反相高效液相色谱中溶质保留过程的热力学研究4: 焓熵补偿

依据液相色谱中溶质计量置换保留模型, 对溶质在反相液相色谱(RPLC)保留过程及其吸附、解吸附过程中的焓熵补偿进行了研究, 证实了在RPLC中焓熵补偿确实存在。从焓熵补偿的定义出发, 从理论上证明了溶质在保留过程中的焓熵补偿温度本质上为溶质保留值的收敛温度, 其数值为Z对1/T线性作图的斜率与截距之比。与惯常计算焓熵补偿温度的方法相比, 本文的方法所得补偿温度更为合理且不受流动相中强溶剂浓度变化的影响。     

Solute Retention in Column Liquid Chromatography. VI. Enthalphy Entropy Compensation: The Column Temperature and Mobile-Phase Composition

Abstract

The concept of enthalpy-entropy compensation in “non-polar” reverse-phase (ODS) column liquid chromatography is examined in terms of the column temperature (10[ddot]-35[ddot]C) as well as mobile-phase composition (acidified water/tetrahydrofuran) for the solutes: phenol, p-nitrophenol, 2,4-dinitrophenol, o-chlorophenol, o-nitrophenol, 2,4-dimethylphenol, 2,4- dinitro-o-cresol, 4-chloro-m-cresol, and 2,4-dichlorophenol. Plots of log (solute capacity factor, k') against reciprocal column temperature, T^?1 exhibit approximate linearity only over the volume-fraction based mobile phase compositional range ?THF = 0 to 0.2. Substantial deviations occur beyond ?THF = 0.3. Plots of In k' against -ΔH[ddot] (compensation plots) yield a random distribution of data, indicating that what enthalpy-entropy compensation may exist for the systems studied is masked both in terms of temperature and mobile-phase composition. It is concluded that interaction(s) of the solutes with the stationary sorbent are more complex than has heretofore been supposed.     

焓-熵补偿在芳香化合物位置异构体保留机理研究中的应用

系统地研究了二氯苯等６种芳香化合物在ＰＳＯ－１１－１８Ｃ６等９支冠醚聚硅氧烷和ＯＶ－１７０１,Ｃａｒｂｏｗａｘ－２０Ｍ毛细管色谱柱上分离过程的热力学参数。利用超热力学方法,考察了这些芳香化合物及其异构体在各种色谱柱上分离过程的焓－熵补偿现象。论述了各种冠醚固定液对芳香化合物的保留机理以及取代基在苯环上位置的变化对溶质与冠醚固定液之间的相互作用参数的影响。     

Concentration of sodium ion as the determining factor for the association of dansyl amino acids with the teicoplanin molecule in reversed-phase liquid chromatography

The mechanism of the binding of D,L dansyl amino acids to teicoplanin was investigated. Na+ was used as an indicator of the interactions between the solutes and teicoplanin. The number (n) of sodium ions, Na+, excluded from the solute-teicoplanin interface when analyte transfer occurred was determined. A thermodynamic study and enthalpy-entropy compensation were performed to further explore the interaction mechanism. From these results, it was shown that teicoplanin was balanced between 2 conformational states characterized by distinct enantioselective properties. This approach indicates that liquid chromatography (LC) is a useful tool to extract physicochemical and molecular information from retention data. Thus, LC can be used as a complementary technique with the conventional techniques of molecular interaction analysis.     

Mechanistic implications of the equality of compensation temperatures in chromatography

A common interpretation of the observation that two processes exhibit similar compensation temperatures in an enthalpy-entropy plot is that the two processes occur via the same "mechanism". We show that this interpretation is not rigorously allowed. In fact, the only thing that can be concluded from the observation of identical compensation temperatures is that the relative contributions of enthalpy and entropy to the overall free energy are the same in the two processes. Since it is possible that two processes occur via different mechanisms that, by chance, result in the same relative blends of enthalpy and entropy, the observation of identical compensation temperatures cannot be used as evidence for mechanistic identity. If two processes exhibit different compensation temperatures, however, it can logically be concluded that the two processes are mechanistically distinct.     

Retention of polycyclic aromatic hydrocarbons on propyl-phenyl stationary phases in reversed-phase high performance liquid chromatography

A study on the retention of PAHs on three Propyl-phenyl stationary phases was conducted, assessing absolute retention, selectivity, retention correlation and thermodynamic behaviour. The chromatographic retention data revealed that each of the three Propyl-phenyl columns exhibited differences in absolute retention, however, comparison of the compensation temperatures derived from enthalpy-entropy compensation plots showed that the underlying processes responsible for the retention on these columns were the same.     

Investigation of the low temperature effect in reversed phase micro high performance liquid chromatography

The low temperature effect has been investigated in reversed phase micro high performance liquid chromatography with various aqueous mobile phase systems by an approach based on enthalpy-entropy compensation. The compensation temperatures, Tc , were determined for these systems, and the results show that the retention mechanism in a lower temperature column is similar to that in a normal temperature column in the reversed phase mode at mobile phase water contents above 9.8%. At lower water contents, the separation mechanism is different from that in the reversed phase mode, but otherwise very similar to that in the normal phase mode.     

Microfluidic thermodynamics of the shift in thermal stability of DNA duplex in a microchannel laminar flow

This paper reports the shift in thermal stability of DNA duplex and its thermodynamics spectroscopically, caused by stretching and orientation of DNA strands in a microchannel laminar flow. For direct spectroscopic measurement of the microchannel, we prepared an in-house temperature-controllable microchannel-type flow cell. The melting curves of DNA oligomers in a microchannel laminar flow were measured. For DNA oligomers with more than 10 base pairs, the melting curve shifted to the high-temperature side with higher flow speed. However, for 8-base-pair DNA oligomers, a change in the melting profile was not observed in batchwise and microchannel flows. We undertook microfluidic thermodynamic analysis to elucidate details of the shift in thermal stability of the DNA duplex in a microchannel laminar flow. Enthalpy-entropy compensation is applicable to the microfluidic thermal stability shift. We studied the relationships between the enthalpy-entropy compensation and DNA strand length or flow speed. Results showed that the enthalpy-entropy compensation was influenced by both DNA strand length and flow speed, and the penalties of enthalpy were 2-12% greater than the benefits of entropy.     

Indirect detection in liquid chromatography I. Response models for reversed-phase ion-pairing systems

Summary Ionic compounds without inherent detectable properties show good response in reversed phase ion-pairing systems containing a detectable ionic compound (probe) in the mobile phase. The basis for the indirect detection is the common interaction effect for solutes and probe caused by the limited binding capacity of the adsorbent. Quantitative models for solutes and probe with the same or opposite charges show a simple dependence of the response on the retention of the solute and the coverage of the binding surface by the probe. Applications of the models have shown good agreement between calculated and found response. The binding to sites of different kinds on the adsorbent and possibly ion-pair formation in the mobile phase have an influence on the indirect response. Uncharged solutes give low response in such systems owing to low binding to the competitive sites.     

Separation of the enantiomers of 2-phenylcyclopropanecarboxylate esters by capillary gas chromatography on derivatized cyclodextrin stationary phases

Summary Separation of the enantiomers of 2-phenylcyclopropanecarboxylate esters has been investigated on derivatized cyclodextrin chiral stationary phases (CD CSPs) to enable direct determination of the enantiomeric purity of the products of enantioselective cyclopropanation. Four stereoisomers of these chiral compounds could be resolved to baseline on permethylated β-cyclodextrin CSP. Some unusual phenomena, iso-enthalpy retention behavior and entropically driven chiral separation, were observed for the enantioseparation of 2-phenylcyclo-propanecarboxylates on the CD CSPs. Thermodynamic parameters were evaluated and an enthalpy-entropy compensation effect was observed forn-alkyl esters of 2-phenylcyclopropanecarboxylate separated on CD CSPs.     

A multidisciplinary approach to probing enthalpy-entropy compensation and the interfacial mobility model

In recent years, interfacial mobility has gained popularity as a model with which to rationalize both affinity in ligand binding and the often observed phenomenon of enthalpy-entropy compensation. While protein contraction and reduced mobility, as demonstrated by computational and NMR techniques respectively, have been correlated to entropies of binding for a variety of systems, to our knowledge, Raman difference spectroscopy has never been included in these analyses. Here, nonresonance Raman difference spectroscopy, isothermal titration calorimetry, and X-ray crystallography were utilized to correlate protein contraction, as demonstrated by an increase in protein interior packing and decreased residual protein movement, with trends of enthalpy-entropy compensation. These results are in accord with the interfacial mobility model and lend additional credence to this view of protein activity.     

Excess free energies and entropies of aqueous solutions of methyl-D-pyranosides at 25°C

The excess Gibbs free energies of four -methyl-D-aldopyranosides in water were determined at 25°C from isopiestic measurements. The excess entropies were obtained by subtracting the free energy values from the enthalpy data. The results show a remarkable enthalpy-entropy compensation. The positive sign of the excess free energies is interpreted as due to the favorable solute-solvent interactions that prevent the favorable solute-solute interaction. The signs and magnitudes of the pair interaction coefficients are attributed to the release of water from the hydration cospheres during the concentration process. The excess properties seem to depend, however, on the stereochemistry of the solutes. This effect is discussed and attributed to the different specific stabilities of the hydration shells.     

Retention mechanism of poly(ethylene oxide) in reversed-phase and normal-phase liquid chromatography

The retention behavior of low- and high-molecular-mass poly(ethylene oxide) (PEO) in reversed-phase (RP) and normal-phase (NP) liquid chromatography was investigated. In RPLC using a C18 bonded silica stationary phase and an acetonitrile-water mixture mobile phase, the sorption process of PEO to the stationary phase showed deltaH(o) > 0 and deltaS(o) > 0. Therefore, PEO retention in RPLC separation is an energetically unfavorable, entropy-driven process, which results in an increase of PEO retention as the temperature increases. In addition, at the enthalpy-entropy compensation point the elution volume of PEO was very different from the column void volume. These observations are quite different from the RPLC retention behavior of many organic polymers. The peculiar retention behavior of PEO in RPLC separation can be understood in terms of the hydrophobic interaction of this class of typical amphiphilic compounds with the non-polar stationary phase, on the one hand, and with the aqueous mobile phase, on the other. The entropy gain due to the release of the solvated water molecules from the PEO chain and the stationary phase is believed to be responsible for the entropy-driven separation process. On the other hand, in NPLC using an amino-bonded silica stationary phase and an acetonitrile-water mixture mobile phase, PEO showed normal enthalpy-driven retention behavior: deltaH(o) < 0 and deltaS(o) < 0, with the retention decreasing with increasing temperature and PEO eluting near the column void volume at the enthalpy-entropy compensation point. Therefore, high-resolution temperature gradient NPLC separation of high-molecular-mass PEO samples can be achieved with relative ease. The molecular mass distribution of high-molecular-mass PEO was found to be much narrower than that measured by size-exclusion chromatography.     

中性芳香族溶质在脂质体电动色谱中的热力学分配行为

脂质体电动色谱是一种理想的评价药物与生物膜相互作用的模型. 在288～323 K范围内测定了中性芳香族溶质在脂质体电动色谱中的分配系数, 通过三项式拟合Van't Hoff图获得了一系列的热力学参数, 研究了溶质在脂质体电动色谱中的热力学分配行为. 结果表明, 分配系数随体系温度的升高和苯环所带亚甲基数目的增加而增大. 从288到323 K, ΔH＞0, －TΔS＜0, ΔG＜0, 溶质在脂质体电动色谱中的分配过程为熵驱动过程. 从288到298 K, 脂质体电动色谱分配系统的ΔC_p为负值, 其表现行为与经典疏水作用一致. 从303到323 K, 脂质体电动色谱分配系统的ΔC_p为正值, 其表现行为与经典疏水作用不完全吻合. ΔH和ΔS呈线性关系, 该分配系统存在焓熵补偿.     

Influence of temperature and mobile phase composition on retention properties of oligomeric bonded phases in reversed-phase liquid chromatography (RPLC)

Summary The effect of temperature and mobile phase composition (methanol-water) on the retention behaviour of an oligomeric series of n-octylsilyl bonded phases in reversed-phase liquid chromatography has been investigated. Plots of lnk against 1/T (van't Hoff plot) and the enthalpy of transfer (ΔH^o) yields linear relationships under the conditions studied. The ΔH^o values of the aromatic hydrocarbons and n-alkyl benzoates are higher than those of the polar compounds due to their higher level of interaction with the stationary phase. A linear plot of ΔH^o vs. ΔS^o suggest that the retention process, which is essentially controlled by non-specific (dispersive) interactions between the solutes and the bonded ligands, is identical for all cases evaluated. The existence of similar retention mechanisms is confirmed by the constant value of the enthalpy-entropy compensation temperature of the columns for a given class of componds. As expected, decreasing the methanol content (% v/v) of the mobile phase results in increased eluite retention times. The methylene and phenyl selectivities are found to be independent of the carbon content of the stationary phases and varied only with the eluent composition. In addition to their high stability under aggressive mobile phase conditions as previously reported, the results of this study generally showed that the solute retention process on oligomeric phases are similar to those exhibited by the conventional reversed phases.     

Chiral discrimination of phenoxypropionic acid herbicides on teicoplanin phase: Effect of mobile phase modifier

Summary The chiral recgonition mechanism for a series of phenoxypropionic acid herbicides was investigated in reversed phase liquid chromatography (RPLC) on a teicoplanin stationary phase over a wide range of column temperatures. Thermodynamic constants, of the transfer of an enantiomer from the mobile to the teicoplanin stationary phases were determined. The van't Hoff plots for all solutes had a break at a critical temperature T^* showing a variation in the enantiomer retention mechanism due to a change in the conformational state of the teicoplanin, structure. Additionally, enthalpy-entropy compensation confirmed both the change in enantiomer interaction mechanism observed for regions T<T^* and T>T^* and the independence of this mechanism from (i) herbicide molecular structure,s i.e. the position of the chloro group on the phenol ring and (ii) the carbon absolute configuration. Moreover, the increasing enantioselectivity with increasing methanol fraction in the mobile phase was attributed to restriction of the solute association in the teicoplain, stationary phase, leading to favorable stereoselective interactions. This behavior was used to optimize chromatographic conditions for separation of herbicide enantiomers on teicoplanin.     

Combined supercritical fluid chromatographic tests to improve the classification of numerous stationary phases used in reversed-phase liquid chromatography

In this paper, we present a combination of a key-solute test based on retention and separation factors of large probe solutes (carotenoid pigments) and a quantitative structure-retention relationship analysis based on the retention factors of small probe solutes (aromatic compounds), both performed in supercritical fluid chromatography, to investigate the different chromatographic behaviour of octadecylsiloxane-bonded stationary phases of all sorts: classical, protected against silanophilic interactions or not, containing polar groups (endcapping groups or embedded groups). The results indicate that the two approaches chosen (carotenoid test and solvation parameter model) are complementary and provide precise information on the chromatographic behaviour of ODS phases. The applicability of the classification to the selection of stationary phases is evidenced with some examples of separations.     

Molecular recognition of alpha-cyclodextrin (CD) to choral amino acids based on methyl orange as a molecular probe

The molecular recognition interaction of alpha-CD to chiral amino acids was investigated by using spectrophotometry based on methyl orange as a molecular probe. The molecular recognition ability depended on the inclusion formation constants. The molecular recognition of alpha-CD to aromatic amino acids was the order: DL-tryptophan > L-tryptophan > L-phenylalanine > L-tyrosine approximately DL-beta-3,4-dihydroxy-phenylalanine; whereas for aliphatic amino acids, the order was: L-iso-leucine > L-leucine approximately L-methionine approximately DL-mehtionine > D-leucine. The effect of temperature on the inclusion interaction was examined and the thermodynamic parameters of inclusion process, delta G, delta H, delta S, were determined. The experimental results indicated that the inclusion process was an exothermic and enthalpy-driven process accompanied with a negative or minor positive entropic contribution. The inclusion interaction between alpha-CD and amino acids satisfied the law of enthalpy-entropy compensation. The compensation temperature was 291 K.     

Comparative study of divalent metals and amines as silanol-blocking agents in reversed-phase liquid chromatography.

In this work we compare the silanol-blocking ability of different alkaline earth metal cations, including calcium, magnesium and barium, and strong amine silanol blockers, such as triethylamine and octylamine, using six basic probe solutes at pH 7 on a conventional octadecylsilane phase. Some amines are better blocking agents than the metal cations but this varies with the amine and analyte structure. Among the metals, barium is the best blocker. For certain solutes barium is as effective at blocking silanols as some of the amine blockers. It produces short retention times and good peak shapes with satisfactory peak symmetry factors. However, amines with long alkyl chains, such as octylamine, are better blocking agents than barium. Peak symmetry is still poor for some solutes even in the presence of the strongest blocking agent in the eluent.