Enantioselective separation of twelve pairs of enantiomers on polysaccharide‐based chiral stationary phases and thermodynamic analysis of separation mechanism

The enantioseparation of twelve pairs of structurally related 1‐aryl‐1‐indanone derivatives was studied in the normal‐phase mode using three different polysaccharide‐type chiral stationary phases, namely Chiralpak IB, Chiralpak IC, and Chiralpak ID. n‐Hexane/2‐propanol and n‐hexane/ethanol were employed as mobile phases. Among all the investigated chiral columns, Chiralpak IC exhibited the most universal and the best enantioseparation ability toward all the racemates, particularly with the mobile phase composed of n‐hexane/2‐propanol (90/10, v/v). Then the effects of column temperature on retention and enantioselectivity were examined in the range of 25–40°C. Satisfactory enantioseparation was obtained at ambient temperature. The natural logarithm of retention and separation factors (ln k and ln α) versus the reciprocal of absolute temperature (1/T) (Van't Hoff plots) were found to be linear for all racemates, indicating that the retention and separation mechanisms were independent of temperature in the range investigated. Then, the thermodynamic parameters (ΔΔH°, ΔΔS°, and ΔΔG°) were calculated from Van't Hoff plots. These values indicated that the solute transfer from the mobile to stationary phase was enthalpically favorable, and the process of enantioseparation was mainly enthalpy controlled. At last, the impact of small changes in molecular structures of the tested 1‐indanone derivatives on enantioseparation was also discussed.     

Retention Prediction System of O‐Aryl,O‐(1‐methylthioethylidene‐amino)phosphates on RP‐HPLC

Using factor analysis and stepwise linear regression methods, two parameters – CMR and ECCR – were selected from eight solute‐related structure parameters as the most retention‐influencing parameters. The relationships between the retention data (k ´) and the two structure parameters were established for 13 O‐aryl,O‐(1‐methylthioethylideneamino)phosphate compounds under a wide range of experimental conditions. The retention data (k ´) of another seven compounds with similar structures were predicted using these QSRR equations. Good agreement was obtained between the experimental k ´ values and predicted ones.     

SBA‐15‐supported N‐coordinate ruthenium(II) materials bearing sulfonamide‐type ligands: Effect of ligand backbones on catalytic transfer hydrogenation of ketones and aldehydes

[RuLCl(p ‐cymene)] (L = N ‐arylsulfonylphenylenediamine) complexes ( 2 _{a – d} ) were synthesized from the reaction between [Ru(p ‐cymene)Cl₂]₂ and ligand. Additionally, SBA‐15–[RuLCl(p ‐cymene)] derived catalysts ( 3 _{a – d} ) were successfully immobilized onto mesoporous silica (SBA‐15) by an easily accessible approach. The structural elucidations of 2 _{a – d} and 3 _{a – d} were carried out with various methods such as ¹H NMR, ¹³C NMR and infrared spectroscopies, elemental analysis, thermogravimetric/differential thermal analysis, nitrogen adsorption–desorption and scanning electron microscopy/energy‐dispersive X‐ray analysis. The Ru(II) complexes and materials were found to be highly active and selective catalysts for the transfer hydrogenation (TH) reaction of aldehydes and ketones. The influence of various 1,2‐phenylenediamines on the reactivity of the catalysts (complexes or materials) was studied and the catalysts ( 2 _d and 3 _d ) with a 4,5‐dichlorophenylenediamine substituent showed the best activity (the maximum turnover frequencies are 2916 and 2154 h⁻¹ for TH of 4‐fluoroacetophenone, and 6000 and 4956 h⁻¹ for TH of 4‐chlorobenzaldehyde).     

The temperature convergence of biopolymers in reversed-phase liquid chromatography

Summary Based on a stoichiometric displacement model for retention (SDM-R) in liquid chromatography, the two linear plots, log/(a constant relating to the affinity of one mole of solute to the stationary phase) andZ (the total moles of the displacing agent released at the interface between stationary phase and solute molecules as one mole of the solute is absorbed by the stationary phase) of small solutes and biopolymers versus the reciprocal of absolute temperature, l/T, in reversed-phase liquid chromatography were theoretically derived and experimentally proved to be linear and to have a common point called the temperature convergent point. The two linear plots could be used instead of the plot of logk to l/T which is normally only valid for small solutes but not for biopolymers for investigations of chromatographic thermodynamics and temperature convergence. The average convergence temperature (T _conv) of biopolymers was theoretically derived to equal the ratio between the slope and the intercept from either one of the two linear plots and to be 130.8±9.3°C for five proteins which are very close to those of biopolymers by calorimetry. A new methodology to investigate chromatographic thermodynamics and possibly for investigating the temperature convergence of biopolymers in the process of protein folding is also presented.     

The unusual fragmentation of long‐chain feruloyl esters under negative ion electrospray conditions

Long‐chain ferulic acid esters, such as eicosyl ferulate ( 1 ), show a complex and analytically valuable fragmentation behavior under negative ion electrospay collision‐induced dissociation ((?)‐ESI‐CID) mass spectrometry, as studied by use of a high‐resolution (Orbitrap) mass spectrometer. In a strong contrast to the very simple fragmentation of the [M + H]⁺ ion, which is discussed briefly, the deprotonated molecule, [M – H]^?, exhibits a rich secondary fragmentation chemistry. It first loses a methyl radical (MS²) and the ortho‐quinoid [M – H – Me]^‐? radical anion thus formed then dissociates by loss of an extended series of neutral radicals, C_nH_2n + 1^? (n = 0–16) from the long alkyl chain, in competition with the expulsion of CO and CO₂ (MS³). The further fragmentation (MS⁴) of the [M – H – Me – C₃H₇]^? ion, discussed as an example, and the highly specific losses of alkyl radicals from the [M – H – Me – CO]^‐? and [M – H – Me – CO₂]^‐? ions provide some mechanistic and structural insights.     

Chromatographic Behavior of C60 and C70 Fullerenes at Subambient Temperature with n-Alkanes Mobile Phases

The influence of temperature on the retention and separation of C60 and C70 fullerenes was studied under HPLC conditions. Particularly, chromatographic experiments were conducted using moderate carbon loaded octadecylsilica stationary phase and homologous series of n-alkanes including n-pentane, n-hexane and n-heptane as the mobile phases. All studies were performed across wide range of subambient temperature from −80 to +20 °C. From practical point of view the best chromatographic conditions for baseline separation of the components of interest were selected. The retention of analytes was strongly affected by temperature and below minus 30 °C strong deviation from van't Hoff behavior was observed. To explore this phenomenon selected thermodynamic parameters including changes of enthalpy (ΔH^o) and changes of entropy (ΔS^o) were estimated. Positive values of the ΔH^o and ΔS^o at low temperature region may indicate the lack of the interaction with the stationary phase ligands. A possible retention mechanism at different temperatures for C60 and C70 molecules has been discussed.     

High‐performance liquid chromatographic enantioseparation of amino alcohol analogues possessing 1,2,3,4‐tetrahydroisoquinoline skeleton on polysaccharide‐based chiral stationary phases

The stereoisomers of 1,2,3,4‐tetrahydroisoquinoline amino alcohol analogues and derivatives thereof were separated in normal‐phase mode on chiral stationary phases based on preprepared silica coated with cellulose tris‐(3,5‐dimethylphenyl carbamate), cellulose tris‐(3‐chloro‐4‐methylphenyl carbamate), cellulose tris‐(4‐methylbenzoate) or cellulose tris‐(4‐chloro‐3‐methylphenyl carbamate). On all the investigated chiral columns, the retention and the enantioseparation were influenced by the nature and the concentrations of the mobile phase components and additives, and also the temperature. Experiments were performed in the temperature range 10–50°C. Thermodynamic parameters were calculated from plots of lnα vs 1/T. On these polysaccharide‐based chiral columns, both enthalpy‐driven separations and entropy‐controlled enantioseparations were observed. The latter was advantageous with regard to the shorter retention and greater selectivity at high temperature. The sequence of elution of the stereoisomers was determined in all cases. Copyright © 2014 John Wiley & Sons, Ltd.     

Liquid chromatographic separation and thermodynamic investigation of stereoisomers of darunavir on Chiralpak AD‐Hcolumn

Liquid chromatographic separation of stereoisomers of darunavir on Chiralpak AD‐H, a column containing the stationary phase coated with amylose tris(3,5‐dimethylphenylcarbamate) as a chiral selector, was studied under normal‐phase conditions at different temperatures between 20 and 50°C. The effect of quality and quantity of different polar organic modifiers viz: methanol, ethanol, 1‐propanol, and 2‐propanol in the mobile phase as well as column temperature on retention, separation, and resolution was investigated and optimized. The optimum separation was accomplished using a mobile phase composed of n‐hexane/ethanol/diethyl amine (80:20:0.1 v/v/v) at 40°C. Apparent thermodynamic parameters ΔH⁰ and ΔS* were derived from the Van't Hoff plots (lnk′ versus 1/T) and used to explain the strength of interactions between the stereoisomers and amylose tris(3,5‐dimethylphenylcarbamate) coated chiral stationary phase.     

Study of mechanism retention of nonvolatile alkylphosphonic acids on unmodified selected polymer-based ion exchanger columns

The retention of alkylphosphonic acids (MPA, EPA, PPA, and BPA) and inorganic anions (Cl⁻, NO₃⁻, and SO₄²⁻) were studied on two anion exchangers, e.g., Dionex IonPac AS4A-SC (250 ± 4 mm I.D.) and Satisfaction P 4000-SAX (50 ± 4.6 mm I.D.) under isocratic elution conditions with evaporative light scattering detector. The plots of retention factors, k, of organic anions vs. the reciprocal of eluent ion concentration show good linearity. The major retention mechanisms are interpreted as ion exchange and some others interactions. The C effect of organic modifier added to the aqueous buffered mobile phase is also investigated.     

High‐performance liquid chromatographic enantioseparation of fluorinated cyclic β3‐amino acid derivatives on polysaccharide‐based chiral stationary phases. Comparison with nonfluorinated counterparts

The stereoisomers of five fluorinated cyclic β³‐amino acid derivatives and their nonfluorinated counterparts were separated on chiral stationary phases containing as chiral selectors cellulose tris‐(3,5‐dimethylphenyl carbamate), cellulose tris‐(3‐chloro‐4‐methylphenyl carbamate), cellulose tris‐(4‐methylbenzoate), cellulose tris‐(4‐chloro‐3‐methylphenyl carbamate), amylose tris‐(3,5‐dimethylphenyl carbamate) or amylose tris‐(5‐chloro‐2‐methylphenyl carbamate). The enantioseparations were carried out in normal‐phase mode with n‐hexane/alcohol/alkylamine mobile phases in the temperature range 5–40 °C. The effects of the mobile phase composition, the nature and concentration of the alcohol and alkylamine additives, the structures of the analytes and temperature on the separations were investigated. Thermodynamic parameters were calculated from plots of ln α vs. 1/T. The Δ(ΔH°) values ranged between ?5.0 and +1.6 kJ/mol, while Δ(ΔS°) varied between ?12.6 and +5.7 J/mol/K. The enantioseparation was enthalpically controlled, the retention factor and the separation factor decreasing with increasing temperature, but entropically controlled separation was also observed. The elution sequence was determined for all of the investigated analytes. Copyright © 2016 John Wiley & Sons, Ltd.     

Pseudonocardides A – G,New γ‐Butyrolactones from Marine‐derived Pseudonocardia sp. YIM M13669

Seven new γ‐butyrolactones, named pseudonocardides A – G ( 1  –  7 ), were isolated from the marine‐derived actinomycete strain Pseudonocardia sp. YIM M13669. Their structures were elucidated on the basis of spectroscopic data including 1D‐ and 2D‐NMR, and HR‐ESI‐MS. The absolute configuration of 1 was determined by X‐ray crystallographic analysis of 1a (4‐bromobenzoate derivative of 1 ). The antibacterial activity against Mycobacterium smegmatis MC²155 and cytotoxicities of compounds 1  –  7 were evaluated in this study.     

Tweaking the affinity of aryl‐substituted diazosalicylato‐ and pyridine ligands towards Zn (II) and its neighbors in the periodic system of the elements,Cu (II) and Cd (II), and their antimicrobial activity

A series of six new Zn (II) compounds, viz., [Zn(HL^ASA)₂(Py)₂] ( 1 ), [Zn(HL^MASA)₂(Py)₂] ( 2 ), [Zn(HL^MASA)₂(4‐MePy)₂] ( 3 ), [Zn(HL^CASA)₂(4‐MePy)₂] ( 4 ), [Zn(HL^BASA)₂(Py)₂] ( 5 ), [Zn(HL^BASA)₂(4‐MePy)₂] ( 6 ) and representative Cu (II) and Cd (II) complexes, viz., [Cu(HL^ASA)₂(Py)₂(H₂O)] ( 7 ) and [Cd(HL^BASA)₂(Py)₃] ( 8 ) [(HL^XASA)^? = para‐substituted 5‐[(E)‐2‐(aryl)‐1‐diazenyl]‐2‐hydroxybenzoate with X = H (ASA), Me (MASA), Cl (CASA) or Br (BASA); Py = pyridine; 4‐MePy = 4‐methylpyridine] have been synthesized and characterized by spectroscopic techniques and single‐crystal X‐ray diffraction analysis. The structural characterization of the compounds revealed distorted tetrahedral ( 1 – 6 ), square‐pyramidal ( 7 ) and pentagonal‐bipyramidal ( 8 ) coordination geometries around the metal atom, in which the aryl‐substituted diazosalicylate ligands are coordinated only through the oxygen atoms of carboxylate groups, either in an anisobidentate or isobidentate mode; meanwhile, the 2‐hydroxy groups of the monoanionic ligand (HL^XASA)^? are involved only in intramolecular O‐H···O hydrogen bonds with the carboxylate function. In the crystal structures of 1 – 8 , the complex molecules are assembled by π‐stacking interactions giving mostly infinite 1D strands. The intermolecular binding in the solid state structures is accomplished by diverse additional non‐covalent contacts including C‐H···O, C‐H···N, C‐H···π, C‐H···Br, O···Br, Br···π and van der Waals contacts. Although the primary and secondary ligands in the Zn (II) complex series 1 – 6 carry different substituents at the periphery (X = H, Me, Cl, Br for (HL^XASA)^? and R = H, Me for 4‐Py‐R), five of the crystal structures were isostructural. Additionally, the antimicrobial activity of the pro‐ligands H₂L^XASA and their Zn (II), Cu (II) and Cd (II) compounds were studied in a comparative manner, showing high sensitivity (IZD ≥ 20) against Bacillus subtilis.     

Styrene‐N‐phenylacrylamide co‐polymer modified silica monolith particles with an optimized mixing ratio of monomers as a new stationary phase for the separation of peptides in high performance liquid chromatography

A stationary phase was prepared by chemical derivatization of the support particles with a layer of copolymer composed of styrene and N‐phenyl acrylamide. Silica monolith particles of ca. 2.6 µm (volume‐based average) have been prepared as the support particles by sol‐gel reaction followed by differential sedimentation. The particles were reacted with 3‐chloropropyl trimethoxysilane followed by sodium diethyldithiocarbamate to introduce an initiator moiety. Then, the copolymer layer was immobilized via reversible addition‐fragmentation transfer polymerization. The resultant phase was packed in glass‐lined stainless‐steel micro‐columns (1 x 150 mm) and evaluated for the separation of a mixture composed of five peptides (Trp‐Gly, Thr‐Tyr‐Ser, angiotensin I, isotocin and bradykinin). The effect of monomer mixing ratio (styrene versus N‐phenyl acrylamide) on the chromatographic separation efficiency of the stationary phase was examined. A number of theoretical plates (N) as high as 33 600 plates/column (224 000 plates/m, 4.46 µm plate height) was achieved using the column packed with the optimized stationary phase. The column‐to‐column reproducibility based on three columns packed with three different batches of stationary phase was found satisfactory in separation efficiency, retention factor, and asymmetry factor.     

Novel aluminium compounds derived from Schiff bases: Synthesis,characterization and catalytic performance in hydroboration

Seven novel aluminium complexes supported by Schiff base ligands derived from o‐diaminobenzene or o‐aminothiophenol were synthesized and characterized. The reactions of AlMe₃ with L¹ (N,N′‐bis(benzylidine)‐o‐phenylenediamine) and L² (N,N′‐bis(2‐thienylmethylene)‐o‐phenylenediamine) gave the complexes L¹AlMe₃ ( 1 ) and L²AlMe₂ ( 2 ), respectively, which involved two types of reaction mechanisms: one was proton transfer and ring closure, and the other was alkyl transfer. Complexes L³AlMe₂ (HL³ = 4‐chlorobenzylidene‐o‐aminothiophenol) ( 3 ), L⁴AlMe₂ (HL⁴ = 2‐thiophenecarboxaldehyde‐o‐aminothiophenol) ( 4 ), L³AlH(NMe₃) ( 5 ), L⁴AlH(NMe₃) ( 6 ) and L⁵AlH(NMe₃) (HL⁵ = 4‐methylbenzylidene‐o‐aminothiophenol) ( 7 ) were prepared by reacting HL^3–5 with equimolar AlMe₃ or H₃Al?NMe₃, respectively. Compounds 3 – 7 feature an organic–inorganic hybrid containing CNAlSC five‐membered ring. All complexes were characterized using ¹H NMR and ¹³C NMR spectroscopy, X‐ray crystal structure analysis and elemental analysis. The efficient catalytic performances of 1 – 7 for the hydroboration of carbonyl groups were investigated, with compound 4 exhibiting the highest catalytic activity among all the complexes.     

Study of the retention behavior of small polar molecules on different types of stationary phases used in hydrophilic interaction liquid chromatography

The retention behavior of a large group of analytes (35) with varied properties (pK_a and logP) was studied on eight hydrophilic interaction LC columns with different surfaces, stationary phase chemistries, and types of particles. The acetonitrile content (5–95%), buffer concentration (0.5–200 mM), and pH of the mobile phase (3.8 and 6.8) were evaluated for their effects on the retention behavior. The type of stationary phase had a significant impact on the selectivity and retention time of the tested analytes. Completely different selectivity was observed on the aminopropyl stationary phase. In this study, the influence of the buffer concentration was similar for all tested columns, except for the aminopropyl stationary phase. Increasing the buffer concentration led to decreased retention times for the basic compounds and increased retention times for the acidic compounds, while the inverse behavior was observed on the aminopropyl stationary phase. The selectivity of the individual stationary phases was evaluated at pH 3.8 and 6.8. Much lower selectivity differences between the stationary phases were observed at pH 6.8 than pH 3.8. Bare silica stationary phases were used in the comparison of the particles (fused‐core and fully porous particles of 3 and 1.7 μm) and the columns provided by different manufacturers.     

High‐performance liquid chromatographic enantioseparation of isoxazoline‐fused 2‐aminocyclopentanecarboxylic acids on a chiral ligand‐exchange stationary phase

The application of a chiral ligand‐exchange column for the direct high‐performance liquid chromatographic enantioseparation of unusual β‐amino acids with a sodium N‐((R)‐2‐hydroxy‐1‐phenylethyl)‐N‐undecylaminoacetate‐Cu(II) complex as chiral selector is reported. The investigated amino acids were isoxazoline‐fused 2‐aminocyclopentanecarboxylic acid analogs. The chromatographic conditions were varied to achieve optimal separation. The effects of temperature were studied at constant mobile phase compositions in the temperature range 5–45°C, and thermodynamic parameters were calculated from plots of lnk or lnα versus 1/T. Δ(ΔH°) ranged from –2.3 to 2.2 kJ/mol, Δ(ΔS°) from –3.0 to 7.8 J mol^?1 K^?1 and –Δ(ΔG°) from 0.1 to 1.7 kJ/mol, and both enthalpy‐ and entropy‐controlled enantioseparations were observed. The latter was advantageous with regard to the shorter retention and greater selectivity at high temperature. Some mechanistic aspects of the chiral recognition process are discussed with respect to the structures of the analytes. The sequence of elution of the enantiomers was determined in all cases.     

The effects of temperature and mobile phase on the retention of aliphatic carboxylic acids in hydrophilic interaction chromatography on zwitterionic stationary phases

The influence of the mobile phase and temperature, on the retention behavior of seven aliphatic acids (pyruvic, gluconic, 2‐oxoglutaric, tartaric, malic, oxalic, and citric acid) in hydrophilic interaction liquid chromatography on zwitterionic stationary phases with sulfobetaine and phosphorylcholine ligands is investigated. In agreement with the van't Hoff model, most acids show linear ln k versus 1/T plots. However, the retention of structurally symmetrical oxalic and tartaric dicarboxylic acids is almost independent of temperature, or slightly increases at rising temperature. The experimental parameters of the van't Hoff plots suggest positive entropic contributions to the retention of these symmetrical acids, possibly connected with changes in molecular symmetry on their adsorption. The type of the zwitterionic stationary phase and the mobile phase composition (the molar concentration of acetate buffer and the volume fraction of acetonitrile) affect the retention and the selectivity of the separation of the acids.     

Dose‐, time‐and lipophilicity‐dependent silver(I)–N‐heterocyclic carbene complexes: Synthesis,characterization and interaction with plasmid and Aedes albopictus DNA

Four new Ag(I)–N‐heterocyclic carbene (NHC) complexes ( 5 – 8 ) bearing symmetrically substituted NHC ligands have been synthesized starting from the corresponding benzimidazolium bromide salts which are accessible in a single step from N ‐substituted benzimidazoles (N ‐alkyl and N ‐aryl) and subsequently reacted with the basic metal source Ag₂O in acetonitrile–methanol. These compounds were characterized using elemental analyses, ¹H NMR, ¹³C NMR, Fourier transform infrared and UV–visible spectroscopic techniques, and molar conductivity. Single‐crystal structural studies for complex 5 show that the Ag(I) centre has a perfectly linear C–Ag–C coordination, with quasi‐parallel pairs of aromatic benzimidazole planes. All the complexes interact with Aedes albopictus DNA via intercalation mode by a large hypochromicity of 22 and 27% and smaller hypochromicity of 16 and 19%. Furthermore, all complexes exhibit efficient DNA cleavage activity via a non‐oxidative mechanistic pathway. The DNase activities of the test compounds revealed a time‐ and concentration‐dependent activity pattern. The Ag(I)–NHC complexes showed considerably higher DNA cleavage activity compared to their respective benzimidazolium salts at a lower concentration. The DNA cleavage of these complexes changed from a moderate effect to a good one, corresponding to the increasing lipophilicity order of the complexes as 5  <  6  <  7  <  8 (1.02, 1.05, 1.78 and 2.06 for 5 – 8 , respectively). This order is further corroborated with the DNA binding study, but with the exception of complex 5 , which shows a better binding ability for DNA (K _b = 3.367 × 10⁶) than complexes 6 – 8 (6.982 × 10⁵, 8.376 × 10⁵ and 1.223 × 10⁶, respectively).     

Phenomenon of dual‐ and single‐retention behaviors of solutes and its validation by computational simulation in linear programmed temperature gas chromatography

The current theory of programmed temperature gas chromatography considers that solutes are focused by the stationary phase at the column head completely and does not explicitly recognize the different effects of initial temperature (T_o) and heating rate (r_T) on the retention time or temperature of a homologue series. In the present study, n‐alkanes, 1‐alkenes, 1‐alkyl alcohols, alkyl benzenes, and fatty acid methyl esters standards were used as model chemicals and were separated on two nonpolar columns, one moderately polar column and one polar column. Effects of T_o and r_T on the retention of nonstationary phase focusing solutes can be explicitly described with isothermal and cubic equation models, respectively. When the solutes were in the stationary phase focusing status, the single‐retention behavior of solutes was observed. It is simple, dependent upon r_T only and can be well described by the cubic equation model that was visualized through four sequential slope analyses. These observed dual‐ and single‐retention behaviors of solutes were validated by various experimental data, physical properties, and computational simulation.     

Determination of the binding constants of modafinil enantiomers with sulfated β‐cyclodextrin chiral selector by capillary electrophoresis using three different linear plotting methods

Binding constants for the enantiomers of modafinil with the negatively charged chiral selector sulfated‐β‐CD (S‐β‐CD) using CE technique is presented. The calculations of the binding constants employing three different linearization plots (double reciprocal, X‐reciprocal and Y‐reciprocal) were performed from the electrophoretic mobility values of modafinil enantiomers at different concentrations of S‐β‐CD in the BGE. The highest inclusion affinity of the modafinil enantiomers were observed for the S‐enantiomer–S‐β‐CD complex, in agreement with the computational calculations performed previously. Binding constants for each enantiomer–S‐β‐CD complex at different temperatures, as well as thermodynamic parameters for binding, were calculated. Host–guest binding constants using the double reciprocal fit showed better linearity (r²>0.99) at all temperatures studied (15–30°C) and compared with the other two fit methods. The linear van't Hoff (15–30°C) plot obtained indicated that the thermodynamic parameters of complexation were temperature dependent for the enantiomers.