α-Helix recognition by rigid-rod β-barrel ion channels with internal arginine-histidine dyads in polarized bilayer membranes

Synthetic multifunctional pores that open and close in response to chemical stimulation

Studies on synthetic multifunctional pores with external and internal active sites for ligand gating and noncompetitive blockage are presented, with emphasis on the contribution of external ligands to the characteristics of pore. A comparison between different synthetic multifunctional pores reveals that the location of functional groups in rigid-rod beta-barrel pores is precisely reflected in the function: molecular recognition at the outer barrel surface results in pore opening, while molecular recognition at the inner barrel surface results in pore closing. Negligible nonspecific leakage, disappearance of pH gating, inhibition of intervesicular pore transfer, and maybe also the flickering of currents of single open pores characterize external ligands as adhesive cushions that liberate the pore from lateral pressure exerted by the surrounding membrane. Refined molecular models show good agreement with pore design and experimental facts with regard to function.

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.

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.

On selectivity and sensitivity of synthetic multifunctional pores as enzyme sensors: discrimination between ATP and ADP and comparison with biological pores

This report delineates scope and limitation of the selectivity of synthetic multifunctional pores as enzyme sensors using glycolytic enzymes as example (G. Das, P. Talukdar, and S. Matile, Science, 2002, Vol. 298, pp. 1600-1602). Unproblematic detectability of hexokinase and phosphofructokinase demonstrates that the selectivity of synthetic multifunctional pore (SMPs) sensors suffices to sense ATP in mixed analytes containing ADP, whereas detection of the isomerization of glucose 6-phosphate into fructose 6-phosphate by phosphoglucose isomerase is not possible with confidence. The sensitivity of SMP sensors is sufficient for end-point detection of one picomole poly-L-glutamate hydrolyzed by papain in unoptimized assay format; the sensitivity of melittin as representative biological pore of similar charge and aggregation number to detect the same reaction is more than four orders of magnitude inferior.

Outer surface modification of synthetic multifunctional pores

The characteristics of pores formed by p-octiphenyl beta-barrels with LWV triads at the outer surface are reported in comparison with the conventional rigid-rod beta-barrels with all-L outer surface. Maintained multifunctionality of tetrameric pores with external LWV triads (inversion of ion selectivity, molecular recognition and transformation) is implicative for intact barrel interior. Increased pore activity supports dominance of high bilayer affinity for W over low affinity for V. Transmembrane p-octiphenyl orientation (from fluorescence depth quenching) supports barrel-stave (rather than toroidal) pores and dominance of transmembrane preference of rigid rods over interfacial preference of W. Destabilization of beta-barrel pores in membranes (from short single-channel lifetimes) and in the media (from 4th-power dependence on monomer concentration) by LWV triads supports dominance of low beta-propensity for W over high beta-propensity for V. The relation between the stability of supramolecular (pre)pores and dependence of activity on monomer concentration is discussed in a more general context.

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.

Enzyme screening with synthetic multifunctional pores: focus on biopolymers

This report demonstrates that a single set of identical synthetic multifunctional pores can detect the activity of many different enzymes. Enzymes catalyzing either synthesis or degradation of DNA (exonuclease III or polymerase I), RNA (RNase A), polysaccharides (heparinase I, hyaluronidase, and galactosyltransferase), and proteins (papain, ficin, elastase, subtilisin, and pronase) are selected to exemplify this key characteristic of synthetic multifunctional pore sensors. Because anionic, cationic, and neutral substrates can gain access to the interior of complementarily functionalized pores, such pores can be the basis for very user-friendly screening of a broad range of enzymes.