Physical organic chemistry on the brain

Probing Noncovalent Interactions in [3,3]Metaparacyclophanes

Arene-arene interactions are fundamentally important in molecular recognition. To precisely probe arene-arene interactions in cyclophanes, we designed and synthesized (2,6-phenol)paracyclophanes and (2,6-aniline)paracyclophanes that possess two aromatic rings in close proximity. Fine-tuning the aromatic character of one aromatic ring by fluorine substituents enables investigations on the intramolecular interactions between the electron-rich phenol and aniline with tetra-H- and tetra-F-substituted benzene. pK_a measurements revealed that the tetra-F-template increases the acidity of the phenol (ΔpK_a = 0.55). X-ray crystallography and computational analyses demonstrated that all [3,3]metaparacyclophanes adopt cofacial parallel conformations, implying the presence of π-π stacking interactions. Advanced quantum chemical analyses furthermore revealed that both electrostatic interactions and orbital interactions provide the key contribution to the structure and stability of [3,3]metaparacyclophanes.

Mechanism of biomolecular recognition of trimethyllysine by the fluorinated aromatic cage of KDM5A PHD3 finger

The understanding of biomolecular recognition of posttranslationally modified histone proteins is centrally important to the histone code hypothesis. Despite extensive binding and structural studies on the readout of histones, the molecular language by which posttranslational modifications on histone proteins are read remains poorly understood. Here we report physical-organic chemistry studies on the recognition of the positively charged trimethyllysine by the electron-rich aromatic cage containing PHD3 finger of KDM5A. The aromatic character of two tryptophan residues that solely constitute the aromatic cage of KDM5A was fine-tuned by the incorporation of fluorine substituents. Our thermodynamic analyses reveal that the wild-type and fluorinated KDM5A PHD3 fingers associate equally well with trimethyllysine. This work demonstrates that the biomolecular recognition of trimethyllysine by fluorinated aromatic cages is associated with weaker cation-π interactions that are compensated by the energetically more favourable trimethyllysine-mediated release of high-energy water molecules that occupy the aromatic cage.

Synthesis and characterization of an unnatural boron and nitrogen-containing tryptophan analogue and its incorporation into proteins

A boron and nitrogen containing unnatural analogue of tryptophan is synthesized and incorporated into proteins.

A boron and nitrogen containing unnatural analogue of tryptophan is synthesized through the functionalization of BN-indole. The spectroscopic properties of BN-tryptophan are reported with respect to the natural tryptophan, and the incorporation of BN-tryptophan into proteins expressed in E. coli using selective pressure incorporation is described. This work shows that a cellular system can recognize the unnatural, BN-containing tryptophan. More importantly, it presents the first example of an azaborine containing amino acid being incorporated into proteins.

Hyaluronic acid-serum albumin conjugate-based nanoparticles for targeted cancer therapy

Multiple carcinomas including breast, ovarian, colon, lung and stomach cancer, overexpress the hyaluronic acid (HA) receptor, CD44. Overexpression of CD44 contributes to key cancer processes including tumor invasion, metastasis, recurrence, and chemoresistance. Herein, we devised novel targeted nanoparticles (NPs) for delivery of anticancer chemotherapeutics, comprised of self-assembling Maillard reaction-based conjugates of HA and bovine serum albumin (BSA). HA served as the hydrophilic block, and as the ligand for actively targeting cancer cells overexpressing CD44. We demonstrate that Maillard reaction-based covalent conjugates of BSA-HA self-assemble into NPs, which efficiently entrap hydrophobic cytotoxic drugs including paclitaxel and imidazoacridinones. Furthermore, BSA-HA conjugates stabilized paclitaxel and prevented its aggregation and crystallization. The diameter of the NPs was < 15 nm, thus enabling CD44 receptor-mediated endocytosis. These NPs were selectively internalized by ovarian cancer cells overexpressing CD44, but not by cognate cells lacking this HA receptor. Moreover, free HA abolished the endocytosis of drug-loaded BSA-HA conjugates. Consistently, drug-loaded NPs were markedly more cytotoxic to cancer cells overexpressing CD44 than to cells lacking CD44, due to selective internalization, which could be competitively inhibited by excess free HA. Finally, a CD44-targeted antibody which blocks receptor activity, abolished internalization of drug-loaded NPs. In conclusion, a novel cytotoxic drug-loaded nanomedicine platform has been developed, which is based on natural biocompatible biopolymers, capabale of targeting cancer cells with functional surface expression of CD44.

The Cation-π Interaction Enables a Halo-Tag Fluorogenic Probe for Fast No-Wash Live Cell Imaging and Gel-Free Protein Quantification

The design of fluorogenic probes for a Halo tag is highly desirable but challenging. Previous work achieved this goal by controlling the chemical switch of spirolactones upon the covalent conjugation between the Halo tag and probes or by incorporating a “channel dye” into the substrate binding tunnel of the Halo tag. In this work, we have developed a novel class of Halo-tag fluorogenic probes that are derived from solvatochromic fluorophores. The optimal probe, harboring a benzothiadiazole scaffold, exhibits a 1000-fold fluorescence enhancement upon reaction with the Halo tag. Structural, computational, and biochemical studies reveal that the benzene ring of a tryptophan residue engages in a cation−π interaction with the dimethylamino electron-donating group of the benzothiadiazole fluorophore in its excited state. We further demonstrate using noncanonical fluorinated tryptophan that the cation−π interaction directly contributes to the fluorogenicity of the benzothiadiazole fluorophore. Mechanistically, this interaction could contribute to the fluorogenicity by promoting the excited-state charge separation and inhibiting the twisting motion of the dimethylamino group, both leading to an enhanced fluorogenicity. Finally, we demonstrate the utility of the probe in no-wash direct imaging of Halo-tagged proteins in live cells. In addition, the fluorogenic nature of the probe enables a gel-free quantification of fusion proteins expressed in mammalian cells, an application that was not possible with previously nonfluorogenic Halo-tag probes. The unique mechanism revealed by this work suggests that incorporation of an excited-state cation−π interaction could be a feasible strategy for enhancing the optical performance of fluorophores and fluorogenic sensors.

Experimental Binding Energies in Supramolecular Complexes

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

UV-photoelectron spectroscopy of BN indoles: experimental and computational electronic structure analysis

We present a comprehensive electronic structure analysis of two BN isosteres of indole using a combined UV-photoelectron spectroscopy (UV-PES)/computational chemistry approach. Gas-phase He I photoelectron spectra of external BN indole I and fused BN indole II have been recorded, assessed by density functional theory calculations, and compared with natural indole. The first ionization energies of these indoles are natural indole (7.9 eV), external BN indole I (7.9 eV), and fused BN indole II (8.05 eV). The computationally determined molecular dipole moments are in the order: natural indole (2.177 D) > fused BN indole II (1.512 D) > external BN indole I (0.543 D). The λ_max in the UV–vis absorption spectra are in the order: fused BN indole II (292 nm) > external BN indole I (282 nm) > natural indole (270 nm). The observed relative electrophilic aromatic substitution reactivity of the investigated indoles with dimethyliminium chloride as the electrophile is as follows: fused BN indole II > natural indole > external BN indole I, and this trend correlates with the π-orbital coefficient at the 3-position. Nucleus-independent chemical shifts calculations show that the introduction of boron into an aromatic 6π-electron system leads to a reduction in aromaticity, presumably due to a stronger bond localization. Trends and conclusions from BN isosteres of simple monocyclic aromatic systems such as benzene and toluene are not necessarily translated to the bicyclic indole core. Thus, electronic structure consequences resulting from BN/CC isosterism will need to be evaluated individually from system to system.

Identification and pharmacological characterization of 3,6-diazabicyclo[3.1.1]heptane-3-carboxamides as novel ligands for the α4β2 and α6/α3β2β3 nicotinic acetylcholine receptors (nAChRs)

We have synthesized a novel series of compounds, 3,6-diazabicyclo[3.1.1]heptane-3-carboxamides, targeting both the α4β2 and α6/α3β2β3 nAChRs. Members of the obtained chemical library are partial or full agonists at both the high sensitivity (α4)2(β2)3 and α6/α3β2β3 nAChRs. 3-(Cyclopropylcarbonyl)-3,6-diazabicyclo[3.1.1]heptane (TC-8831 or compound 7 herein) demonstrated a safe in vitro pharmacological profile and the potential for reducing or preventing L-dopa-induced dyskinesias (LID) in several in vivo animal models [1-4]. In vivo metabolism studies in rat and in vitro metabolism studies in liver microsomes from human, rat, dog and monkey showed TC-8831 to be relatively stable. In vivo pharmacokinetic analysis in the rat confirmed brain penetration, with an average brain:plasma ratio of approximately 0.3 across time points from 0.5 to 4 h. Docking into homology models predicted alternative binding modes for TC-8831 and highlighted the importance of the cationic center, hydrogen-bond acceptor, and hydrophobic aliphatic features in promoting binding affinity to both nAChRs. Pharmacophore elucidation confirmed the importance of these key interactions. QSAR modeling suggested that binding affinity is primarily driven by ligand shape, relative positive charge distribution onto the molecular surface, and molecular flexibility. Of the two subtypes, ligand binding to α6β2β3 appears to be more sensitive to bulkiness and flexibility.

Selective ligand behaviors provide new insights into agonist activation of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are a diverse set of ion channels that are essential to everyday brain function. Contemporary research studies selective activation of individual subtypes of receptors, with the hope of increasing our understanding of behavioral responses and neurodegenerative diseases. Here, we aim to expand current binding models to help explain the specificity seen among three activators of α4β2 receptors: sazetidine-A, cytisine, and NS9283. Through mutational analysis, we can interchange the activation profiles of the stoichiometry-selective compounds sazetidine-A and cytisine. In addition, mutations render NS9283--currently identified as a positive allosteric modulator--into an agonist. These results lead to two conclusions: (1) occupation at each primary face of an α subunit is needed to activate the channel and (2) the complementary face of the adjacent subunit dictates the binding ability of the agonist.

Use of anion-aromatic interactions to position the general base in the ketosteroid isomerase active site

Although the cation-pi pair, formed between a side chain or substrate cation and the negative electrostatic potential of a pi system on the face of an aromatic ring, has been widely discussed and has been shown to be important in protein structure and protein-ligand interactions, there has been little discussion of the potential structural and functional importance in proteins of the related anion-aromatic pair (i.e., interaction of a negatively charged group with the positive electrostatic potential on the ring edge of an aromatic group). We posited, based on prior structural information, that anion-aromatic interactions between the anionic Asp general base and Phe54 and Phe116 might be used instead of a hydrogen-bond network to position the general base in the active site of ketosteroid isomerase from Comamonas testosteroni as there are no neighboring hydrogen-bonding groups. We have tested the role of the Phe residues using site-directed mutagenesis, double-mutant cycles, and high-resolution X-ray crystallography. These results indicate a catalytic role of these Phe residues. Extensive analysis of the Protein Data Bank provides strong support for a catalytic role of these and other Phe residues in providing anion-aromatic interactions that position anionic general bases within enzyme active sites. Our results further reveal a potential selective advantage of Phe in certain situations, relative to more traditional hydrogen-bonding groups, because it can simultaneously aid in the binding of hydrophobic substrates and positioning of a neighboring general base.

Exploring and exploiting polar-π interactions with fluorinated aromatic amino acids

Fluorination has become an increasingly attractive strategy in protein engineering for both basic research and biomedical applications. Thus researchers would like to understand the consequences of fluorination to the structure, stability, and function of target proteins. Although a substantial amount of work has focused on understanding the properties of fluorinated aliphatic amino acids, much less is known about fluorinated aromatic residues. In addition, polar-π interactions, often referred to as aromatic interactions, may play a significant role in protein folding and protein-protein interactions. Fluorination of aromatic residues presents an ideal strategy for probing polar-π interactions in proteins. This Account summarizes the recent studies of the incorporation of fluorinated aromatic amino acids into proteins. Herein we discuss the effects of fluorinating aromatic residues and rationalize them in the context of polar-π interactions. The results strongly support the proposal that polar-π interactions are energetically significant to protein folding and function. For example, an edge-face interaction of a pair of phenylalanines contributes as much as -1 kcal/mol to protein stability, while cation-π interactions can be much stronger. Furthermore, this new knowledge provides guidelines for protein engineering with fluorination. Importantly, incorporating perfluorinated aromatic residues into proteins enables novel mechanisms of molecular recognition that do not exist in native proteins, such as arene-perfluoroarene stacking. Such novel mechanisms can be used for programming protein folding specificity and engineering peptide-based materials.

The impact of introducing a histidine into an apolar cavity site on docking and ligand recognition

Simplified model binding sites allow one to isolate entangled terms in molecular energy functions. Here, we investigate the effects on ligand recognition of the introduction of a histidine into a hydrophobic cavity in lysozyme. We docked 656040 molecules and tested 26 highly and nine poorly ranked. Twenty-one highly ranked molecules bound and five were false positives, while three poorly ranked molecules were false negatives. In the 16 X-ray complexes now known, the docking predictions overlaid well with the crystallographic results. Although ligand enrichment was high, the false negatives, the false positives, and the inability to rank order illuminated weaknesses in our scoring, particularly overweighed apolar and underweighted polar terms. Adjusting these led to new problems, reflecting the entangled nature of docking scoring functions. Changes in ligand affinity relative to other lysozyme cavities speak to the subtleties of molecular recognition even in these simple sites and to their relevance for testing different models of recognition.

Emerging approaches to probing ion channel structure and function

Ion channels, as membrane proteins, are the sensors of the cell. They act as the first line of communication with the world beyond the plasma membrane and transduce changes in the external and internal environments into unique electrical signals to shape the responses of excitable cells. Because of their importance in cellular communication, ion channels have been intensively studied at the structural and functional levels. Here, we summarize the diverse approaches, including molecular and cellular, chemical, optical, biophysical, and computational, used to probe the structural and functional rearrangements that occur during channel activation (or sensitization), inactivation (or desensitization), and various forms of modulation. The emerging insights into the structure and function of ion channels by multidisciplinary approaches allow the development of new pharmacotherapies as well as new tools useful in controlling cellular activity.

Ondansetron and granisetron binding orientation in the 5-HT(3) receptor determined by unnatural amino acid mutagenesis

The serotonin type 3 receptor (5-HT(3)R) is a ligand-gated ion channel found in the central and peripheral nervous systems. The 5-HT(3)R is a therapeutic target, and the clinically available drugs ondansetron and granisetron inhibit receptor activity. Their inhibitory action is through competitive binding to the native ligand binding site, although the binding orientation of the drugs at the receptor has been a matter of debate. Here we heterologously express mouse 5-HT(3)A receptors in Xenopus oocytes and use unnatural amino acid mutagenesis to establish a cation-π interaction for both ondansetron and granisetron to tryptophan 183 in the ligand binding pocket. This cation-π interaction establishes a binding orientation for both ondansetron and granisetron within the binding pocket.

Variations in binding among several agonists at two stoichiometries of the neuronal, α4β2 nicotinic receptor

Drug-receptor binding interactions of four agonists, ACh, nicotine, and the smoking cessation compounds varenicline (Chantix) and cytisine (Tabex), have been evaluated at both the 2:3 and 3:2 stoichiometries of the α4β2 nicotinic acetylcholine receptor (nAChR). Previous studies have established that unnatural amino acid mutagenesis can probe three key binding interactions at the nAChR: a cation-π interaction, and two hydrogen-bonding interactions to the protein backbone of the receptor. We find that all drugs make a cation-π interaction to TrpB of the receptor. All drugs except ACh, which lacks an N(+)H group, make a hydrogen bond to a backbone carbonyl, and ACh and nicotine behave similarly in acting as a hydrogen-bond acceptor. However, varenicline is not a hydrogen-bond acceptor to the backbone NH that interacts strongly with the other three compounds considered. In addition, we see interesting variations in hydrogen bonding interactions with cytisine that provide a rationalization for the stoichiometry selectivity seen with this compound.

The use of hammett constants to understand the non-covalent binding of aromatics

Non-covalent interactions of aromatics are important in a wide range of chemical and biological applications. The past two decades have seen numerous reports of arene-arene binding being understood in terms Hammett substituent constants, and similar analyses have recently been extended to cation-arene and anion-arene binding. It is not immediately clear why electrostatic Hammett parameters should work so well in predicting the binding for all three interactions, given that different intermolecular forces dominate each interaction. This review explores such anomalies, and summarizes how Hammett substituent constants have been employed to understand the non-covalent binding in arene-arene, cation-arene and anion-arene interactions.

Using mutant cycle analysis to elucidate long-range functional coupling in allosteric receptors

Functional coupling of residues that are far apart in space is the quintessential property of allosteric receptors. Data from functional studies of allosteric receptors, such as whole-cell dose-response relations, can be used to determine if mutation to a receptor significantly impacts agonist potency. However, the classification of perturbations as primarily impacting binding or allosteric function is more challenging, often requiring detailed kinetic studies. This protocol describes a simple strategy, derived from mutant cycle analysis, for elucidating long-range functional coupling in allosteric receptors (ELFCAR). Introduction of a gain-of-function reporter mutation, followed by a mutant cycle analysis of the readily measured macroscopic EC(50) values can provide insight into the role of many physically distant targets. This new method should find broad application in determining the functional roles of residues in allosteric receptors.

Docking studies of benzylidene anabaseine interactions with α7 nicotinic acetylcholine receptor (nAChR) and acetylcholine binding proteins (AChBPs): application to the design of related α7 selective ligands

AChBPs isolated from Lymnaea stagnalis (Ls), Aplysia californica (Ac) and Bulinus truncatus (Bt) have been extensively used as structural prototypes to understand the molecular mechanisms that underlie ligand-interactions with nAChRs [1]. Here, we describe docking studies on interactions of benzylidene anabaseine analogs with AChBPs and α7 nAChR. Results reveal that docking of these compounds using Glide software accurately reproduces experimentally-observed binding modes of DMXBA and of its active metabolite, in the binding pocket of Ac. In addition to the well-known nicotinic pharmacophore (positive charge, hydrogen-bond acceptor, and hydrophobic aromatic groups), a hydrogen-bond donor feature contributes to binding of these compounds to Ac, Bt, and the α7 nAChR. This is consistent with benzylidene anabaseine analogs with OH and NH(2) functional groups showing the highest binding affinity of these congeners, and the position of the ligand shown in previous X-ray crystallographic studies of ligand-Ac complexes. In the predicted ligand-Ls complex, by contrast, the ligand OH group acts as hydrogen-bond acceptor. We have applied our structural findings to optimizing the design of novel spirodiazepine and spiroimidazoline quinuclidine series. Binding and functional studies revealed that these hydrogen-bond donor containing compounds exhibit improved affinity and selectivity for the α7 nAChR subtype and demonstrate partial agonism. The gain in affinity is also due to conformational restriction, tighter hydrophobic enclosures, and stronger cation-π interactions. The use of AChBPs structure as a surrogate to predict binding affinity to α7 nAChR has also been investigated. On the whole, we found that molecular docking into Ls binding site generally scores better than when a α7 homology model, Bt or Ac crystal structure is used.

Substituent effects on non-covalent interactions with aromatic rings: insights from computational chemistry

Non-covalent interactions with aromatic rings pervade modern chemical research. The strength and orientation of these interactions can be tuned and controlled through substituent effects. Computational studies of model complexes have provided a detailed understanding of the origin and nature of these substituent effects, and pinpointed flaws in entrenched models of these interactions in the literature. Here, we provide a brief review of efforts over the last decade to unravel the origin of substituent effects in π-stacking, XH/π, and ion/π interactions through detailed computational studies. We highlight recent progress that has been made, while also uncovering areas where future studies are warranted.

Boron in disguise: the parent "fused" BN indole

"Fused" BN indoles are an emerging class of boron-containing indole mimics, featuring geometric structure and electophilic aromatic substitution reactivity similar to those of indoles but exhibiting distinct electronic structure, leading to unique optoelectronic properties. Herein we report the synthesis of the parent N-H BN indole and provide a head-to-head comparison of the structural features, pK(a) values, and optoelectronic properties of this hybrid organic/inorganic indole with the classic natural indole.

Inherently chiral phosphonatocavitands as artificial chemo- and enantio-selective receptors of natural ammoniums

Inherently chiral phosphonatocavitands with various bridging moieties at their wide rim were synthesized. Optical resolution by chiral HPLC was performed with cavitand 8 to afford enantiopure compounds (+)-8 and (-)-8. The molecular structures of hosts 8 and 12 were determined by X-ray diffraction. The host properties were investigated by (1)H and (31)P NMR spectroscopy. The phosphonatocavitands form inclusion complexes with chiral ammonium neurotransmitters, some presenting enantioselectivity towards the right or left-handed host enantiomers.

Substituent effects in cation/pi interactions and electrostatic potentials above the centers of substituted benzenes are due primarily to through-space effects of the substituents

Substituent effects in cation/pi interactions have been examined using the M05-2X DFT functional and CCSD(T) paired with triple-zeta-quality basis sets. In contrast to popular, intuitive models, trends in substituent effects are explained primarily in terms of direct through-space interactions with the substituents. While there is some scatter in the data, which is attributed to pi polarization, the trend in substituent effects in cation/pi interactions is captured by an additive model in which the substituent is isolated from the aryl ring. Similarly, changes in the electrostatic potential at a point above the center of a substituted benzene arise largely from through-space effects of the substituents; pi polarization is not the dominant underlying cause.

Relative substituent position on the strength of pi-pi stacking interactions

It was observed that the relative position of the arene substituents have a profound influence on the strength of pi-pi stacking in the 9-benzyl substituted triptycene system. A new series of model compounds (3a-i) capable of revealing quantitatively pi-pi stacking interactions was studied. This series of compounds (3a-i) has an ortho substituted methyl group in one of the two interacting arenes and the syn/anti ratios were determined and compared to a series previously studied compounds (4a-i) that have a para methyl group on the corresponding arene. A greater than 50% increase in the strength of pi-pi stacking interactions was observed with the methyl group in the ortho position comparing to that in the para position. No difference in pi-pi stacking interactions was observed when the other aromatic ring was a pentafluorobenzoate group.

Wrestling with native chemical ligation

An improved method for the semisynthesis of a potassium channel involving native chemical ligation allows the introduction of short sequences containing non-canonical amino acids at any position within the polypeptide chain. The work enhances the technology available for a range of fundamental investigations of membrane proteins and for applications of membrane channels and pores in biotechnology.

Site-dependent excited-state dynamics of a fluorescent probe bound to avidin and streptavidin

The excited-state dynamics of biotin-spacer-Lucifer-Yellow (LY) constructs bound to avidin (Avi) and streptavidin (Sav) was investigated using femtosecond spectroscopy. Two different locations in the proteins, identified by molecular dynamics simulations of Sav, namely the entrance of the binding pocket and the protein surface, were probed by varying the length of the spacer. A reduction of the excited-state lifetime, stronger in Sav than in Avi, was observed with the long spacer construct. Transient absorption measurements show that this effect originates from an electron transfer quenching of LY, most probably by a nearby tryptophan residue. The local environment of the LY chromophore could be probed by measuring the time-dependent polarisation anisotropy and Stokes shift of the fluorescence. Substantial differences in both dynamics were observed. The fluorescence anisotropy decays analysed by using the wobbling-in-a-cone model reveal a much more constrained environment of the chromophore with the short spacer. Moreover, the dynamic Stokes shift is multiphasic in all cases, with a approximately 1 ps component that can be ascribed to diffusive motion of bulk-like water molecules, and with slower components with time constants varying not only with the spacer, but with the protein as well. These slow components, which depend strongly on the local environment of the probe, are ascribed to the motion of the hydration layer coupled to the conformational dynamics of the protein.

Probing the role of the cation-pi interaction in the binding sites of GPCRs using unnatural amino acids

We describe a general application of the nonsense suppression methodology for unnatural amino acid incorporation to probe drug-receptor interactions in functional G protein-coupled receptors (GPCRs), evaluating the binding sites of both the M2 muscarinic acetylcholine receptor and the D2 dopamine receptor. Receptors were expressed in Xenopus oocytes, and activation of a G protein-coupled, inward-rectifying K(+) channel (GIRK) provided, after optimization of conditions, a quantitative readout of receptor function. A number of aromatic amino acids thought to be near the agonist-binding site were evaluated. Incorporation of a series of fluorinated tryptophan derivatives at W6.48 of the D2 receptor establishes a cation-pi interaction between the agonist dopamine and W6.48, suggesting a reorientation of W6.48 on agonist binding, consistent with proposed "rotamer switch" models. Interestingly, no comparable cation-pi interaction was found at the aligning residue in the M2 receptor.

Nicotine binding to brain receptors requires a strong cation-pi interaction

Nicotine addiction begins with high-affinity binding of nicotine to acetylcholine (ACh) receptors in the brain. The end result is over 4,000,000 smoking-related deaths annually worldwide and the largest source of preventable mortality in developed countries. Stress reduction, pleasure, improved cognition, and other CNS effects are strongly associated with smoking. But, if nicotine activated ACh receptors found in muscle as potently as it does brain receptors, smoking would cause intolerable and perhaps fatal muscle contractions. Despite extensive pharmacological, functional, and structural studies of ACh receptors, the basis for the differential action of nicotine on brain vs. muscle ACh receptors has not been determined. Here we show that at the α4β2 brain receptors thought to underlie nicotine addiction, the high affinity of nicotine is the result of a strong cation-п interaction to a specific aromatic amino acid of the receptor, TrpB. In contrast, the low affinity of nicotine at the muscle-type receptor is largely due to the fact that this key interaction is absent, even though the immediate binding site residues, including the key TrpB, are identical in the brain and muscle receptors. At the same time a hydrogen bond from nicotine to the backbone carbonyl of TrpB is enhanced in the neuronal receptor relative to the muscle-type. A point mutation near TrpB that differentiates α4β2 and muscle-type receptors appears to influence the shape of the binding site, allowing nicotine to interact more strongly with TrpB in the neuronal receptor. ACh receptors are established therapeutic targets for Alzheimer’s disease, schizophrenia, Parkinson’s disease, smoking cessation, pain, attention deficit-hyperactivity disorder, epilepsy, autism, and depression1. Along with solving a chemical mystery in nicotine addiction, our results provide guidance for efforts to develop drugs that target specific types of nicotinic receptors.

Use of calculated cation-pi binding energies to predict relative strengths of nicotinic acetylcholine receptor agonists

Agonists and antagonists of the nicotinic acetylcholine receptor (nAChR) are used to treat nicotine addiction, neuromuscular disorders, and neurological diseases. In designing small molecule therapeutics with the nAChR as a target, it is useful to identify chemical parameters that correlate with ability to activate the receptor. Previous studies have shown that cation-pi interactions at the transmitter binding sites of the nAChR are important for receptor activation by strong agonists such as acetylcholine. We hypothesized that a calculated estimate of cation-pi binding ability could be used to predict the efficiency for channel opening (i.e., the gating efficiency) associated with activation of the acetylcholine receptor by a series of structurally related organic cations. We demonstrate that the calculated cation-pi energy is strongly correlated with gating efficiency but only weakly correlated with closed-state binding affinity. Our results suggest that cation-pi interactions contribute significantly to the open-state affinity of these cations and that the calculated cation-pi energy will be a useful parameter for designing nAChR agonists and antagonists.