept. The layered structure of the block can be accounted for by using an average magnetic susceptibility. Sample and support materials contribute to that average according to their volume filling factors. If the sample material is anisotropic at the molecular level, as e.g. lipid bilayers are, the resulting anisotropy of the block is reduced by the filling factor of the sample material.