2H-NMR study and molecular dynamics simulation of the location, alignment, and mobility of pyrene in POPC bilayers

Thermodynamic and structural study of the main phospholipid components comprising the mitochondrial inner membrane

Cardiolipin (CL) is a phospholipid found in the energy-transducing membranes of bacteria and mitochondria and it is thought to be involved in relevant biological processes as apoptosis. In this work, the mixing properties of CL and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocoline (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) at the air-water interface, have been examined using the thermodynamic framework analysis of compression isotherms. Accordingly, the values of the Gibbs energy of mixing, the more stable monolayers assayed were: POPC:CL (0.6:0.4, mol:mol) and POPE:CL (0.8:0.2, mol:mol). The results reflect that attractive forces are the greatest contributors to the total interaction in these compositions. Supported planar bilayers (SPBs) with such compositions were examined using atomic force microscopy (AFM) at different temperatures. With the POPC:CL mixture, rounded and featureless SPBs were obtained at 4 degrees C and 24 degrees C. In contrast, the extension of the POPE:CL mixture revealed the existence of different lipid domains at 24 degrees C and 37 degrees C. Three lipid domains coexisted which can be distinguished by measuring the step height difference between the uncovered mica and the bilayer. While the low and intermediate domains were temperature dependent, the high domain was composition dependent. When cytochrome c (cyt c) was injected into the fluid cell, the protein showed a preferential adsorption onto the high domain of the POPC:CL. These results suggest that the high domain is mainly formed by CL.

Computer simulation of partitioning of ten pentapeptides Ace-WLXLL at the cyclohexane/water and phospholipid/water interfaces

BACKGROUND

Peptide-membrane interactions play a key role in the binding, partitioning and folding of membrane proteins, the activity of antimicrobial and fusion peptides, and a number of other processes. To gain a better understanding of the thermodynamics of such interactions, White and Wimley created an interfacial hydrophobicity scale based of the transfer free energy from water to octanol or lipid bilayers of a series of synthetic peptapeptides (Ace-WLXLL, with X being any of the twenty natural amino acids) (White and Wimley (1996) Nat. Struct. Biol. 3, 842-848). In this study, we performed molecular dynamics simulations of a representative set of ten of these peptides (X = D, K, R, N, A, T, S, I, F and W) in two membrane mimetic interfaces: water-cyclohexane (10 ns) and a fully solvated dioleoylphosphatidylcholine (DOPC) bilayer (50 ns) using both constant pressure and constant area ensembles. We focus on partitioning of the ten peptides at the cyclohexane/water and lipid/water interfaces.

RESULTS

The peptides rapidly equilibrate (< 2 ns) and partition at the cyclohexane/water interface. The X3 guest residue assumes average orientations that depend on the nature of the side chain. At the DOPC/water interface, dynamics is much slower and convergence is difficult to achieve on a 50 ns timescale. Nonetheless, all peptides partition to the lipid/water interface with distributions with widths of 1-2 nm. The peptides assume a broad range of side chain and backbone orientations and have only a small effect on the area of the unit cell. On average, hydrophobic guest residues partition deeper into the hydrophobic core than hydrophilic residues. In some cases the peptides penetrate sufficiently deep to somewhat affect the distribution of the C=C double bond in DOPC. The relative distribution of the X3 guest residue compared to W1 and L5 is similar in the water/cyclohexane and water/lipid simulations. Snapshots show mostly extended backbone conformations in both environments. There is little difference between simulations at a constant area of 0.66 nm2 and simulations at constant pressure that approximately yield the same average area of 0.66 nm2.

CONCLUSION

These peptides were designed to assume extended conformations, which is confirmed by the simulations. The distribution of the X3 side chain depends on its nature, and can be determined from molecular dynamics simulations. The time scale of peptide motion at a phospholipids-water interface is too long to directly calculate the experimentally measured hydrophobicity scale to test and improve the simulation parameters. This should be possible at the water/cyclohexane interface and likely will become feasible in the future for the phospholipids/water case.

Effect of Temperature, Cholesterol Content, and Antioxidant Structure on the Mobility of Vitamin E Constituents in Biomembrane Models Studied by Laterally Diffusion-Controlled Fluorescence Quenching

Kinetic parameters relevant for the antioxidant activity of the vitamin E constituents (alpha, beta, gamma, and delta homologues of tocopherols and tocotrienols) and of an amphiphilic vitamin C derivative, l-ascorbyl 6-palmitate, were determined. Fluorescence quenching experiments of 2,3-diazabicyclo[2.2.2]oct-2-ene in homogeneous acetonitrile-water mixtures afforded reactivity trends in terms of intermolecular quenching rate constants, while the quenching of Fluorazophore-L in liposomes provided the lateral diffusion coefficients relevant for understanding their biological activity in membranes. The reactivity in homogeneous solution was not influenced by the nature of the isoprenoid tail (tocopherol versus tocotrienol), but was dependent on the methylation pattern. The resulting order (alpha > beta = gamma > delta) was found to be in line with their reactivities toward peroxyl radicals as well as the phenolic O-H bond dissociation energies. The mutual lateral diffusion coefficient in POPC liposomes was the same, within error, for different tocopherols and tocotrienols (D(L) = (1.6 +/- 0.2) x 10(-7) cm(2) s(-1)). l-Ascorbyl 6-palmitate exhibited a reactivity similar to that of delta-tocopherol in homogeneous solution, but displayed a 1 order of magnitude lower fluorescence quenching efficiency in liposomes than the vitamin E constituents. Temperature effects on the laterally diffusion-controlled fluorescence quenching were large, with activation energies of 44 +/- 6 kJ mol(-1). The addition of cholesterol (0-30%) to POPC liposomes resulted only in slightly reduced diffusion coefficients. The combined results demonstrate that Fluorazophore-L can provide important physicochemical parameters for the understanding of antioxidant activity in biological environments.

Orientation and Dynamics of Benzyl Alcohol and Benzyl Alkyl Ethers Dissolved in Nematic Lyotropic Liquid Crystals. 2H NMR and Molecular Dynamics Simulations

Most drugs have to cross cell membranes to reach their final target. A better understanding of the distribution, interactions, and dynamics of biologically active molecules in model bilayers is of fundamental importance in understanding drug functioning and design. 2H NMR quadrupole splittings (delta nu(Q)) and longitudinal relaxation times (T1) from the aromatic ring of benzyl alcohol-d5 (C0), a commonly used anesthetic, and a series of linear alkyl benzyl-d5 ethers with chain lengths from 1 to 12 carbon atoms (C1-C12), were measured. The molecules were dissolved in a nematic discotic lyotropic liquid crystal solution made of tetradecyltrimethylammonium chloride (TTAC)/decanol (DeOH)/NaCl/H2O. Values of delta nu(Q) and T1 from 1,1-dideuteriodecanol (15% enriched) and DHO (H2O with 0.2% D2O) were also measured. Delta nu(Q) of DeOH and DHO remained constant throughout the series. The value of delta nu(Q) of the para position of the ring (delta nu(p)) in C1 is 30% smaller than the delta nu(p) of C0. This is attributed to the existence of an H-bond between the alcohol hydroxyl proton and the solvent, which influences the average orientation of the ring. The relaxation data show that T1o,m is always longer than T1p and both decrease with the increase in alkyl chain length. Molecular dynamics simulations of the experimentally studied systems were performed. The aggregate was represented as a bilayer. The distribution, average orientation, and order parameters of the aromatic ring of the guest molecules in the bilayer were examined. Rotational correlation functions of all the C-D bonds and the OH bond from H2O were evaluated, allowing an estimate of the correlation times and T1. According to these results all spins relax in extreme narrowing conditions, except DeOH. Experimental and calculated T1 values differ at most by a factor of 3. However, the order of magnitude and the observed trends are well reproduced by the calculations. The aromatic ring of C0 possesses a unique average orientation in the bilayer. For the ether series, the orientation is modified and the C2 symmetry axis of the aromatic ring is exchanging between two orientations averaging the quadrupole splittings from the ortho and meta positions. The simulation supports the existence of an H-bond between C0 and the solvent not found in the ethers, which should be responsible for the observed differences.

Arginine magic with new counterions up the sleeve

The elusive questions how arginine-rich sequences allow peptides and proteins to penetrate cells or to form voltage-gated ion channels are controversial topics of current scientific concern. The possible contributions of exchangeable counterions to these puzzling processes remain underexplored. The objective of this report is to clarify scope and limitations of certain counteranions to modulate cellular uptake and anion carrier activity of oligo/polyarginines. The key finding is that the efficiency of counteranion activators depends significantly on many parameters such as activator-membrane and activator-carrier interactions. This finding is important because it suggests that counteranions can be used to modulate not only efficiency but also selectivity. Specifically, activator efficiencies are found to increase with increasing aromatic surface of the activator, decreasing size of the transported anion, increasing carrier concentration as well as increasing membrane fluidity. Efficiency sequences depend on membrane composition with coronene > pyrene >>fullerene > calix[4]arene carboxylates in fluid and crystalline DPPC contrasting to fullerene > calix[4]arene approximately coronene > pyrene carboxylates in EYPC with or without cholesterol or ergosterol. In HeLa cells, the efficiency of planar activators (pyrene) exceeds that of spherical activators (fullerenes, calixarenes). Polyarginine complexes with pyrene and coronene activators exhibit exceptional excimer emission. Decreasing excimer emission with increasing ionic strength reveals dominant hydrophobic interactions with the most efficient carboxylate activators. Dominance of ion pairing with the inefficient high-affinity sulfate activators is corroborated by the reversed dependence on ionic strength. These findings on activator-carrier and activator-membrane interactions are discussed as supportive of arene-templated guanidinium-carboxylate pairing and interface-directed translocation as possible origins of the superb performance of higher arene carboxylates as activators.