Retention parameters compounds

Relationships between lipophilicity and retention parameters obtained by RPLC methods using isocratic or gradient condition are reviewed. Advantages and limitations of the two approaches are also pointed out, and general guidelines to determine partition coefficients in 1-octanol-water are proposed. Finally, more recent literature data on Hpophilicity determination by capillary electrophoresis of neutral compounds and neutral forms of ionizable compounds are compiled. Quotation is restricted to key references for every method presented - an exhaustive listing is only given for the last few years. 

It has been shown for many RPLC methods that correlations between log Pod and retention parameters were improved by separating compounds in two classes, i.e. H-bond acceptor and donor compounds. Minick et al. [23] propose to add 0.25% (v/v) of 1-octanol in the organic porhon of the mobile phase (methanol was preferred in this study) and to prepare the aqueous portion with 1-octanol-saturated water to minimize this discriminahon regarding H-bond properties. For a set of heterogenous neutral compounds, the addition of 0.25% (v/v) of 1-octanol in methanol and the use of water-saturated 1-octanol to prepare mobile phase improve the correlahon between log few obtained on the LC-ABZ column and log Poc, [13]. 

Lombardo F, Shalaeva MY, Tupper KA et al. (2001) A tool for lipophilicity determination in drug discovery. 2. Basic and neutral compounds. J Med Chem 44 2490-2497 Nasal A, Siluk D, Kahszan R (2003) Chromatographic retention parameters in medicinal chemistry and molecular pharmacology. Curr Med Chem 10 381—426 OECD (1989) Guideline for testing of chemicals 117, (http //www.oecd.org)... 

QSAR, QSPR and similar techniques are used to relate one block of properties to another. In this dataset, six chromatographic retention parameters of 13 compounds are used to predict a biological property (Al). 

The dimensionless retention parameter X of all FFF techniques, if operated on an absolute basis, is a function of the molecular characteristics of the compounds separated. These include the size of macromolecules and particles, molar mass, diffusion coefficient, thermal diffusion coefficient, electrophoretic mobility, electrical charge, and density (see Table 1, Sect. 1.4.1.) reflecting the wide variablity of the applicable forces [77]. For detailed theoretical descriptions see Sects. 1.4.1. and 2. For the majority of operation modes, X is influenced by the size of the retained macromolecules or particles, and FFF can be used to determine absolute particle sizes and their distributions. For an overview, the accessible quantities for the three main FFF techniques are given (for the analytical expressions see Table l,Sect. 1.4.1) ... 

Assuming LEER, one can determine the relative inputs of individual structural groups, fragments or features to a property measured for a series of compounds in various chemical, physical, and biological experiments. Such obtained structural parameters (descriptors) can then be related to retention parameters. 

In Eq. (11.10) kp is the retention parameter of a parent compound, is the corresponding value for the derivative carrying n substituents and r, are retention increments due to individual substituents i. Having appropriate values for functional groups of interest one needs only to determine the retention of the parent structure and can next calculate the retention of a derivative. To get reliable predictions, the correction factors are introduced in Eq. (11.10) accounting for mutual interactions between substituents (electronic, steric, hydrogen bonding) [41,70], In cases of polyfunctional analytes the interactions between substituents make retention predictions of rather limited value. 

The driving force in chromatography for the. separation of an analyte is the equilibrium between the stationary and the mobile phases. As it was di.scus.sed in Chapter 11 in more detail, the chromatographic equilibrium can be related to the chemical potential of the compound. Unfortunately, the relationship between retention parameters and the quantities related to the chemical structure cannot be solved in. strictly thermodynamic terms. Therefore, the extra-thermodynamic approach is applied to reveal the relationships. During chromatography we do not achieve a proper equilibrium, the separation is still a result of the difference of equilibrium constants for the compounds in the stationaiy and the mobile phases. The.se equilibrium con.stants can be related to measured retention data as was discussed in the previous chapter. So whenever our chromatographic system (the stationary and the mobile phase) can be considered as two immiscible phases the retention data (equilibrium data) will provide a partition coefficient. 

The first exploitation of this relationship in a biological context was by Boyce and Milbarrow [28], who showed a relationship between the molluscicidal activity of some A -alkyltritylamines and their Rm values on TLC plates. Many publications have reported the application of TLC to determine the relative lipophilicity of compounds. The first chapter of the book Chromatographic Determination of Molecular Interactions Applications in Biochemistry, Chemistry, and Biology [29[ summarises the theory and presents the major application fields of the TLC method. The advantage of the method for the determination of lipophilicity is that the layers can be easily covered by octanol and by using an aqueous buffer the retention parameter would be directly proportional to the octanol-water partition coefficients. The drawbacks of this method are the limited reproducibility and precision. 

Application of RP-HPLC in isocratic mode. In HPLC, the most frequently used chromatographic retention parameter for characterising hydrophobicity is the logarithmic value of the retention factor (logA = log((fR - Ud/tn). where /r is the retention time of the analyte and to is the retention time of the unretained compound). The retention factor can be related directly to the chromatographic partition coefficients (A chr) according to Eiq. (12.6) ... 

Both O-alkyl hydroxylamines and, especially, arylhydrazines, are slightly oxidized compounds. The presence of any oxidizers in the reaction mixtures must be excluded. Nevertheless, in real practice, these mixtures very often contain some by-products (e.g., ArNH2, ArOH, ArH, etc.). Usually, there are no problems to reveal their chromatographic peaks, because aU of them have lower retention parameters than those for the initial reagents and, moreover, all target derivatives. The condensation reaction of the considered type can be characterized by statistically processed differences of retention indices of products and initial substrates. This mode of additive scheme permits us to estimate these analytical parameters for any new derivatives on standard nonpolar polydimethyl siloxanes. For the simplest reaction scheme, Ah— —>Bh—, AMW = MW(B) -MW(A) and ARl, = RI(B) RI(A) ... 

This set of ART values illustrate that the simplest alkylhydrazones (methyl, ethyl, dimethyl, etc.) have appropriate GC retention parameters and, theoretically, can be recommended as the derivatives for carbonyl compounds. However, in real analytical practice, these hydrazones are not used because of their low yields, especially for the aliphatic ketones. 

The formation of acetals from carbonyl compounds requires acid catalysis and (sometimes) the presence of water-coupling reagents (for instance, anhydrous CUSO4). The conversion of aliphatic aldehydes into dimethyl acetals slightly increases the retention parameters of analytes (ARI = 189 17). The cyclic ethylene derivatives (1,3-dioxolanes, ARI = 212 7, this value is valid only for acyclic carbonyl compounds) are more stable to the hydrolysis and used in GC practice... 

It is interesting to note that by analogy with chromatographic retention parameters, the values of some other properties of organic compounds may be pre-2 sented in the linear interpolated form relative to the set of reference compounds. These equivalent to indices forms are known for boiling points [6], molecular I weights [7], and molar refractions, MR , = (MW/d)(n -1 l)/( + 2), where MW is the molecular weight, is the... 

The investigation of chromatographic retention is one of the most active areas for QSRR studies using various topological indices. Many papers have been written for this important area of analytical chemistry.The first topological indices used for the prediction of retention parameters or lipophilic parameters were Randi s indices (molecular connectivity indices), the Wiener index, and the Balaban (7b) index. Selected applications of topological indices for the prediction of retention parameters of compounds separated by HPLC are covered later in this work. 

The acidity constant of the nucleoside analogue (44) has been reported. The pA a = 1 was obtained by the spectroscopic method. The high-pressure liquid chromatographic (HPLC) retention parameter for this compound has also been determined <92J0C1579>. 

RI values do not depend on column dimension or flow rate. They must be obtained in isothermal mode, and they are less dependent on temperature than other retention parameters (fc, K). Because there is a linear relationship between the logarithm of the adjusted retention times, obtained under isothermal conditions, of the components of a homologous series and their number of carbon atoms, n-alkanes that differ in two carbon atoms (z and z + 2) can be used as standards in Equation (3) with only a small loss in accuracy. Other homologous series can be used instead of n-alkanes when these compounds are not appropriate — for instance, in columns with high-polarity stationary phases. 

A second gradient was run at pH 7 to obtain the retention parameter 5, and ky, for all compounds. From the values obtained for the last-eluting peak, a composition of 32% acetonitrile was estimated to lead to an analysis time of... 

The retention factor k was defined as a retention parameter that is in contrast to the retention time fg independent of column dimensions and the mobile phase flow rate F. It is characteristic for the given analyte under given stationary and mobile phase conditions as well as temperature. The retention factor can be derived from the peak retention time fg and the column hold-up time (elution time of a nonretained compound, the so-called inert marker) and equals the ratio of the so-called net retention time fg (difference of retention time and hold-up time) and the hold-up time ... 

Both 0-ethers of oximes and some disubstituted hydrazones of asymmetrical carbonyl compounds exist in two isomeric structures with slightly different GC retention parameters (Fig. 5). 

Retention in chromatographic systems can be connected with the properties of the chromatographed compounds. It should manifest itself in quantitative structure-retention relationships (QSRR) equations, correlating retention parameters (log k) with the properties of analytes and chromatographic system revealed by molecular descriptors dipolarity/polarizability, ability to donate H-bonds, measure of analyte H-bond accepting potency, analyte molecular volume, and others. 

Besides the optimization of retention parameters and peak shaping, an additional function of derivatization is the protection of analytes from chemical transformation, which is most important for 5-analogues of hydroxy compounds. 

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