Interaction between quaternary ammonium ions and dipeptides: positive anion allosteric effect

A molecular container providing supramolecular protection against acetylcholine hydrolysis

Alzheimer's disease (AD) is characterized by cognitive decline, often attributed to the deficiency of acetylcholine, which can undergo hydrolysis by acetylcholinesterase (AChE) within the biological milieu. Here, we report a supramolecular strategy that takes advantage of confinement effects to inhibit such a hydrolysis process, shedding some light on AD therapy. A water-soluble and bowl-shaped molecule, hexacarboxylated tribenzotriquinacene (TBTQ-C6), was employed to shield acetylcholine (G1) from enzymatic degradation through host-guest binding interactions. Our study revealed highly efficient host-guest interactions with a binding ratio of 1 : 3, resulting in a significant reduction in acetylcholine hydrolysis from 91.1% to 7.4% in the presence of AChE under otherwise identical conditions. Furthermore, TBTQ-C6 showed potential for attenuating the degradation of butyrylcholine (G2) by butyrylcholinesterase (BChE). The broader implications of this study extend to the potential use of molecular containers in various biochemical and pharmacological applications, opening new avenues for research in the field of neurodegenerative diseases.

The minimal pharmacophore for silent agonism of the α7 nicotinic acetylcholine receptor

The minimum pharmacophore for activation of the human α7 nicotinic acetylcholine receptor (nAChR) is the tetramethylammonium cation. Previous work demonstrated that larger quaternary ammonium compounds, such as diethyldimethylammonium or 1-methyl quinuclidine, were α7-selective partial agonists, but additional increase in the size of the ammonium cation or the quinuclidine N-alkyl group by a single carbon to an N-ethyl group led to a loss of efficacy for ion channel activation. We report that although such compounds are ineffective at inducing the normal channel open state, they nonetheless regulate the induction of specific conformational states normally considered downstream of channel activation. We synthesized several panels of quaternary ammonium nAChR ligands that systematically varied the size of the substituents bonded to the central positively charged nitrogen atom. In these molecular series, we found a correlation between the molecular volume of the ligand and/or charge density, and the receptor's preferred distribution among conformational states including the closed state, the active state, a nonconducting state that could be converted to an activated state by a positive allosteric modulator (PAM), and a PAM-insensitive nonconducting state. We hypothesize that the changes of molecular volume of an agonist's cationic core subtly impact interactions at the subunit interface constituting the orthosteric binding site in such a way as to regulate the probability of conversions among the conformational states. We define a new minimal pharmacophore for the class of compounds we have termed "silent agonists," which are able to induce allosteric modulator-dependent activation but not the normal activated state.

The cation-π interaction at protein-protein interaction interfaces: developing and learning from synthetic mimics of proteins that bind methylated lysines

First discovered over 60 years ago, post-translational methylation was considered an irreversible modification until the initial discoveries of demethylase enzymes in 2004. Now researchers understand that this process serves as a dynamic and complex control mechanism that is misregulated in numerous diseases. Lysine methylation is most often found on histone proteins and can effect gene regulation, epigenetic inheritance, and cancer. Because of this connection to disease, many enzymes responsible for methylation are considered targets for new cancer therapies. Although our understanding of the biology of post-translational methylation has advanced at an astonishing rate within the last 5 years, chemical approaches for studying and disrupting these pathways are only now gaining momentum. In general, enzymes methylate lysine and arginine residues with very high specificity for both the location and methylation state. Each methylated target serves as the focused hot spot for an inducible protein-protein interaction (PPI). Conceptually, lysine or arginine methylation is a subtle modification that leads to no change in charge and small changes in size, but it significantly alters the hydration energies and hydrogen bonding potential of these side chains. Nature has evolved a special motif for recognizing the methylation states of lysine, called the "aromatic cage", a collection of aromatic protein residues, often accompanied by one or more neighboring anionic residues. The combination of favorable cation-π, electrostatic, and van der Waals interactions, as well as size matching, gives these proteins a high degree of specificity for the methylation state. This Account summarizes the development of various supramolecular host system scaffolds developed to recognize and bind to ammonium cations, such as trimethyllysine, on the basis of their methylation state. Early systems bound to their targets in pure, buffered water but failed to achieve biochemically relevant affinities and selectivities. Surprisingly, the use of the simple and very well-known p-sulfonatocalix[4]arene provides protein-like affinities and selectivities for trimethyllysine in water. New analogs, created by synthetic modification of the same scaffold, allow for further tuning of affinities and selectivities for trimethyllysine. Our studies of each family of hosts paint a consistent picture: cation-π interactions and electrostatics are important, and solvation effects are complex. Rigidity is especially important for host-guest systems that function in pure water. Despite their simplicity, synthetic systems that take these lessons into account can achieve affinities that rival or surpass those of their naturally evolved counterparts. The stage is now set for the next act: the use of such compounds as tunable and adaptable tools for modern chemical biology.

Modular incorporation of 1-benzyltryptophan into dipeptide hosts that bind acetylcholine in pure water

Proteins that recognize and bind quaternary ammonium ions depend on “aromatic-cage” structural motifs that use multiple aromatic residues to engage the side chain’s ammonium cation. We introduce herein the use of 1-benzyltryptophan (Trp(Bn)) residues as synthetic, unnatural partial analogues of natural aromatic cages. We demonstrate the modular incorporation of these building blocks into simple dipeptide hosts and show that they are capable of binding quaternary ammonium ions in buffered water and in chloroform.

Synthesis of a new π-deficient phenylalanine derivative from a common 1,4-diketone intermediate and study of the influence of aromatic density on prolyl amide isomer population

Enantiopure (2S)-N-(Boc)-3-(6-methylpyridazinyl)alanine (14) has been synthesized to serve as a phenylalanine analog lacking significant pi-donor capability. Two approaches were developed to furnish the target compound from L-aspartic acid as chiral educt in respectively six and nine steps and 13% and 12% yields. In both routes, a key homoallylic ketone intermediate was synthesized by a copper-catalyzed cascade addition of vinylmagnesium bromide to a carboxylic ester. Dipeptide models Ac-Xaa-Pro-NHMe (21a-c) were prepared and the relative populations of prolyl cis- and trans-amide isomers were measured in chloroform, dimethylsulfoxide, and water by proton NMR spectroscopy in order to assess the significance of the electron density of the neighboring aromatic residue on the prolyl amide geometry.

Interaction between Biphenols and Anions: Selective Receptor for Dihydrogenphosphate

Biphenol was shown to bind dihydrogenphosphate (H2PO4-) selectively over various other anions (MeCO2-, Cl-, Br-, I-, NO3-, HSO4-). The highly selectivity of biphenol toward dihydrogenphosphate is explained in terms of the basicity and shape of the guest anion.