The nicotinic pharmacophore: thermodynamics of the hydrogen-bonding complexation of nicotine, nornicotine, and models

The thermodynamics of the hydrogen-bonding complexation of the acetylcholine agonists nicotine and nornicotine and of model pyridines, pyrrolidines, and N-methylpyrrolidines has been measured in CCl(4) by FTIR spectrometry toward a reference hydrogen-bond donor, 4-fluorophenol. Various methods are devised for measuring separately the hydrogen-bond acceptor strength of each nitrogen of nicotine and nornicotine: variation of the stoichiometry of complexation; correlations with electrostatic potentials on nitrogens and with substituent constants in the series of 3-substituted pyridines, 2-substituted pyrrolidines, and 2-substituted N-methylpyrrolidines; and linear free energy relationships between 4-fluorophenol and hydrogen fluoride hydrogen-bonded complexes. It is consistently found that nicotine and nornicotine have two active hydrogen-bond acceptor sites, the pyridine and pyrrolidine nitrogens, and that ca. 90% (for nicotine) and 80% (for nornicotine) of the 1:1 hydrogen-bonded complexes are formed to the pyridine nitrogen, although the pyrrolidine nitrogen is the first protonation site of nicotine and nornicotine in water. The low hydrogen-bond basicity of the pyrrolidine nitrogen in nicotine is mainly explained by the inductive electron-withdrawing and steric effects of the 2-(3-pyridyl) substituent. The partition of the Gibbs energy of the isomerism of complexation (AH...Nsp(2) <==> AH...Nsp(3)) into enthalpic and entropic contributions shows that the selectivity in favor of the pyridine nitrogen is driven by entropy. It is important to recognize the bifunctionality of nicotine in hydrogen bonding for understanding its lipophilicity and molecular recognition in non protonic media. When monoprotonated on their sp(3) nitrogen, nicotine and nornicotine keep, through their sp(2) nitrogen, a significant hydrogen-bond basicity which is greater than that of the ester group of acetylcholine.     

Interference between the hydrogen bonds to the two rings of nicotine

The biochemical transport and binding of nicotine depends on the hydrogen bonding between water and binding site residues to the pyridine ring and the protonated pyrrolidinium ring. To test the independence of these two moderately separated hydrogen-bonding sites, we have calculated the structures of clusters of protonated nicotine with water and a bicarbonate anion, benzene, indole, or a second water molecule. Unprotonated nicotine-water clusters have also been studied for contrast. The potential energy surfaces are first explored with an intermolecular anisotropic atom-atom model potential. Full geometry optimizations are then carried out using density functional theory to include nonadditive terms in the interaction energies. The presence of the charge on the pyrrolidine nitrogen removes the conventional hydrogen-bonding site on the pyridine ring. The hydrogen-bond ability of this site is nearly recovered when the protonated pyrrolidinium ring is bound to a bicarbonate anion, whereas its interaction with benzene shows a much smaller effect. Indole appears to partially restore the hydrogen-bond ability of the pyridine nitrogen, although indole and benzene both pi-bond to the pyrrolidinium ring. A second hydrogen-bonding water produces a significant conformational distortion of the nicotine. This demonstrates the limitations of the conventional qualitative predictions of hydrogen bonding based on the independence of molecular fragments. It also provides benchmarks for the development of atomistic modeling of biochemical systems.     

Neuronal nicotinic receptor agonists: a multi-approach development of the pharmacophore

Based on the results obtained with different automated computational approaches as applied to the study of eleven high-affinity agonists of the neuronal nicotine acetylcholine receptor (nAChR), belonging to different chemical classes, new relevant features were detected which complement the existing pharmacophores. Convergent results from DISCO (Distance Comparison), QXP (Quick Explore), Catalyst/HipHop, and MIPSIM (Molecular Interaction Potential Similarity) allowed us to identify and locate, in a well defined spatial arrangement, three geometrically independent key structural features: (i) a positively charged nitrogen atom for ionic or hydrogen bond interactions, (ii) a lone pair of the pyridine nitrogen or a specific lone pair of a carbonyl oxygen, as a hydrogen bond acceptor, and (iii) a centre of a hydrophobic area generally occupied by aliphatic cycles. The pharmacophore presented herein, along with predictive 2D and 3D QSAR models recently developed in our group, could represent valuable computational tools for the design of new nAChR agonists having therapeutical potential.     

Site of protonation of nicotine and nornicotine in the gas phase: pyridine or pyrrolidine nitrogen?

The gas-phase basicities (GBs) of nornicotine, nicotine, and model pyrrolidines have been measured by FT-ICR. These experimental GBs are compared with those calculated (for the two sites of protonation in the case of nicotine and nornicotine) at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) level, or those estimated from substituent effects on the GBs of 2-substituted pyrrolidines, 2-substituted N-methylpyrrolidines, and 3-substituted pyridines. It is found that, in contrast to the Nsp(3) protonation in water, in the gas phase nornicotine is protonated on the pyridine nitrogen, because the effects of an intramolecular CH.Nsp(3) hydrogen bond and of the polarizability of the 3-(pyrrolidin-2-yl) substituent add up on the Nsp(2) basicity, while the polarizability effect of the 2-(3-pyridyl) substituent on the Nsp(3) basicity is canceled by its field/inductive electron-withdrawing effect. The same structural effects operate on the Nsp(3) and Nsp(2) basicities of nicotine, but here, the polarizability effect of the methyl group puts the pyrrolidine nitrogen basicity very close to that of pyridine. Consequently, protonated nicotine is a mixture of the Nsp(3) and Nsp(2) protonated forms.     

Micromechanical measurement of AChBP binding for label-free drug discovery

A potential binding assay based on binding-driven micromechanical motion is described. Acetylcholine binding protein (AChBP) was used to modify a microcantilever. The modified microcantilever was found to bend on application of the naturally occurring agonist (acetylcholine) or the antagonist (nicotine and d-tubocurarine). Control experiments show that microcantilevers modified without AChBP do not respond to acetylcholine, nicotine, and d-tubocurarine. K(d) values obtained for acetylcholine, nicotine, and d-tubocurarine are similar to those obtained from radio-ligand binding assays. These results suggest that the microcantilever system has potential for use in label free, drug screening applications.     

Synthetic Methods for the Preparation of Conformationally Restricted Analogues of Nicotine

In the context of naturally occurring nitrogen heterocycles, nicotine is a chiral alkaloid present in tobacco plants, which can target and stimulate nicotinic acetylcholine receptors (nAChRs), a class of ligand-gated ion channels commonly located throughout the human brain. Due to its well-known toxicity for humans, there is considerable interest in the development of synthetic analogues; in particular, conformationally restricted analogues of nicotine have emerged as promising drug molecules for selective nAChR-targeting ligands. In the present mini-review, we will describe the synthesis of the conformationally restricted analogues of nicotine involving one or more catalytic processes. In particular, we will follow a systematic approach as a function of the heteroarene structure, considering: (a) 2,3-annulated tricyclic derivatives; (b) 3,4-annulated tricyclic derivatives; (c) tetracyclic derivatives; and (d) other polycyclic derivatives. For each of them we will also consider, when carried out, biological studies on their activity for specific nAChR subunits.     

The determination of nicotine and cotinine by ion pair reversed-phase chromatography

An ion chromatographic method is described for the determination of nicotine and cotinine in aqueous solutions. This method is based on a type of reversed-phase chromatography involving ion pair formation of protonated nicotine, cotinine, pyridine, and pyridine derivatives. Detection is accomplished by measuring the UV absorption at 262 nm. Detection limits for nicotine and cotinine are 8 ng/mL and 2 ng/mL, respectively. Analyses of environmental samples and spiked environmental samples by both this ion chromatographic method and a previously reported gas chromatographic method have been used to demonstrate the accuracy and precision of this technique. The results of the analyses of both sets of samples by the two methods are in excellent agreement with a linear correlation coefficient of 0.97.     

Unbinding of nicotine from the acetylcholine binding protein: steered molecular dynamics simulations

The ligand binding/unbinding process is critical to our understanding of the pharmacology of both the nicotinic acetylcholine receptor (nAChR) and the acetylcholine binding protein (AChBP). Steered molecular dynamics simulations were performed to learn about the unbinding process of the full agonist nicotine. Three different pulling models were designed to investigate the possible binding/unbinding pathways: radial and tangent models, and also a mixed model. Of the three, the tangent pulling model finally failed to dissociate nicotine from the ligand binding pocket. The efficiency of the pulling force profiles was superior, and the opening of the C-loop was smaller in the mixed pulling model than that in the radial model. The most favorable pathway for the cholinergic agonist nicotine to enter or leave the binding pocket is through the principal binding side, following a curvilinear track. Noticeably, it has been seen that the unbinding of the nicotine is concomitant with a global rotation of the protein-ligand complex which could be caused by the interactions of the ligand with protein at the tangent direction.     

Regioselective C-2 and C-6 substitution of (S)-nicotine and nicotine derivatives

[reaction: see text] Regioselective deprotonations of (S)-nicotine and derivatives at the C-2 and C-6 positions of the pyridine ring were performed in good to excellent yields. These methodologies allow the direct introduction of a plethora of functional groups onto the pyridine ring of nicotine.     

Starker Tobak

The active ingredient of tobacco, nicotine, is originally biosynthesized by plants as a protection against pests. Nicotine survives the harsh conditions in a burning cigarette and binds to certain acetylcholine receptors in the smoker's nerve system within seconds after the first puff. It is the nicotine that makes smoking so extremely addictive. On the other side, the majority of tobacco‐related diseases are not caused by nicotine but by other smoke components. Therefore smoke‐free products like electronic cigarettes have been developed that potentially pose less risk. It is now up to the individual smoker to make an informed choice between different nicotine delivery products and/or smoking at all.     

The Helical Coordination Polymer Ag(Nic)2(NO3)

Single crystals of Ag(Nic)₂(NO₃) were obtained from an aqueous solution of silver nitrate and nicotine as plate‐like colourless crystals. The crystal structure (monoclinic, P2₁, Z = 2, a = 933.3(2), b = 1136.8(2), c = 1024.3(2) pm, β = 94.49(2)°) consists of helical chains in which one nicotine molecule bridges with both the pyridine‐N and the pyrrol‐N coordinating and with a second nicotine molecule terminally coordinating with the pyridine‐N. A monodentate nitrate‐O is completing the coordination sphere of Ag⁺ to a distorted tetrahedron. Ag–N distances (229‐240 pm) attest for a rather strong attraction of the nicotine molecules to Ag(I) and thereby constitute essentially a one‐dimensional, helical coordination polymer according to the formulation Ag(Nic1)_2/2(Nic2)_1/1(NO₃)_1/1.     

Experimental observation of the transition between gas-phase and aqueous solution structures for acetylcholine, nicotine, and muscarine ions

Structural information on acetylcholine and its two agonists, nicotine, and muscarine has been obtained from the interpretation of infrared spectra recorded in the gas-phase or in low pH aqueous solutions. Simulated IR spectra have been obtained using explicit water molecules or a polarization continuum model. The conformational space of the very flexible acetylcholine ions is modified by the presence of the solvent. Distances between its pharmacophoric groups cover a lower range in hydrated species than in isolated species. A clear signature of the shift of protonation site in nicotine ions is provided by the striking change of their infrared spectrum induced by hydration. On the contrary, structures of muscarine ions are only slightly influenced by the presence of water.     

Electronic absorption spectra of pyridine and nicotine in aqueous solution with a combined molecular dynamics and polarizable QM/MM approach

The electronic absorption spectra of pyridine and nicotine in aqueous solution have been computed using a multistep approach. The computational protocol consists in studying the solute solvation with accurate molecular dynamics simulations, characterizing the hydrogen bond interactions, and calculating electronic transitions for a series of configurations extracted from the molecular dynamics trajectories with a polarizable QM/MM scheme based on the fluctuating charge model. Molecular dynamics simulations and electronic transition calculations have been performed on both pyridine and nicotine. Furthermore, the contributions of solute vibrational effect on electronic absorption spectra have been taken into account in the so called vertical gradient approximation. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.     

Binding characteristics of pyridine on Ag(110)

A combination of low-temperature scanning tunneling microscopy and density functional theory calculations was used to determine the binding characteristics of single pyridine molecules at a low coverage on a silver surface. The results indicated that pyridine binds to silver through the nitrogen atom in either a perpendicular or a parallel configuration with the latter structure being more prevalent. Both configurations are produced predominantly through electrostatic interaction between nitrogen and silver atoms. This is induced by charge redistribution in the pyridine molecule and nearby silver atoms upon pyridine adsorption.     

Role of the pyridine nitrogen in pyridoxal 5'-phosphate catalysis: activity of three classes of PLP enzymes reconstituted with deazapyridoxal 5'-phosphate

Pyridoxal 5'-phosphate (PLP; vitamin B(6))-catalyzed reactions have been well studied, both on enzymes and in solution, due to the variety of important reactions this cofactor catalyzes in nitrogen metabolism. Three functional groups are central to PLP catalysis: the C4' aldehyde, the O3' phenol, and the N1 pyridine nitrogen. In the literature, the pyridine nitrogen has traditionally been assumed to be protonated in enzyme active sites, with the protonated pyridine ring providing resonance stabilization of carbanionic intermediates. This assumption is certainly correct for some PLP enzymes, but the structures of other active sites are incompatible with protonation of N1, and, consequently, these enzymes are expected to use PLP in the N1-unprotonated form. For example, aspartate aminotransferase protonates the pyridine nitrogen for catalysis of transamination, while both alanine racemase and O-acetylserine sulfhydrylase are expected to maintain N1 in the unprotonated, formally neutral state for catalysis of racemization and β-elimination. Herein, kinetic results for these three enzymes reconstituted with 1-deazapyridoxal 5'-phosphate, an isosteric analogue of PLP lacking the pyridine nitrogen, are compared to those for the PLP enzyme forms. They demonstrate that the pyridine nitrogen is vital to the 1,3-prototropic shift central to transamination, but not to reactions catalyzed by alanine racemase or O-acetylserine sulfhydrylase. Not all PLP enzymes require the electrophilicity of a protonated pyridine ring to enable formation of carbanionic intermediates. It is proposed that modulation of cofactor electrophilicity plays a central role in controlling reaction specificity in PLP enzymes.     

Enhanced methylation rate within a foldable molecular receptor

A N,N-(dimethylamino)pyridine monomer is incorporated into the backbone of a m-phenyleneethynylene oligomer such that the pyridine nitrogen is located on the interior surface of the binding cavity in the folded structure of the oligomer. For an oligomer having a chain length of 13 monomer units, competitive inhibition experiments reveal that methyl iodide binds weakly within the oligomer cavity with an association constant K(a) = 2 M(-1), and the oligomer-methyl iodide complex reacts with unimolecular rate constant k(u) = 0.082 s(-1) to provide the methylated product. The effective molarity is calculated to be 230 M by comparison of k(u) for the 13-mer with the second-order rate constant for a 3-mer that is too short to fold and thus unable to bind methyl iodide.     

Optimization study for the reversed-phase ion-pair liquid chromatographic determination of nicotine in commercial tobacco products.

The availability of published methods for the determination of nicotine in commercial tobacco products based on state-of-the-art chromatographic methods is limited. Nicotine is a diprotic base with pKa's of 3.12 (pyridine ring) and 8.02 (pyrrolidine ring). Other monoprotic and diprotic bases are also present in commercial tobacco including anatabine, nornicotine, anabasine, and cotinine. In this paper, the chromatography of nicotine and the minor tobacco alkaloids under reversed-phase ion-pairing conditions is thoroughly studied. The results of this study are used to understand the retention mechanisms of the tobacco alkaloids, to examine their observed elution order with respect to fundamental analyte properties (size, functionality, and acid-base strength), and to select optimum chromatographic conditions for the determination of nicotine in commercial tobacco products.     

Adsorption study of acetylcholine and cholinergic and anticholinergic drugs onto an electrode surface

As a type of interaction of acetylcholine and cholinergic and anticholinergic drugs with a charged surface, their adsorption onto a gold electrode has been examined by measuring the specular reflectivity and differential capacity of the electrode-solution interface.Based on the adsorption potential profiles, the drugs have been divided into three groups: (I) carbamylcholine and methacholine; (II) tetraethylammonium ion and hexamethonium; (III) nicotine, atropine, scopolamine and pilocarpine. Group I includes cholinergic drugs. Group II and III include antichilinergic drugs whose adsorptivity is stronger than that of acetylcholine. The results of the adsorption of acetylcholine and drugs onto a charged electrode surface should provide information about the competitive inhibitory effects on the attachement of acetylcholine to receptor sites.     

Expedient five-step synthesis of SIB-1508Y from natural nicotine

Altinicline (SIB-1508Y), an anti-Parkinson's agent, was prepared in five steps from natural nicotine in 32% overall yield via a regioselective substitution of the pyridine ring of (S)-nicotine.