Istvan Kocsis

A class of carbonic anhydrase I – selective activators

Abstract A series of ureido and bis-ureido derivatives were prepared by reacting histamine with alkyl/aryl-isocyanates or di-isocyanates. The obtained derivatives were assayed as activators of the enzyme carbonic anhydrase (CA, EC 4.2.1.1), due to the fact that histamine itself has this biological activity. Although inhibition of CAs has pharmacological applications in the field of antiglaucoma, anticonvulsant, anticancer, and anti-infective agents, activation of these enzymes is not yet properly exploited pharmacologically for cognitive enhancement or Alzheimer’s disease treatment, conditions in which a diminished CA activity was reported. The ureido/bis-ureido histamine derivatives investigated here showed activating effects only against the cytosolic human (h) isoform hCA I, having no effect on the widespread, physiologically dominant isoform hCA II. This is the first report in which CA I-selective activators were identified. Such compounds may constitute interesting tools for better understanding the physiological/pharmacological effects connected to activation of this widespread CA isoform, whose physiological function is not fully understood.

Squalyl Crown Ether Self-Assembled Conjugates: An Example of Highly Selective Artificial K(+) Channels.

The natural KcsA K+ channel, one of the best-characterized biological pore structures, conducts K+ cations at high rates while excluding Na+ cations. The KcsA K+ channel is of primordial inspiration for the design of artificial channels. Important progress in improving conduction activity and K+ /Na+ selectivity has been achieved with artificial ion-channel systems. However, simple artificial systems exhibiting K+ /Na+ selectivity and mimicking the biofunctions of the KcsA K+ channel are unknown. Herein, an artificial ion channel formed by H-bonded stacks of squalyl crown ethers, in which K+ conduction is highly preferred to Na+ conduction, is reported. The K+ -channel behavior is interpreted as arising from discreet stacks of dimers resulting in the formation of oligomeric channels, in which transport of cations occurs through macrocycles mixed with dimeric carriers undergoing dynamic exchange within the bilayer membrane. The present highly K+ -selective macrocyclic channel can be regarded as a biomimetic alternative to the KcsA channel.

Columnar Self-Assemblies of Triarylamines as Scaffolds for Artificial Biomimetic Channels for Ion and for Water Transport

Triarylamine molecules appended with crown-ethers or carboxylic moieties form self-assembled supramolecular channels within lipid bilayers. Fluorescence assays and voltage clamp studies reveal that the self-assemblies incorporating the crown ethers work as single channels for the selective transport of K+ or Rb+. The X-ray crystallographic structures confirm the mutual columnar self-assembly of triarylamines and crown-ethers. The dimensional fit of K+ cations within the 18-crown-6 leads to a partial dehydration and to the formation of alternating K+ cation-water wires within the channel. This original type of organization may be regarded as a biomimetic alternative of columnar K+-water wires observed for the natural KcsA channel. Supramolecular columnar arrangement was also shown for the triarylamine-carboxylic acid conjugate. In this latter case, stopped-flow light scattering analysis reveals the transport of water across lipid bilayer membranes with a relative water permeability as high as 17 μm s-1.

Oriented chiral water wires in artificial transmembrane channels

Chiral dipolar oriented water wires are observed inside artificial water channels embedded in supported bilayer membranes. Aquaporins (AQPs) feature highly selective water transport through cell membranes, where the dipolar orientation of structured water wires spanning the AQP pore is of considerable importance for the selective translocation of water over ions. We recently discovered that water permeability through artificial water channels formed by stacked imidazole I-quartet superstructures increases when the channel water molecules are highly organized. Correlating water structure with molecular transport is essential for understanding the underlying mechanisms of (fast) water translocation and channel selectivity. Chirality adds another factor enabling unique dipolar oriented water structures. We show that water molecules exhibit a dipolar oriented wire structure within chiral I-quartet water channels both in the solid state and embedded in supported lipid bilayer membranes (SLBs). X-ray single-crystal structures show that crystallographic water wires exhibit dipolar orientation, which is unique for chiral I-quartets. The integration of I-quartets into SLBs was monitored with a quartz crystal microbalance with dissipation, quantizing the amount of channel water molecules. Nonlinear sum-frequency generation vibrational spectroscopy demonstrates the first experimental observation of dipolar oriented water structures within artificial water channels inserted in bilayer membranes. Confirmation of the ordered confined water is obtained via molecular simulations, which provide quantitative measures of hydrogen bond strength, connectivity, and the stability of their dipolar alignment in a membrane environment. Together, uncovering the interplay between the dipolar aligned water structure and water transport through the self-assembled I-quartets is critical to understanding the behavior of natural membrane channels and will accelerate the systematic discovery for developing artificial water channels for water desalting.

Artificial water channels—deconvolution of natural Aquaporins through synthetic design

Artificial Water Channels (AWCs) have been developed during the last decade with the hope to construct artificial analogues of Aquaporin (AQP) proteins. Their osmotic water permeability are in the range of natural transporters, making them suitable candidates that can potentially transport water at lower energy and operating cost. Compared to AQPs, AWCs would have several potential advantages, such as improved stability, simple and scalable fabrication and higher functional density when confined in 2D membrane arrays. The first knowledge gap between AWCs and AQPs is in the mimicry of the complete set of functionality, in terms of obtaining systems capable of simultaneous water permeation and salt rejection, while not forfeiting the advantage of simplicity. Despite incipient developments, major problems still remain unsolved, such as their up-scaling preparation procedures from laboratory studies to square meters needed for large industrial membrane applications. However, the flow of structural information from molecular level through nanoscale dimensions, towards highly ordered ultradense macroscopic arrays of AWCs is conceptually possible. Successfully transitioning from synthetic molecules to functional channels and materials could lead to a new generation of membranes for water purification. Moving AWCs into products in the commercial arena is now the main objective of research in this new-born field.

Imidazole derivatives as artificial water channel building-blocks: structural design influence on water permeability

A series of mono- and di-ureidoethylimidazole derivatives were tested as self-assembled supramolecular channels for water transport. Several structural behaviours were compared in order to gain insight on the structure-water transport activity relationship. The three main features that are critical to tailor artificial water channel building blocks are: (i) the selectivity of the hydrophilic head, (ii) the H-bonding scaffold favouring the directional self-assembly, and (iii) the lipophilic tail for the compatibility with the hydrophobic environment of the lipid bilayer. The designed compounds bear one or two imidazole heads, one or two urea moieties, and different lipophilic tails. Water transport experiments were performed in order to assess the critical parameters. For that, large unilamellar vesicles (LUV) were fabricated using a mixture of phosphatidylcholine, phosphatidylserine and cholesterol. The bilayer of the LUV constituted a membrane between an intra and an extra vesicular medium. The artificial water channel candidates are put in the presence of this membrane to improve its water permeability. The permeation of elements other than water is ideally maintained to a minimum in order to achieve selective water filtration. In this study the effect of additional urea moieties, as well as its absence, was evidenced as detrimental for the permeation and the influence of the tail was also investigated.

Controlled supramolecular structure of guanosine monophosphate in the interlayer space of layered double hydroxide

Guanosine monophosphates (GMPs) were intercalated into the interlayer space of layered double hydroxides (LDHs) and the molecular arrangement of GMP was controlled in LDHs. The intercalation conditions such as GMP/LDH molar ratio and reaction temperature were systematically adjusted. When the GMP/LDH molar ratio was 1:2, which corresponds to the charge balance between positive LDH sheets and GMP anions, GMP molecules were well-intercalated to LDH. At high temperature (100 and 80 °C), a single GMP molecule existed separately in the LDH interlayer. On the other hand, at lower temperature (20, 40 and 60 °C), GMPs tended to form ribbon-type supramolecular assemblies. Differential scanning calorimetry showed that the ribbon-type GMP assembly had an intermolecular interaction energy of ≈101 kJ/mol, which corresponds to a double hydrogen bond between guanosine molecules. Once stabilized, the interlayer GMP orientations, single molecular and ribbon phase, were successfully converted to the other phase by adjusting the external environment by stoichiometry or temperature control.