Supramolecular Pharmacy 4. Artificial anion transporters Ondřej Jurček 1 NS N H N S N H N S N H H H H CF3 F3C CF3 NS N H N S N H N S N H H H H F3C CF3 CF3 CF3 CF3 CF3 Cyclohexane tris-thioureasDecalin bis-squaramides 12 13 14 17 18 15 16 13 + Me4N+Cl(X-ray crystal structure) Selectivity of cation complexation 2 Molecular recognition is based on strong and selective binding: • size complementarity between the cation and host cavity • electrostatic complementarity between the cation and the host binding site • degree of preorganisation and donor group orientation • chelate ring size • cation binding kinetics • solvent (polarity, H-bonding, coordination), and cation and host • free energies of solvation • counteranion and its interactions with solvent, cation (and host) • enthalpic and entropic contributions to the cation-host interaction • based on the complex stability and cation exchange kinetics the hosts may be classified as cation receptors (slow kinetics, large stability constant) and cation carriers (fast kinetics, lower stability) Introduction to anion complexation 3 • Katapinands (1968) – macrobicyclic hosts capable of binding anions (du Pont, C. H. Park and H. E. Simmons) • Later F. P. Schmidtchen, J.-M. Lehn, etc. • 1980s boom in development of anion receptors Anion importance : • Chlorides - major component of the oceans and it is the dominant anion in biological extra-cellular fluids • Nitrate (from N2 oxidation) and sulfate (from burning organosulfur compound containing fossil fuels) are key components in acid rain • Hydrogen carbonate and carboxylates are also key biological anions, while carbonates, phosphates and silicates are the major anions in biomineralised materials such as the exoskeletons of radiolarian, and in bone Anion importance 4 • Phosphates and nitrates in fertilizers are beneficial to agriculture but also major pollution hazards since such bioavailable sources of phosphorus and nitrogen are often biolimiting, i.e., rate of microorganism growth is limited by the amount of these elements that are present • Highly soluble and mobile 99TcO4 - and ClO4 - , or arsenate: technetium-99 is a β-emitter with a half-life of 213,000 years and is a product of the nuclear fuel cycle, formed in ca. 6 % fission yield and can leach from nuclear waste storage facilities. Perchlorate was used extensively as an explosive and rocket propellant. • Anions are crucial in biological systems – perhaps this is why imbalances in their concentration have such serious effects. 70-75 % of enzyme substrates and cofactors are anions, very often phosphate residues (as in ATP and ADP) or as inorganic phosphate (H2PO4 -). Chloride anion is the major extracellular anion, and it is responsible for the maintenance of ionic strength (cystic fibrosis). Cystic fibrosis 5 • inherited disorder that causes severe damage to the lungs, digestive system and other organs in the body • no functional copies (alleles) of the gene cystic fibrosis transmembrane conductance regulator (CFTR) • the product of this gene (the CFTR protein) is a chloride ion channel important in creating sweat, digestive juices, and mucus. https://www.mayoclinic.org/diseases-conditions/cystic-fibrosis/symptoms-causes/syc-20353700 • it regulates flow of Cl- and H2O • developing supramolecular chloride transporters to treat the conditions Challenges in anion transport 6 • Development in the area is relatively slower than cation transport • Primarily based on preorganisation, complementarity, solvation and size and shape effects • BUT anions are relatively large and therefore require receptors of considerably greater size than cations, e.g., one of the smallest anions, F- is comparable in ionic radius to K+ • Even simple inorganic anions occur in a range of shapes and geometries, e.g. spherical (halides), linear (SCN-, N3 -), planar (NO3 -, PtCl4 2-), tetrahedral (PO4 3-, SO4 2-), octahedral (PF6 -, Fe(CN)6 3-) as well as more complicated examples as in the case of biologically important oligophosphate anions. Challenges in anion transport 7 • In comparison to cations of similar size, anions have high free energies of solvation and hence anion hosts must compete more effectively with the surrounding medium • Many anions exist only in a relatively narrow pH window, which can cause problems especially in the case of receptors based upon polyammonium salts where the host may not be fully protonated in the pH region in which the anion is present in the desired form. • Anions are usually coordinatively saturated and therefore bind only via weak forces such as hydrogen bonding and van der Waals interactions, although they can form dative bonds. Hofmeister series 8 • Easing the situation is division of anions based on their hydrophobicity • Ability of anions to „salt out“ proteins, which is also correlated with hydration of anions • Any cationic molecule that acts as a host for anions is in competition with the counter-cation (particularly in non-polar solvents where ion pairing can be very significant) • Citrate increases solvent surface tension and decrease the solubility of nonpolar molecules (salt out); in effect, they strengthen the hydrophobic interaction Hofmeister series 9 Anion receptors/transporters in nature 10 • At least 14 mitochondrial anion transport systems have been identified including systems involving flux of ADP, ATP, citrate, phosphates, glutamate, fumarate, maleate, oxaloacetate and halides. • The structure of a chloride channel protein was reported in 2002 and, along with earlier work on potassium channels, led to the award of a share of the 2003 Nobel Prize in Chemistry to Roderick MacKinnon • In biochemical anion binding, the enzyme or protein host is always part of a functioning biological system, e.g. in biocatalysis or anion transport. • Natural anion binding systems must not only have high affinity for their target anion and low affinity for other species present in the cell or extracellular fluid (thermodynamic selectivity) but must also complex and release their substrates rapidly and at the appropriate time (kinetic selectivity). Anion binding proteins 11 • Phosphate binding protein (PBP) binds HPO4 2- which is held in place by a total of 12 hydrogen bonding interactions with N/O…O distances between 2.62 and 2.92 Å. Seven are from NH groups from the protein main chain or arginine side chain residues, four are from OH groups (two serine and two threonine), and one involves an oxygen atom from a carboxylate anion (Asp56), which acts as a hydrogen bond acceptor (crystal structure). • Arginine fork motif • Guanidinium, the protonated form of guanidine, is an excellent anion binding site because it remains protonated over an extremely wide pH range • Three amide NH pocket = nest Concepts in anion host design 12 • Preorganization - significant contribution to anion binding comes from dispersion and electrostatic interactions, which are non-directional, and so in some sense the whole host is a binding site • Challenge – coordinatively saturated nature of anions or lack of specific HB functionality, especially for halides, which behave approximately like spherical charges and have a highly versatile coordination geometry without specific binding sites Katapinands – low binding constants, low preorganization Important issues to anion binding 13 • The negative charge suggests that both neutral, and especially positively charged, hosts will bind anions. • Ion pair is held together through electrostatic interactions, which are nondirectional and so all anions will be attracted to hosts on an electrostatic basis – competition between original counter cation and the host. • The vast majority of anions are Lewis bases, thus hosts containing Lewis acidic atoms, such as organo boron, mercury or tin compounds, or metal cations in general, might make the basis of a suitable host by formation of coordinate bonds (HB also possible N-H···Cl, e.g. katapinands). • Anions are highly polarisable and so van der Waals interactions will be significant (importance of contact surface area). • Anions generally have high solvation energies and so, to an even greater degree than cation binding, the solvent effects the binding constant (generally, 102-103 M-1 in water) (increasing H2O