Ionization behaviour and tautomerism-dependent lipophilicity of pyridine-2(1H)-one cardiotonic agents

Spectral Properties Echoing the Tautomerism of Milrinone and Its Application to Fe3+ Ion Sensing and Protein Staining

Knowledge on the spectral properties of the tautomers of milrinone (MLR) in solvents and solid-state, as well as under light conditions is of critical importance from both theoretical and practical points of view. Herein, we investigated the spectral properties of MLR in different conditions using UV-Vis and fluorescence spectroscopies. The experimental results demonstrated that MLR can undergo the tautomerization reaction induced by solvent polarity, light and pH, eliciting four tautomeric structures (enol, keto, anion, and cation forms). The interesting multi-functional groups in MLR enable it to coordinate with metal ions or to recognize gust molecules by H-bonding. In the use of MLR as an excited-state intermolecular proton transfer (inter-ESPT) fluorescent probe, a highly sensitive and selective analysis of Fe³⁺ was developed, which offered a sensitive detection of Fe³⁺ with the detection limit of 3.5 nM. More importantly, MLR exhibited the ability of anchoring proteins and led to the recognition-driven turn-on inter-ESPT process, highlighting the potential for the probe to image proteins in electrophoresis gels. The spectral experimental results revealed the possible degradation mechanism, so that we can better understand the side effects of oral preparations. The use of the available drug as an inter-ESPT fluorescent probe is simple and accurate, providing a good method for Fe³⁺ ion sensing and protein staining.

Synthesis of 1,6-Dihydropyridine-3-carbonitrile Derivatives via Lewis Acid-Catalyzed Annulation of Propargylic Alcohols with (E)-3-Amino-3-phenylacrylonitriles

A novel Lewis acid-catalyzed, highly efficient, practical, and atom-economical protocol for the synthesis of functionalized 1,2-dihydropyridine-3-carbonitrile derivatives in the presence of Bi(OTf)₃ (10 mol %) in tetrahydrofuran (2.0 mL) at 80 °C for 8 h in air is described, starting from readily accessed propargylic alcohols and (E)-3-amino-3-phenylacrylonitriles. This cycloaddition protocol, which is scalable and proceeds under mild conditions, is amenable to the gram-scale construction of valuable 1,2-dihydropyridine-3-carbonitriles. Furthermore, the good functional group compatibility and broad scope of this strategy were demonstrated by a broad range of propargylic alcohols and (E)-3-amino-3-phenylacrylonitriles, with yields ranging from 34 to 96%.

A convenient method for synthesis of terpyridines via a cooperative vinylogous anomeric based oxidation

The presented study is the first report of the synthesis of terpyridines in the presence of a nanomagnetic catalyst instead of harmful reagents. Herein, Fe3O4@O2PO2(CH2)2NH3 +CF3CO2 - as a retrievable nanocatalyst with magnetic properties was applied for the multi-component reaction between acetylpyridine derivatives (2 or 3 or 4-isomer), aryl aldehydes and ammonium acetate under conventional heating conditions in the absence of any solvent. The derived terpyridines were obtained with acceptable yields and brief reaction times via a cooperative vinylogous anomeric based oxidation route. Fe3O4@O2PO2(CH2)2NH3 +CF3CO2 - showed a high capability for recovery and reuse in the mentioned reaction.

Interplay between Ionization and Tautomerism in Bioactive β-Enamino Ester-Containing Cyclic Compounds: Study of Annulated 1,2,3,6-Tetrahydroazocine Derivatives

Depending on the chemical scaffold, a bioactive species could reflect the interplay between ionization and tautomerism, which is often complicated by the possibility of populating different conformational states, in the case of flexible ligands. In this context, theoretical methods can be valuable to discern the role of these factors, as shown here for β-enamino esters of 1,2,3,6-tetrahydroazocino-fused ring systems, some of which had proven to be suitable scaffolds for designing novel acetylcholinesterase inhibitors. The compounds investigated herein form two clusters with distinctive experimental pK_a values (i.e., α,β-diesters and β-esters ranging within 6.1-7.3 and 8.2-9.0 pK_a intervals, respectively), which implies a drastic difference in the most populated species at physiological conditions. While chemoinformatic tools did not provide a consistent description of the actual pK_a values, the theoretical analysis performed for the protonated and neutral species of these compounds revealed a marked change in the tautomeric preference of the tetrahydroazocine moiety upon (de)protonation. Excellent agreement between the calculated and experimental pK_a values was found when the tautomeric preference of the protonated and neutral species was considered. Overall, this study highlights the potential use of high-level computational methods to disclose the mutual influence between ionization, tautomerism, and conformational preferences in multifunctional (bio)organic compounds.

Experimental and pKa prediction aspects of tautomerism of drug-like molecules

Molecules that can tautomerize are a challenge to scientists because one must consider the possible tautomers in most tasks involving chemical structures: for example, searching databases, interpreting experimental property measurements, calculating properties, virtual screening, and analyzing structure-bioactivity relationships. The challenge in interpreting property measurements such as pK_a values feeds into the general lack of extensive information not only of the relative tautomer stability in water but also the properties of the individual tautomers. This lack of information results in the lack of reliability of computational predictions of tautomer stability or properties. In spite of these problems, pK_a calculations are reliable enough that they can be used to filter out high-energy tautomers from databases used for virtual screening. Continuous improvements in both pK_a prediction software and theoretical calculations promise further improvements in solving the challenges of tautomers. The expected availability of high-resolution structures of many more tautomer-protein complexes will help guide the selection of the bioactive tautomer when the structure of the complex is not known.

Solvent and structural effects in tautomeric 2(6)-hydroxy-4-methyl-6(2)-oxo-1-(substituted phenyl)-1,2(1,6)-dihydropyridine-3-carbonitriles: UV, NMR and quantum chemical study

The state of the tautomeric equilibria of 2(6)-hydroxy-4-methyl-6(2)-oxo-1-(substituted phenyl)-1,2(1,6)-dihydropyridine-3-carbonitriles, 2-PY/6-PY, was evaluated using experimental and theoretical methodology. The experimental data were interpreted with the aid of time-dependent density functional (TD-DFT) method. Electron charge density was obtained by the use of Quantum Theory of Atoms in Molecules, i.e. Bader's analysis. Linear solvation energy relationships (LSER) rationalized solvent influence on tautomeric equilibrium. Linear free energy relationships (LFERs) were applied to the substituent-induced NMR chemical shifts (SCS) using SSP (single substituent parameter) and DSP (dual substituent parameter) model. Theoretical calculations and obtained correlations gave insight into the influence of molecular conformation on the transmission of substituent electronic effects, as well as on different solvent-solute interactions, and the state of tautomeric equilibrium.

Effects of donor-acceptor groups on the structural and electronic properties of 4-(methoxymethyl)-6-methyl-5-nitro-2-oxo-1,2-dihydropyridine-3-carbonitrile

Quantum chemical calculations on the geometric parameters, harmonic vibrational wavenumbers and 1H and 13C nuclear magnetic resonance (NMR) chemical shifts values of 4-(methoxymethyl)-6-methyl-5-nitro-2-oxo-1,2-dihydropyridine-3-carbonitrile [C9H9N3O4] molecule in ground state were performed using the ab initio HF and density functional theory (DFT/B3LYP) methods with 6-311++G(d,p) basis set. The results of optimized molecular structure were presented and compared with X-ray diffraction results. The theoretical vibrational frequencies and 1H and 13C NMR chemical shifts values were compared with experimental values of the investigated molecule. The observed and calculated values were found to be in good agreement. Since the title compound contains different electron-donor and -acceptor groups as well as lone pair electrons, and multiple bonds, the effects of these groups on the structural and electronic properties are found out. In addition, conformational, natural bond orbital (NBO), nonlinear optical (NLO) analysis, frontier molecular orbital energies, molecular surfaces, Mulliken charges and atomic polar tensor based charges were investigated using HF and DFT methods.

2-Amino-6-(dimethylamino)pyridine-3,5-dicarbonitrile

The title compound, C₉H₉N₅, is slightly twisted from planarity, with a maximum deviation of 0.0285 (13) Å from the pyridine plane for the C atom bearing the amino group. The cyano groups are on different sides of the pyridine plane, with C- and N-atom deviations of 0.072 (3)/0.124 (4) and −0.228 (4)/−0.409 (5) Å from the pyridine plane. In the crystal, N—H⋯N and C—H⋯N hydrogen bonds connect the mol­ecules into zigzag chains running along the c axis.

Ultrafast photodynamics of drugs in nanocavities: cyclodextrins and human serum albumin protein

In this feature article, we discuss recent advances in studying ultrafast dynamic and structural aspects of host-guest interactions. Steady-state and time-resolved techniques exploring events from the femto- to nanosecond regime were used to examine the ultrafast photodynamics and subsequent events in selected nanostructures of the formed complexes. These consist of aromatic systems, biologically relevant molecules, and drugs trapped within cyclodextrins (CD) and human serum albumin (HSA) protein pockets. We examine the effects exerted by these chemical and biological cavitands on internal twisting motions, proton transfer and charge transfer, and cis-trans isomerization reactions that may occur in the confined molecular systems. In addition, the influence of a restricting environment on the interaction of guest molecules with biological water is considered. The dynamic details of the complexes (diffusion, early interactions, formation, stability, internal guest diffusion, and conformational changes) and the excited-state relaxation pathways, rate constants of the involved processes, and changes in the electronic distribution within encapsulated guests gave clues to elucidate their photobehavior and are relevant to the photostability and delivery of drugs when using nanocarriers.

2-(Morpholin-4-yl)-6-(1H-pyrrol-1-yl)-pyridine-3,5-dicarbonitrile

In the title compound, C(15)H(13)N(5)O, the morpholine ring adopts a chair conformation. The dihedral angle between the pyrrole ring and the pyridine ring is 28.93 (14)°. In the crystal, the molecules are linked by C-H⋯O hydrogen bonds occur, and aromatic weak π-π stacking [centroid-centroid separation = 4.178 (2) Å] and C-H⋯π inter-actions consolidate the packing.

Ultrasound-assisted one-pot, four component synthesis of 4-aryl 3,4-dihydropyridone derivatives

The condensation of Meldrum's acid, aromatic aldehydes, alkyl acetoacetates and ammonium acetate in glacial acetic acid under ultrasound irradiation at room temperature results in the formation of 3,4-dihydropyridone derivatives (4a-p) in 85-96% yields. Compared with conventional methods, the main advantages of the present procedure are milder conditions, shorter reaction times and higher yields.

Synthesis of some biologically active 2,4'-bipyridine-5-carbonitriles carrying the 4-hydroxyphenylthio moiety

A series of new 4-aryl-2'-[(4-hydroxyphenyl)thio]-6oxo-1,6-dihydro- 2,4'-bipyridine-5-carbonitriles (3a-k) and 6-amino-4aryl-2'-[(4-hydroxyphenyl) thio]-2,4'-bipyridine-5-carbonitriles (4a-h) were synthesized from 4-hydroxythiophenol (1). The reaction of 4-hydroxythiophenol with 4-acetyl-2-chloropyridine yielded 1-{2-[(4-hydroxyphenyl)thio]pyridin-4-yl}ethanone (2). Further treatment of 2 with ethyl cyanoacetate in the presence of ammonium acetate with various aromatic aldehydes furnished the compounds 3a-k. On the other hand, condensation of 2 with aromatic aldehydes in the presence of alcoholic malononitrile in ammonium acetate gave compounds 4a-h. The structures of the newly synthesized compounds were established on the basis of their elemental analysis, as well as their IR, 1H- and 13C-NMR and mass spectral data. All the title compounds were subjected to in vitro antibacterial testing against two strains and antifungal screening against two fungi. Some of the compounds showed promising activity.

In silico prediction of ionization constants of drugs

Most pharmacologically active molecules contain one or more ionizing groups, and it is well-known that knowledge of the ionization state of a drug, indicated by the pKa value, is critical for understanding many properties important to the drug discovery and development process. The ionization state of a compound directly influences such important pharmaceutical characteristics as aqueous solubility, permeability, crystal structure, etc. Tremendous advances have been made in the field of experimental determination of pKa, in terms of both quantity/speed and quality/accuracy. However, there still remains a need for accurate in silico predictions of pKa both to estimate this parameter for virtual compounds and to focus screening efforts of real compounds. The computer program SPARC (SPARC Performs Automated Reasoning in Chemistry) was used to predict the ionization state of a drug. This program has been developed based on the solid physical chemistry of reactivity models and applied to successfully predict numerous physical properties as well as chemical reactivity parameters. SPARC predicts both macroscopic and microscopic pKa values strictly from molecular structure. In this paper, we describe the details of the SPARC reactivity computational methods and its performance on predicting the pKa values of known drugs as well as Pfizer internal discovery/development compounds. A high correlation (r2=0.92) between experimental and the SPARC calculated pKa values was obtained with root-mean-square error (RMSE) of 0.78 log unit for a set of 123 compounds including many known drugs. For a set of 537 compounds from the Pfizer internal dataset, correlation coefficient r2=0.80 and RMSE=1.05 were obtained.

Effect of Cyclodextrin Nanocavity Confinement on the Photorelaxation of the Cardiotonic Drug Milrinone

We report on steady-state UV-visible absorption, emission, and picosecond emission studies of milrinone (MIR) drug in neutral water and complexed to cyclodextrins (alpha-, beta-, gamma-CD and dimethyl-beta-CD (DM-beta-CD)). The results reveal that MIR forms a 1:1 inclusion complex with CD. Upon encapsulation the emission intensity increases and the fluorescence lifetime changes from approximately 65 ps to 240-350 ps, indicating a confinement effect of the nanocages on the photophysical behavior of the drug. Due to its methyl groups, the DM-beta-CD complex shows the largest effect. The time-anisotropy experiments support the formation of 1:1 inclusion complexes and indicate motion of the drug inside the nanocavity. Furthermore, results of PM3 calculations combined with spectral and dynamical data show that the drug is not fully embedded into the cavities, and the conformation of the included complex explains the relatively short lifetimes and low emission quantum yields of these entities.

Fast Relaxation Dynamics of the Cardiotonic Drug Milrinone in Water Solutions

The fast relaxation dynamics of 1,6-dihydro-2-methyl-6-oxo-3,4'-bipyridine-5-carbonitrile (milrinone, MIR), a cardiotonic drug, has been characterized in water solutions at different pH. In acidic media, a blue emission reflects a charge-transfer state in the cation (C) leading to a more stabilized structure with an emission lifetime of 90 ps. The emission lifetimes of the keto (K) and anion (A) structures are approximately 65 and 310 ps, respectively. Reasons for efficient nonradiative channels are discussed in terms of hydrogen-bonding interactions, intramolecular charge transfer (ICT), and twisting motion. A blue nanosecond-emission observed in almost all the studied pH range is suggested to be due to relaxed K due to an ICT reaction. B3LYP (6-31+G**) calculations showed that, in a water cavity, K is more stable than the enol form by 7 kcal/mol, and the ICT may take place within the pyridone moiety. At the physiological pH, the inotropic K structure is the dominant species (approximately 100%).

Insights into structure-activity relationships from lipophilicity profiles of pyridin-2(1H)-one analogs of the cardiotonic agent milrinone

The pH-dependent distribution profiles of a series of pyridin-2(1H)-one analogs of the inotropic/vasodilator agent milrinone, determined in 1-octanol/water (and for a number of them also in chloroform-water) using a pH-metric technique, showed that partition coefficients of the neutral forms (logP(N)) significantly encode for 2-pyridone/2-hydroxypyridine tautomerism. A comparison between experimental and calculated logP (CLOG P) values indicated that electron-withdrawing substituents, at the C(6) position, and to a lesser extent at the C(3) and C(5) positions, push up logPs toward the values of the more lipophilic 2-hydroxypyridine tautomers. RP-HPLC parameters (log k'omega) carry for large part similar information related to tautomerism-dependent lipophilicity, but they were also found to reasonably correlate with the solute molar volumes (r2 = 0.75). Investigating the implications of ionization and partition properties in modulating the in vitro cardiotonic activity of the examined compounds revealed that a high fraction of the neutral species at physiological pH, predominantly in the more polar pyridone (OX) tautomer, increases the positive inotropic potency.

Synthesis and pharmacological characterization of functionalized 2-pyridones structurally related to the cardiotonic agent milrinone

A new class of cardiotonic agents characterized by a 2-pyridone structure was synthesized. Appropriate sym-2-dimethylaminomethylene-1,3-diones reacted with methylcyanoacetate to afford the desired compounds. These derivatives were evaluated for their ability in inducing cardiotonic response on guinea pig isolated myocardial preparations. Compound 8b increased atrial contractility to an extent which is significantly higher than that of milrinone, the parent drug used as a reference compound. The pharmacological characterization and the docking studies performed on 8b highlighted its selective mechanism of action via type 3 PDE (PDE3) inhibition.