Flow injection chemiluminescence study of acridinium ester stability and kinetics of decomposition

Decomposition of phenyl acridinium-9-carboxylate is monitored using electrogenerated chemiluminescence in a flow system. The formation of the pseudobase from the acridinium ester [AE] is described by rate = k′₁[AE] + k″₁[AE][OH^?]^0.5, where k′₁ = 0.020 ± 0.006 s^?1 and k″₁ = 2.1 ± 0.8 (L/mol)^?0.5 s^?1. Irreversible decomposition of the pseudobase is described by rate = k′₂[AE][OH^?], where k′₂ = 20.1 ± 3.8 (L/mol s). These kinetic equations, plus measurement of variation in emission intensity for constant acridinium ester concentration, are used to predict the resulting emission intensity v. pH behaviour given various contact times (in the 0.25 to 25 s range) for the acridinium ester to be in an alkaline solution prior to initiation of the chemiluminescence reaction.     

Quantitative Structure-Activity Relationships for Antifungal 3-(3,5-Dichloropheny)-2,4-imidazolidinediones

The antifungal activity of 441-acyl derivatives of 3-(3,5-dichlorophenyl)-2,4-imidazol- idinedione against Botrytis cinerea, and of 10 1-sulfonyl compounds against Aiternaria kikuchi- ana were assayed by the agar medium dilution method. The structure-activity relationships for the substituents of the acyl and sulfonyl moieties were analyzed with such physicochemical parameters as hydrophobic π, inductive electronic σ₁, and steric E′^c_s and B₁ values by multiple regression. The activity of the acyl derivatives against B. cinerea was related parabolically to the hydrophobicity of the substituents. The stronger the electron-donating power, the larger the overall steric bulkiness, and the smaller the minimum width in the direction perpendicular to the bond axis of the substituents, the greater was the activity. The activity of the sulfonyl derivatives against A. kikuciana was related only to the hydrophobicity of the substituents.     

Partial Agonism: Mechanisms Based on Ligand-Receptor Interactions and on Stimulus-Response Coupling

Abstract

Substances eliciting, at very high concentrations, a lower maximal response of a particular biological system than a defined standard, are defined as partial agonists. The convention rests on the definition of a standard substance that achieves a ‘full’ maximal response; partial agonism being, therefore, relative. Various mechanisms lie behind this phenomenon: 1. Receptor-related mechanisms: the agonist-receptor complex exists in several conformational states from which only one, or only a few, activate the cell signaling pathway. This may occur when the receptor itself, or the agonist, exists in multiple states (e.g., in the form of enantiomers or stereoisomers), or when the agonist-receptor complex changes its conformation (receptor switch: two-state model of receptor activation). Furthermore, a steric hindrance by a ‘wrong-way binding’ of a part of the agonist's molecules may prevent the full ‘correct’ occupancy of receptors. 2. Mechanisms based on the efficacy of the stimulus-response coupling. The efficacy is then proportional to the sum of probabilities that receptors in individual states activate the cell-signaling pathway. Doses (concentrations) eliciting the half maximal response (EC₅₀), or similar response sensitivity parameters, are not included in the definition of partial agonism. However, tight correlations exist between maximal response and EC₅₀ in many, but not all, generic groups of agonistically acting substances. These relationships are frequently linear; intercepts and slopes of these ‘E, K_E plots’ are characteristic for individual, putative mechanisms. Dose-response curves of partial agonists are akin to those obtained for a response to a full agonist after a stepwise partial inactivation of receptors by an irreversible inhibitor. Also, the E, K_E plots obtained in these instances are similar to those of partial agonists. The receptor reserve, rather vaguely defined in early reports, is therefore closely linked to the phenomenon of partial agonism.     

Conformational interconversions in peptide β-turns: Discrimination between enantiomeric conformations by chiral perturbation

The crystal structure of the model tripeptide Boc-Aib-Gly-Leu-OMe ( 1 ) reveals two independent molecules in the asymmetric unit that adopt “enantiomeric” type I and type I′ β-turn conformations with the Aib and Gly residues occupying the corner (i + 1 and i + 2) positions. ¹³C cross polarization and magic angle sample spinning spectra in the solid state also support the coexistence of two conformational species. ¹³C-nmr in CDCl₃ establishes the presence of a single species or rapid exchange between conformations. 400 MHz ¹H-nmr provides evidence for conformational exchange involving a major and minor species, with β-turn conformations supported by the low solvent exposure of Leu(3) NH and the observation of N_iH ↔ N_i+1H nuclear Overhauser effects. CD bands in the region 190–230 nm are positive, supporting a major population of type I′ β-turns. The isomeric peptide, Boc-Gly-Leu-Aib-OMe ( 2 ), adopts an “open” type II′ β-turn conformation in crystals. Solid state and solution nmr support population of a single conformational species. Chiral perturbation introduced outside the folded region of peptides may provide a means of modulating screw sense in achiral sequences. © 1998 John Wiley & Sons, Inc. Biopoly 45: 191–202, 1998     

Computer simulation of the conformational properties of retro–inverso peptides. I. Empirical force field calculations of rigid and flexible geometries of N-acetylglycine-N′- methylamide,bis(acetamido) methane,and N,N′- dimethylmalonamide and their corresponding Cα-methylated analogs

Rigid and flexible geometry calculations are described for N-acetylglycine-N′-methylamide, N-acetylalanine-N′-methylamide, and their retro-inverso analogs, bis(acetamido) methane, 1,1-bis(acetamido) ethane, N,N′-dimethylmalonamide, and N,N′-dimethyl-2-methyl-malonamide. The significance of relaxing all degrees of freedom, especially angular flexibility is demonstrated. The flexible geometry approach yields energy maps similar to those from rigid geometry, but the energy barriers between minima are substantially reduced, leading in general, to more probable transitions and a higher volume of accessible conformational space. Whereas the glycine and alanine derivatives exhibit their lowest energy minima in the C region, the gem-diaminoalkyl and malonyl residues show their lowest minima in the “α-helical” regions. With respect to the effect of side chains (H versus CH₃), the greatest conformational influence appears with the gem-diaminoalkyl residues. These results indicate significantly different conformational behavior of retro peptides and the implications of these pairwise incorporations of retro-inverso residues in peptide chains, are discussed.     

Control of transpiration in an irrigated Eucalyptus globulus Labill. plantation

Stomatal conductance and transpiration were measured concurrently in an irrigated Eucalyptus globulus Labill. plantation. Canopy stomatal conductance, canopy boundary layer conductance and the dimensionless decoupling coefficient (Ω) were calculated (a) summing the conductance of three canopy layers (g_c) and (b) weighting the contribution of foliage according to the amount of radiation received (g_c′). Canopy transpiration was then calculated from g_c and g_c′ for Ω = 1 (E_eq), Ω = 0 (E_imp) and by weighting E_eq and E_imp using Ω (E_Ω). E_eq, E_imp and E_Ω were compared to transpiration estimated from measurements of heat pulse velocity. The mean value of Ω was 0·63. Transpiration calculated using g_c and assuming perfect coupling (12·5 ± 0·9 mmol m^?2 s^?1) significantly overestimated measured values (8·7 ± 0·8 mmol m^?2 s^?1). Good estimates of canopy transpiration were obtained either (a) calculating E_Ω separately for the individual canopy layers or (b) treating the canopy as a single layer and using g_c′ in a calculation of E_imp (Ω = 0). The latter approach only required measurement of stomatal conductance at a single canopy position but would be unsuitable for use in combined models of canopy transpiration and assimilation. It should however, be suitable for estimating transpiration in forests regardless of the degree of coupling.     

CRYSTAL STRUCTURE OF 1,3,5-TRIMETHYL-N4-HYDROXYCYTOSINE,AND ITS RELEVANCE TO THE MECHANISM OF HYDROXYLAMINE MUTAGENESIS

Abstract

1,3,5-Trimethyl-N⁴-hydroxycytosine, an analogue of the promutagenic N⁴-hydroxycytosine and 5-methyl-N⁴-hydroxycytosine nucleosides, crystallizes in the monoclinic space group P 2₁/n with cell dimensions at ?147°C: a = 7.1481(7), b = 9.2565(5), c = 13.3086(12) Å, β = 97.90(2)°, V = 872.24(13) ų, ρ_c = 1.426 Mg m^?3, Z = 4, F(000) = 401.39, μ = 0.91 mm^?1, λ(Cu) = 1.54056 Å, 20(max) = 139.3°. The crystal structure has been solved by X-ray difraction and refined to R = 3.7 % for 1457 reflections. Notwithstandin the steric hindrance imposed by methyl groups at both N(3) and C(5), the exocyclic N⁴-OH group is located essentially in the plane of the ring, giving rise to an “overcrowded” molecule, like that of 1,5,N⁴,N⁴-tetramethylcytosine. The conformational parameters have also been compared with those of a number of related and previously reported N(1)-substituted cytosines. In the present compound the N⁴-OH rotamer is in the anti conformation relative to the ring N(3), hence similar to that of one of the rotamers in N(1)-substituted N⁴-hydroxycytosine, which permits normal Watson-Crick base pairing of the latter, relevant to the mechanism of hydroxylamine mutagenesis.     

The effect of β‐methylation on the conformation of α, β‐dehydrophenylalanine: a DFT study

Dehydroamino acids are non‐coded amino acids that offer unique conformational properties. Dehydrophenylalanine (ΔPhe) is most commonly used to modify bioactive peptides to constrain the topography of the phenyl ring in the side chain, which commonly serves as a pharmacophore. The Ramachandran maps (in the gas phase and in CHCl₃ mimicking environments) of ΔPhe analogues with methyl groups at the β position of the side chain as well as at the C‐terminal amide were calculated using the B3LYP/6‐31 + G** method. Unexpectedly, β‐methylation alone results in an increase of conformational freedom of the affected ΔPhe residue. However, further modification by introducing an additional methyl group at C‐terminal methyl amide results in a steric crowding that fixes the torsion angle ψ of all conformers to the value 123°, regardless of the Z or E position of the phenyl ring. The number of conformers is reduced and the accessible conformational space of the residues is very limited. In particular, (Z)‐Δ(βMe)Phe with the tertiary C‐terminal amide can be classified as the amino acid derivative that has a single conformational state as it seems to adopt only the β conformation. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Estradiol dependent decrease of binding inhibition by anti-estrogens (a possible test of receptor activation)

The interaction of the uterine estrogen receptor (R) with three anti-estrogens, Dimethylstilbestrol¹, Tamoxifen and Nafoxidine, have been studied either indirectly by competitive experiments or directly using radioactive compounds. The affinity of these anti-estrogens for R was found much higher when determined directly without estradiol (E₂) than when evaluated by competitive experiments. In the presence of E₂, but not in its absence, the inhibitory activity of the anti-estrogens decreased slowly with time. The present report strongly suggests that R is transformed by E₂ into a form less sensitive to anti-estrogen. This “desensitization” of R to the estrogen antagonists is proposed as another $\text{in vitro}$ test for the E₂ induced “activation” of its receptor.     

Molecular basis of the protonophoric and uncoupling activities of the potent uncoupler SF-6847 ((3,5-di-tert-butyl-4-hydroxybenzylidene)malononitrile) and derivatives. Regulation of their electronic structures by restricted intramolecular rotation

We reported recently (Yoshikawa, K. and Terada, H. (1982) J. Am. Chem. Soc. 104, 7644–7646) that the potent uncoupler of oxidative phosphorylation SF-6847 ((3,5-di-tert-butyl-4-hydroxybenzylidene)malononitrile) shows unique intramolecular restricted rotation of the malononitrile moiety. In this study, values for the activation energy E_a of the restricted rotation of SF-6847 derivatives with the same alkyl chain R in both ortho positions of the phenolic hydroxyl group were determined from the temperature-dependent change in the¹H-NMR signals of their aromatic protons. The E_a values of the neutral forms of these derivatives were found to be the same irrespective of R, but those of the anionic forms increased with increase in the alkyl chain length of R. It was found that the restricted rotation of the malononitrile moiety regulates its electron-withdrawing ability in such a way as to keep the acid dissociability of these derivatives similar, overcoming the effect of steric hindrance by R. The protonophoric activity of these derivatives, in a phospholipid bilayer membrane and their uncoupling activity in rat-liver mitochondria were both found to depend on E_a of their anionic forms. The stability of the uncoupler anions regulated by the restricted rotation of the malononitrile group in a nonpolar membrane environment was found to be important for exhibition of these activities. The hydrophobicity of the anionic forms of these derivatives was suggested also to be affected by the intramolecular rotation.     

Microbiological Redox Potential Control to Improve the Efficiency of Chalcopyrite Bioleaching

The effect of controlling the redox potential (E_h) on chalcopyrite bioleaching kinetics was studied as a new aspect of redox control during chalcopyrite bioleaching, and its mechanism was investigated by employing the “normalized” solution redox potential (E_normal) and the reaction kinetics model. Different E_h ranges were established by use of different acidophiles (Sulfobacillus acidophilus YTF1; Sulfobacillus sibiricus N1; Acidimicrobium ferrooxidans ICP; Acidiplasma sp. Fv-AP). Cu dissolution was very susceptible to real-time change in E_h during the reaction. It was found that efficiency of bioleaching of chalcopyrite can be effectively evaluated on the basis of E_normal, since it is normalized for real-time fluctuations of concentrations of major metal solutes during bioleaching. For steady Cu solubilization during bioleaching at a maximum rate, it was important to maintain a redox potential range of 0 ≤ E_normal ≤ 1 (?0.35 mV optimal) at the mineral surface by employing a “weak” ion-oxidizer. This led to a copper recovery of > 75%. At higher E_normal levels (E_normal > 1 by “strong” microbial Fe²⁺ oxidation), Cu solubilization was slowed by diffusion through the product film at the mineral surface (< 50% Cu recovery) caused by low reactivity of the chalcopyrite and by secondary passivation of the chalcopyrite surface, mainly by jarosite.     

The new anticancer drug [(2R )-aminomethylpyrrolidine](1,1-cyclobutanedicarboxylato)platinum(II) and its toxic S enantiomer do interact differently with nucleic acids

 The interaction of the two chiral isomers of the new anticancer agent [Pt(ampyr)(cbdca)] (ampyr=aminomethylpyrrolidine, cbdca=cyclobutanedicarboxylate) with 5′-GMP and with short G-containing oligonucleotides has been studied using ¹H and ³¹P NMR, UV-vis spectroscopy and molecular modelling. Each isomer loses the cbdca ligand upon binding to the DNA fragments. Two geometrical isomers of the DNA adducts are formed owing to the presence of the unsymmetric ampyr ligand. These isomers prove to be GG-N7,N7 chelates for d(GpG), d(pGpG) and d(CpGpG). A slight preference for the formation of one geometrical isomer is found in the case of DNA fragments having a phosphate moiety and/or a C base at the 5′-site of the GG sequence. H-bonding interactions from the NH₂ moiety towards the 5′-phosphate group and/or the O atom of the C base clearly favour the formation of one geometrical isomer. The presence of these H-bonds, together with the bulky pyrrolidine ring, has resulted in the unique observation (by ¹H NMR) of NH protons of coordinated amines that do not rapidly exchange in a 99.95% D₂O solution. Temperature-dependence studies show an extremely slow stack ⇄ destack conformational change for the CGG adducts of the S isomer, which could be related to these stable H-bonds of the amine protons towards the oligonucleotide. For the R isomer this stack ⇄ destack conformational change is faster, probably owing to more steric hindrance of the pyrrolidine ring as deduced from the NOESY data, and as also suggested by molecular modelling. The observation of extremely slow rotation around the Pt-N7 bond for [Pt(R-ampyr)(GMP-N7)₂] provides further evidence for increased steric hindrance of the R isomer compared to the S isomer. The rate of binding of the drug to G bases proved to be second order for both isomers; in fact the (toxic) S isomer is about two times more reactive than the (non-toxic) R isomer, as seen from k ₂ values of 0.17±0.01 M^–1s^–1 for [Pt(S-ampyr)(cbdca)] and 0.09±0.01 M^–1s^–1 for [Pt(R-ampyr)(cbdca)]. No solvent-assisted pathway is involved in these reactions, since the complexes prove to be stable in solution for weeks and therefore only a direct attack of the G base on the Pt must be involved. Because hardly any intermediate species can be detected during the reaction, coordination of the second G base must occur much faster than the binding of the first G base. Since direct attack of the nucleobases takes place, steric interactions become extremely important and therefore are likely to determine the reactivity, activity and even the toxicity of such Pt complexes. Received: 12 January 1999 / Accepted: 17 June 1999     

Effects of sap velocity on the daytime increase of stem CO2 efflux from stems of Schima superba trees

Stem CO₂ efflux (E _s) has been estimated from a temperature-related equation, but sap flux often affects measurements of E _s, which leads to misunderstanding real stem respiration. In order to observe the relationship between E _s and stem temperature and to analyze the effect of sap velocity on E _s, stem temperature, E _s and sap flux were measured from a subtropical Schima superba plantation in South China on three trees for consecutive 3 days in July and October 2009. Stem temperature, E _s and sap velocity were significantly higher in July than in October. Stem temperature could explain 17–41 and 54–75% variations of E _s in July and October, respectively. A negative relationship between E _s and stem temperature was found during 1800–2300 hours in July. The daytime E _s was 9.2, 4.3 and 2.4% higher than the predicted for three trees in July, and this occurred only on Tree 1 in October. Sap velocity was positively correlated with E _s for three trees in July, and the increase of E _s with the increase of sap velocity was only observed on Tree 1 in October. These results demonstrated that the occurrence of sap flux could account for the increase of daytime E _s, and the effect of sap velocity on E _s varied with the seasons from the S. superba stem.     

Some mechanistic aspects on chiral discrimination of organic acids by immobilized bovine serum albumin (BSA)

Although chiral anionic compounds, notably a large number of organic acids, have been found to be readily separated into enantiomers on BSA-based columns, the structural requirements for an efficient enantiomer discrimination by the protein is still not very well known. Since it is often observed that very hydrophobic acids, like many of the antiinflammatory “profens,” can be resolved with large separation factors for the enantiomers, a systematic study of a series of racemic α-substituted alkanoic acids was made. The series of analytes was prepared from α-amino acids, RCH(NH₂)CO₂H (where R = C₁-C₆), by reaction with N-(chloroformyl)-carbazole. A rapid increase in the capacity ratios of both enantiomers was found with increasing length of R. The effect, however, was larger for the last eluted enantiomer, leading to a substantial increase in the separation factor; this being 7.3 for R = C₆ in 20 mM phosphate buffer (pH 8.0) with 30% of acetonitrile. Further, the separation factor also increased with decreasing organic modifier content. Thus when the R = C₆-analyte was run at a mobile phase concentration of 20% acetonitrile and a flow rate of 1.5 ml/min, the time difference between the two eluted enantiomers exceeded 20 hr. A reasonable interpretation of our results seems to be that enantioselectivity is promoted by increased hydrophobic interaction. Since the anionic charge of the analyte is also taking part in the retention mechanism, a tight binding of the analyte will result from simultaneous electrostatic and hydrophobic interaction. When the latter is increased, less conformational freedom will be left for the analyte and the steric configuration at the α-carbon atom will become more and more important. Steric hindrance by the α-substituent in the first eluted enantiomer will counteract the tight binding caused by the combined binding interactions and lead to a smaller increase in the capacity ratio.     

Nonlocal nature of the electron energy spectrum in a glow discharge in pure O2: I. Nonlocal character of the electron distribution function

Abstrac A study is made of the nonlocal nature of the electron energy distribution function in the positive column of a glow discharge in a tube filled with pure oxygen. The distribution function and the axial (E _z) and radial (E _r) electric fields as functions of radius are measured using an array of mobile probes. The experimentally obtained spatial profiles of the distribution function are used to test the applicability of the two-term approximation to the distribution function of the electrons with a nonlocal energy spectrum. The distribution function in a specified electric field E=E _z +E _r (where E _z ⊥ E _r) is calculated by solving the coordinate-dependent Boltzmann equation in the two-term approximation and by directly integrating the equations of electron motion using the Monte Carlo method. A comparison between the experimental data and the results of simulations carried out for a broad parameter range shows that, in the case of a highly nonlocal electron energy spectrum, the two-term approximation makes it possible to calculate the electron distribution function with a fairly good accuracy, in which case, however, in imposing the boundary conditions, the electron losses at the plasma surface should be treated in the kinetic approximation. It is shown that using the reflection coefficient of the plasma surface for electrons instead of the loss cone in space makes it possible to accurately calculate the electron energy distribution function over the entire parameter range under consideration, including the transient region in which the electron-energy relaxation length is comparable to the characteristic plasma dimension. __________ Translated from Fizika Plazmy, Vol. 26, No. 11, 2000, pp. 1038–1045. Original Russian Text Copyright ? 2000 by Ivanov, Klopovskii, Lopaev, Rakhimov, Rakhimova.     

On the “filterable aggregates and other particles” interpretation of the extraordinary regime of polyelectrolytes

Quasi-elastic light scattering studies on some polyelectrolyte systems exhibit a somewhat “bizarre” behavior in the profile of the apparent diffusion coefficient D_app as a function of the salt concentration C_s. As C_s is decreased, D_app first increases in accordance with polyelectrolyte theories, and then undergoes a precipitous drop in value by over an order of magnitude at a well-defined critical value C_s = C. This “transition” from C_s > C (ordinary) to C_s < C (extraordinary) is referred to as the “ordinary-extraordinary” (o-e) transition. Ghosh, Peitzsch, and Reed [(1992) Biopolymers, Vol. 32, pp. 1105–1122] proposed a “filterable aggregate” (FA) and “other particle” interpretation for the o-e transition and its reversibility in regard to ionic strength changes. The present communication examines in detail the FA model as applied to the o-e transition. It is shown that the FA model fails to account of the established characteristics of the o-e transition. © 1993 John Wiley & Sons, Inc.     

Multiple Molecular Forms of Acetylcholinesterase in the Nematode Caenorhabditis elegans

Abstract: Extracts of the nematode Caenorhabditis elegans contain five molecular forms of acetylcholinesterase (AChE) activity that can be separated by a combination of selective solubilization, velocity sedimentation, and ion-exchange chromatography. These are called form IA (5.2s), form IB (4.9.s), form II (6.7s), form III (11.3s), and form IV (13.0s). All except form III are present in significant amounts in rapidly prepared extracts and are probably native; form III is probably derived autolytically from form IV. Most of forms IA and IB can be solubilized by repeated extractions without detergent, whereas forms II, III, and IV require detergent for effective solubilization and may therefore be membrane-bound. High salt concentrations are not required for, and do not aid in, the solubilization of these forms. For all forms, molecular weights and frictional ratios have been estimated by a combination of gel permeation chromatography and velocity sedimentations in both H₂O and D₂O. The molecular weight estimates range from 83,000 to 357,000 and only form II shows extensive asymmetry. The separated forms have been characterized with respect to substrate affinity, substrate specificity, inhibitor sensitivity, thermal inactivation, and detergent sensitivity. Judging by these properties, C. elegans is like other invertebrates in that none of its cholinesterase forms resembles either the “true” or the “pseudo” cholinesterase of vertebrates. However, internal comparison of the C. elegans forms clearly distinguishes forms IA, III, and IV as a group from forms IB and II; the former are therefore designated “class A” forms, the latter “class B” forms. Genetic evidence indicates that separate genes control class A and class B forms, and that these two classes overlap functionally. Several factors, including kinetic properties, molecular asymmetry, molecular size, and solubility, all suggest that a molecular model of the multiple cholinesterase forms observed in vertebrate electric organs probably does not apply in C. elegans. Potential functional roles and subunit structures of the multiple AChE forms within each C. elegans class are discussed.     

Helix-coiled transition in heteropolymers. I. Ground-state energy

A new technique is presented for treating the ground state of an heteropolymer with a random sequence of components. An exact system of equations is found for determining the ground state energy E which is equal to the polymer free energy f in the lowest-order approximation in T/V (V/2 is the large “surface” energy arising at the boundaries between coiled and “helical” sections: V ? T, U_k; U₁ and –U₂ are the free energies of the components counted from the corresponding coiled state energies). These equations are essentially simplified at certain fixed values of the ratio U₁/U₂. For integer values of U₂/U₁ and U₁/U₂ a solution is obtained with an accuracy exp(–V/U_k). The ground-state energy as a function of U₁ and U₂ is shown to be highly irregular: its derivatives have jumps at an infinite number of points. These jumps provide a fine structure of the melting curves. A smoothed over the jumps function E′ is found by way of analytic continuation from the integer values of U₁/U₂ and U₂/U₁. The accuracy of the approximation f ≈ E is estimated and the correctional term of order T/V is determined.     

Liquid chromatographic direct resolution of flecainide and its analogs on a chiral stationary phase based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid

Flecainide, an antiarrythmic agent, and its analogs were resolved on a high performance liquid chromatographic chiral stationary phase (CSP) based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid with the use of a mobile phase consisting of methanol‐acetonitrile‐trifluoroacetic acid‐triethylamine (80/20/0.1/0.3, v/v/v/v). The chiral resolution was quite successful, the separation factors (α) and the resolutions (R_S) for 20 analytes including flecainide being in the range of 1.19–1.82 and 1.73–6.80, respectively. The ortho‐substituent of the benzoyl group of analytes was found to cause decrease in the retention times of analytes probably because of the conformational deformation of analytes originated from the steric hindrance exerted by the ortho‐substituent. Chirality, 2010. © 2009 Wiley‐Liss, Inc.