Outer surface modification of synthetic multifunctional pores

Best of Two Worlds? How MD Simulations of Amphiphilic Helical Peptides in Membranes Can Complement Data from Oriented Solid-State NMR

The membrane alignment of helical amphiphilic peptides in oriented phospholipid bilayers can be obtained as ensemble and time averages from solid state ²H NMR by fitting the quadrupolar splittings to ideal α-helices. At the same time, molecular dynamics (MD) simulations can provide atomistic insight into peptide-membrane systems. Here, we evaluate the potential of MD simulations to complement the experimental NMR data that is available on three exemplary systems: the natural antimicrobial peptide PGLa and the two designer-made peptides MSI-103 and KIA14, whose sequences were derived from PGLa. Each peptide was simulated for 1 μs in a DMPC lipid bilayer. We calculated from the MD simulations the local angles which define the side chain geometry with respect to the peptide helix. The peptide orientation was then calculated (i) directly from the simulation, (ii) from back-calculated MD-derived NMR splittings, and (iii) from experimental ²H NMR splittings. Our findings are that (1) the membrane orientation and secondary structure of the peptides found in the NMR analysis are generally well reproduced by the simulations; (2) the geometry of the side chains with respect to the helix backbone can deviate significantly from the ideal structure depending on the specific residue, but on average all side chains have the same orientation; and (3) for all of our peptides, the azimuthal rotation angle found from the MD-derived splittings is about 15° smaller than the experimental value.

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.

Self-assembly of crown ether-based amphiphiles for constructing synthetic ion channels: the relationship between structure and transport activity

Efficient ion transport was achieved from crown ether amphiphile-based ion channels by simply regulating the molecular structures.

Translocation of hydrophilic molecules across lipid bilayers by salt-bridged oligocholates

Macrocyclic oligocholates were found in a previous work (Cho, H.; Widanapathirana, L.; Zhao, Y. J. Am. Chem. Soc.2011, 133, 141-147) to stack on top of one another in lipid membranes to form nanopores. Pore formation was driven by a strong tendency of the water molecules in the interior of the amphiphilic macrocycles to aggregate in a nonpolar environment. In this work, cholate oligomers terminated with guanidinium and carboxylate groups were found to cause efflux of hydrophilic molecules such as glucose, maltotriose, and carboxyfluorescein (CF) from POPC/POPG liposomes. The cholate trimer outperformed other oligomers in the transport. Lipid-mixing assays and dynamic light scattering ruled out fusion as the cause of leakage. The strong dependence on chain length argues against random intermolecular aggregates as the active transporters. The efflux of glucose triggered by these compounds increased significantly when the bilayers contained 30 mol% cholesterol. Hill analysis suggested that the active transporter consisted of four molecules. The oligocholates were proposed to fold into "noncovalent macrocycles" by the guanidinium-carboxylate salt bridge and stack on top of one another to form similar transmembrane pores as their covalent counterparts.

Synthetic pores with sticky π-clamps

In this report, we describe design, synthesis, evaluation and molecular dynamics simulations of synthetic multifunctional pores with pi-acidic naphthalenediimide clamps. Experimental evidence is provided for the formation of unstable but inert, heterogeneous and acid-insensitive dynamic tetrameric pores that are sensitive to base and ionic strength. Blockage experiments reveal that the introduction of aromatic electron donor-acceptor interactions provides access to the selective recognition of pi-basic intercalators within the pore. This breakthrough is important for the application of synthetic pores as multianalyte sensors.

Synthetic ion channels and pores (2004-2005)

This critical review covers synthetic ion channels and pores created between January 2004 and December 2005 comprehensively. The discussion of a rich collection of structural motifs may particularly appeal to organic, biological, supramolecular and polymer chemists. Functions addressed include ion selectivity and molecular recognition, as well as responsiveness to light, heat, voltage and membrane composition. The practical applications involved concern certain topics in medicinal chemistry (antibiotics, drug delivery), catalysis and sensing. An introduction to principles and methods is provided for the non-specialist; some new sources of inspiration from fields beyond chemistry are highlighted.

Contributions of Lipid Bilayer Hosts to Structure and Activity of Multifunctional Supramolecular Guests

The question of whether or not the surrounding lipid bilayer host contributes to structure and activity of included functional guests is a general topic of current scientific concern. We report that synthetic multifunctional pores are of use to address this elusive question, because the detection of their catalytic activity is membrane independent. According to their salt-rate profiles, unstable multifunctional supramolecules with permanent internal charges show highest membrane sensitivity, and the dependence of membrane sensitivity on the acidity of internal cations exceeds that on supramolecule stability. These results can, with all appropriate caution, be interpreted as indications for the existence of long-range EMP-ICR interactions (EMP: external membrane pressure, ICR: internal charge repulsion) between membrane hosts and functional guests that can, for instance, prevent the 'explosion' and promote the 'implosion' of over- and undercharged transmembrane barrel-stave supramolecules, respectively.

The depth of molecular recognition: voltage-sensitive blockage of synthetic multifunctional pores with refined architecture

Voltage-sensitive blockage by ADP, ATP and phytate (IP6) demonstrates that active-site contraction toward the middle of newly synthesized rigid-rod beta-barrels provides a general strategy to rationally create and modulate the voltage sensitivity (and to increase the efficiency) of molecular recognition by synthetic multifunctional pores.

Synthetic Ion Channel Activity Documented by Electrophysiological Methods in Living Cells

Hydraphiles are synthetic ion channels that use crown ethers as entry portals and that span phospholipid bilayer membranes. Proton and sodium cation transport by these compounds has been demonstrated in liposomes and planar bilayers. In the present work, whole cell patch clamp experiments show that hydraphiles integrate into the membranes of human embryonic kidney (HEK 293) cells and significantly increase membrane conductance. The altered membrane permeability is reversible, and the cells under study remain vital during the experiment. Control compounds that are too short (C(8)-benzyl channel) to span the bilayer or are inactive owing to a deficiency in the central relay do not induce similar conductance increases. Control experiments confirm that the inactive channel analogues do not show nonspecific effects such as activation of native channels. These studies show that the combination of structural features that have been designed into the hydraphiles afford true, albeit simple, channel function in live cells.

Thermodynamic and Kinetic Stability of Synthetic Multifunctional Rigid-Rod β-Barrel Pores: Evidence for Supramolecular Catalysis

The lessons learned from p-octiphenyl beta-barrel pores are applied to the rational design of synthetic multifunctional pore 1 that is unstable but inert, two characteristics proposed to be ideal for practical applications. Nonlinear dependence on monomer concentration provided direct evidence that pore 1 is tetrameric (n = 4.0), unstable, and "invisible," i.e., incompatible with structural studies by conventional methods. The long lifetime of high-conductance single pores in planar bilayers demonstrated that rigid-rod beta-barrel 1 is inert and large (d approximately 12 A). Multifunctionality of rigid-rod beta-barrel 1 was confirmed by adaptable blockage of pore host 1 with representative guests in planar (8-hydroxy-1,3,6-pyrenetrisulfonate, KD = 190 microM, n = 4.9) and spherical bilayers (poly-L-glutamate, KD < or = 105 nM, n = 1.0; adenosine triphosphate, KD = 240 microM, n = 2.0) and saturation kinetics for the esterolysis of a representative substrate (8-acetoxy-1,3,6-pyrenetrisulfonate, KM = 0.6 microM). The thermodynamic instability of rigid-rod beta-barrel 1 provided unprecedented access to experimental evidence for supramolecular catalysis (n = 3.7). Comparison of the obtained kcat = 0.03 min(-1) with the kcat approximately 0.18 min(-1) for stable analogues gave a global KD approximately 39 microM3 for supramolecular catalyst 1 with a monomer/barrel ratio approximately 20 under experimental conditions. The demonstrated "invisibility" of supramolecular multifunctionality identified molecular modeling as an attractive method to secure otherwise elusive insights into structure. The first molecular mechanics modeling (MacroModel, MMFF94) of multifunctional rigid-rod beta-barrel pore hosts 1 with internal 1,3,6-pyrenetrisulfonate guests is reported.

Programmed assembly of rigid-rod ?-barrel pores: Thermal inversion of chirality

The programmed assembly of p-octiphenyl rods carrying six complementary tripeptide strands was studied in the presence of bilayer membranes using circular dichroism (CD) spectroscopy. Thermal CD experiments demonstrated programmed assembly of anionic and cationic rods into supramolecules at low temperature that irreversibly transform into more stable supramolecules at intermediate and high temperature. Higher activation energies for programmed assembly with rods containing multiple guanidinium rather than ammonium cations was consistent with stabilization by guanidinium-anion complexes. Qualitative thermal inversion of supramolecular chirality during programmed assembly was detected continuously. Inversion of supramolecular chirality occurred with decreasing solvent polarity as well.