TY - JOUR
AU - Kurbatova, Svetlana, V
AB - Abstract The retention of 16 quinoline and tetrahydroquinoline derivatives was investigated under liquid chromatography conditions using porous graphitized carbon (PGC), octadecyl silica (ODS) and hypercrosslinked polystyrene (HCLP) stationary phases. For most of the analytes, retention on PGC was greater than on ODS, while retention on HCLP was even greater than on both ODS and PGC. The non-linearity of retention dependencies on acetonitrile content in the eluent was observed for compounds containing carboxy, hydrazo and methoxy groups. The relationships between quinolines structure and their retention factors were investigated. It was found that sorption on different sorbents correlated with different descriptors. Retention on ODS was found to be highly correlated with lipophilicity only while on HCLP it depended on both lipophilicity and polarizability of the sorbates. The feature of PGC was a good correlation of retention factors with topological and geometrical parameters. Additionally, ability of PGC to form OH…π bonds with hydroxymethylquinolines was found. The observed regularities allow one to rationally optimize the chromatographic analysis of structurally similar compounds, which is very important for bioactive substances. Introduction Studying of the sorption mechanism is crucial for the development of analysis technique, but it is a complicated task in the case of liquid chromatography because of many types of intermolecular interactions in the system (1). Under constant ambient conditions liquid chromatography retention depends on the structure of analytes and on properties of mobile and stationary phases (2). Therefore, sorption models usually connect retention with structural descriptors of sorbates (2–9) and with characteristics of sorbent materials and eluent composition (10–12). Modified silica gel, porous graphitized carbon (PGC) and polymer network-based materials are often used under reversed-phase high-performance liquid chromatography (RP HPLC). Each of them has its own peculiarities, which influence on the sorption mechanism. In accordance with the solvophobic theory (13), retention in reversed-phase mode is determined by hydrophobic interactions between sorbates and polar eluent and correlates well with solutes lipophilicity (14). However, the adsorption process on modified silica is complicated by the presence of residual silanol groups (15); hypercrosslinked polystyrene (HCLP) is involved in different types of π interactions with aromatic and heteroatoms-containing solutes (16, 17); retention on PGC, in addition to dispersion and π interactions, is determined by electrostatic forces (18). Pereira in the review (19) highlighted two factors determining retention on PGC under reversed-phase conditions: hydrophobic interactions and the interaction of polarizable or polarized structural fragments of analyte with the graphite, additional to the normal dispersive interactions. Due to inductive interactions between solutes and graphite surface, polar molecules on PGC retains much stronger than less polar ones, whereas the contrary is observed with octadecylsilica (ODS) and polystyrene-divinylbenzene copolymers. It was observed by Forgacs and co-workers that the retention of barbituric acid, aniline and phenol derivatives on PGC correlates with electronic parameters of the sorbates, their sterical parameters and hydrogen donor capacity while their lipophilicity does not affect significantly on the sorption characteristics and that the position of the substituents also has a considerable effect on retention (20–24). These findings are consistent with results obtained for the heterocyclic compounds by Polyakova and Row—that their retention is determined by the interaction of the polarized or polarizable functional groups with PGC (25, 26). Bassler et al. (27, 28) in their pioneering investigation of adsorption on PGC from nonpolar phase have found that the decisive factor for the description of retention is ability of a solute to participate in donor–acceptor or dipole-induced dipole interactions, at that a localized polar segments of a molecule are responsible for retention rather than aromatic π systems. In the case of homologous series, separation of PGC shows much greater sensitivity to methylene group and methyl substitution in analytes structure comparing with ODS. Another feature of PGC is high stereoselectivity for planar and non-planar isomers due to its flat surface (19). West with co-authors in the review (29) have compared chromatographic properties of PGC and many other sorbent materials and, particularly, have noted that the graphite shows selectivity similar to that of aromatic stationary phases (phenyl-, naphtyl- or pyrenyl-bonded silica), which indicates that the special chromatographic properties of PGC could be simply related to its aromatic character. These results indicate that the retention mechanism on PGC significantly differs from that of traditional reversed-phase mode. In the group of polymer stationary phases, polystyrene-divinylbenzene copolymers (PS-DVB) are most studied adsorbent materials. Retention on PS-DVB correlates well with lipophilicity of solutes; adding of parameter accounting ability of sorbates to form hydrogen bonds enhances the goodness of the fit (30). However, one of the main disadvantages of PS-DVB is that they are too compressible and can collapse in some solvents (16). HCLP (16) is devoid of these shortcomings (31). Nevertheless, regularities of retention on this sorbent under reversed-phase mode remain almost unexplored—only few studies concerned this topic (16, 31–34). Particularly, Sychov et al. (31) have found that HCLP displays two particular retention mechanisms: first, involves π interactions, which exceed that for PGC, and second, usual for RP HPLC dispersive interactions and solvation effects. It should be noted that the objects of most of structure–retention studies are compounds with rather simple structure, which does not allow to evaluate the effect of a combination of several functional groups and sorption centers in molecules on the sorption regularities. Understanding of liquid chromatography behavior is especially important for bioactive compounds, because they are widely analyzed applying this method (35, 36). In contrast, most of the biologically active substances have complex polyfunctional structure and their sorption behavior does not fall into line with traditional regularities mostly due to specific interactions with components of chromatographic system (37–39). From this point of view, investigation of sorption of heterocycles containing quinoline moiety is of great interest because it is a very common structural motif of drug agents (40, 41) and at the same time they are able to realize different types of intermolecular interactions during the sorption process. Currently quinoline derivatives are often analyzed using RP HPLC (42–45). However, only few studies concerned regularities of their retention investigation under reversed-phase (46, 47) and thin-layer chromatography (48). The purpose of this work was to investigate and to compare regularities of quinoline and tetrahydroquinoline derivatives sorption on different sorbents—octadecyl silica, PGC and HCLP—under RP HPLC conditions. The structures of the investigated compounds contain polar and hydrophobic functional groups, which together with the abovementioned peculiarities of the sorbents allow us to expect deviations of chromatographic behavior from conventional regularities. Materials and methods Chromatographic experiment The quinoline derivatives used as analytes were received from (49, 50). Their structures are presented in Table I. Table I. Structural Formulas of the Investigated Compounds Number Formula Number Formula 1 9 2 10 3 11 4 12 5 13 6 14 7 15 8 16 Number Formula Number Formula 1 9 2 10 3 11 4 12 5 13 6 14 7 15 8 16 Table I. Structural Formulas of the Investigated Compounds Number Formula Number Formula 1 9 2 10 3 11 4 12 5 13 6 14 7 15 8 16 Number Formula Number Formula 1 9 2 10 3 11 4 12 5 13 6 14 7 15 8 16 Chromatographic experiment was performed using liquid chromatograph Varian with UV spectrophotometry detector ProStar. Detection was carried out with 254 nm wavelength. PGC (Hypercarb, 50 mm × 3 mm, pore size (d) = 5 μm, surface area (S) ~120 m2/g), octadecylsilica (ODS) (Waters Resolve C18, 150 mm × 3.9 mm, d = 5 μm, S ~ 320 m2/g) and microporous HCLP (synthesized and packed into columns in the laboratory of INEOS RAS under the guidance of Prof. VA Davankov, 150 mm × 4.6 mm, d = 3.2 μm, S ~ 1,000–1,200 m2/g) were used as sorbent materials. The column temperature was set at 25°C. Analyses were performed in isocratic mode. The eluent flow rate was set to 500 μL/min. Water–acetonitrile mixture with acetonitrile concentration 40–90% (volume) was applied as mobile phases. Tridistilled water and HPLC-gradient grade acetonitrile (Panreac, Spain) were used for the eluent preparation. Degassing of prepared water–acetonitrile mixture was performed by means of ultrasound treatment. The sorbates solutions were prepared by dissolving of dry individual substances in acetonitrile. Injected sample volume was 20 μL. Chromatograms processing was carried out using Galaxie program. The value of retention factor k was used as the measure of chromatographic retention. The dead time (tm) was determined from the “primary” chromatographic peak. The results were obtained from five parallel measurements. The k values were not corrected for the extra column volume of chromatographic system. Calculations Conformers' search was performed through computation of potential energy surface while turning rotatable bonds around its axis using the Gaussian09 software. Conformers geometry optimization was performed using density functional theory with B3LYP functional (51, 52) in 6–31 G(d) basis set. The Boltzmann averaging procedure was performed as described in (53). Dipole moment, polarizability, solvation energy and Gibbs energy values were calculated applying 6–31 G(d,p) basis set. Water-acetonitrile environment was specified by PCM-model (Polarizable Continuum model) (54). Values of molecular volume were calculated for the optimized structures using CrystalExplorer software while magnitudes of molecule projection area, molar refraction and lipophilicity were computed using Marvin program (ChemAxon). Topological descriptors were calculated by means of RdKit library (www.rdkit.org). The obtained molecular descriptors are given in Table S1. To reveal intermolecular interactions during adsorption process on PGC the corresponding sorption complexes were modeled and optimized using Gaussian09 at B3LYP/6–31 G(d) level. The layer consisted of 112 carbon atoms in hexagonal arrangement was used as a model of graphite surface. The boundary bonds were saturated with 28 hydrogen atoms. Internal layers were not considered since as it was shown in (55). that they do not affect significantly on the relative values of the adsorption energy. Grimme’s dispersion correction D3 (56) was applied for taking into account Van der Waals intermolecular interactions because of their important role in the sorption processes. Results The values of retention factors for 16 analytes on ODS, PGC and HCLP, are given in Table S2. Figure 1 shows the relationships between retention (log k) of quinolines and mobile phase composition on PGC in the form of Snyder–Soczewinski model (10), with corresponding regression equations and values of their correlation coefficients (R2). The dependencies of retention on acetonitrile concentration on ODS and HCLP are of a similar nature as the dependence on PGC. Figure 1. View largeDownload slide The relationships between retention (log k) of quinolines and mobile phase composition on PGC (φ, volume fraction of acetonitrile). Figure 1. View largeDownload slide The relationships between retention (log k) of quinolines and mobile phase composition on PGC (φ, volume fraction of acetonitrile). The retention dependence on acetonitrile content in the eluent was non-linear for compounds containing carboxy (Compound 1), hydrazo (Compounds 2 and 11) and methoxy (Compounds 6 and 12) groups. Due to non-linearity of retention–mobile phase composition relationship, we cannot use the value of the retention factor extrapolated to 100% water for chromatographic data treatment. The retention factors on ODS and PGC at content of acetonitrile in mobile phase >50% were <1 for most compounds while on HCLP many of them were sorbed irreversibly. Therefore, we compare the retention data for PGC and ODS at 40% of acetonitrile at the eluent, for PGC and HCLP at 60% content of the modifier and for ODS and HCLP at 60% acetonitrile concentration. Discussion The investigated compounds can be contingently divided into two groups: tetrahydroquinoline derivatives (Compounds 1–12) and quinoline derivatives (Compounds 13–16). The main feature of tetrahydroquinoline structure is the presence of condensed aromatic and saturated systems. It leads to the appearance of structural deformations and geometry distortion. Substituents contribute to additional structural changes and redistribution of the electron density in the molecules, which affects their physicochemical properties. In general, tetrahydroquinolines combine properties of hydrogen and electron donor; their aromatic fragment is capable to strong dispersion and π–π interactions and the heterocyclic nitrogen atom can participate in various specific interactions. Additionally, to the structural differences of the main fragment, the investigated compounds differ by the functional groups at position 4 and by substituents at positions 1, 2 and 6. It makes them capable to form various types of intermolecular interactions during chromatographic process. It was observed that the quinoline derivatives retain stronger than tetrahydroquinolines under all studied experimental conditions. This is most clearly seen for Compounds 12 and 13, which have the same substituents and differ only by basic frame structure. At the same time, tetraquinolines containing functional groups capable to specific interaction with the eluent components are characterized by the smallest retention. All of the investigated quinoline and tetrahydroquinoline derivatives, except Compound 16, were more strongly retained on PGC than ODS over the 40% content of acetonitrile in mobile phase. The similar results were previously obtained by De Matteis et al. (57)—for polar benzene derivatives retention on PGC was higher than on ODS, while for nonpolar compounds the opposite trend was observed. This peculiarity of chromatographic behavior on PGC is known as a polar retention effect on graphite (25). Given the polar nature of quinolines and tetrahydroquinolines, manifestation of this feature for them could indicate the presence of polar retention effect and, therefore, it denotes the importance of inductive interactions of sorbate–PGC in the studied system. For Compound 1, containing carboxylic group, the largest increase in retention on PGC was observed as compared to ODS. This was also found for acidic benzene derivatives in the work of De Matteis et al. (57), which was explained by the strong polar retention effect. Compound 13, in contrast to its tetrahydroquinoline analog—Compound 12, was irreversibly retained on PGC, which indicate the ability of PGC to participate in π interactions with quinolines. Conjugated system of tetrahydroquinolines is significantly less than that of quinolines, which together with their non-planar structure leads to less important role of π interactions. This type of forces was also manifested in the large increase of retention of phenyl-containing analytes on PGC compared to ODS. In addition to π interactions, significant contribution to retention of Compounds 13 and 14 on PGC could be made by specific interactions with their hydroxymethyl group. Theoretical results of geometry optimization of the corresponding sorption complexes (Figure 2) suggest a possibility of OH…π bonds formation. Similar results were obtained in (58)—the authors have observed possibility of hydrogen bonds formation between benzyl alcohol and graphitized thermal black. It is interesting that for Compounds 6 and 12, which also contain hydroxymethyl group, such interactions were not observed because of their non-planar structure (Figure 3). Figure 2. View largeDownload slide Optimized structures of the sorption complexes “Compound 13—PGC” (а) and “Compound 14—PGC” (b). OH…C(π) distances are 2.633 and 2.319 Å. Figure 2. View largeDownload slide Optimized structures of the sorption complexes “Compound 13—PGC” (а) and “Compound 14—PGC” (b). OH…C(π) distances are 2.633 and 2.319 Å. Figure 3. View largeDownload slide Optimized structure of complex “Compound 12—PGC”. OH…C(π) distance is 2.82 Å. Figure 3. View largeDownload slide Optimized structure of complex “Compound 12—PGC”. OH…C(π) distance is 2.82 Å. Positively charged moiety of Compound 15 was strongly retained over all experimental conditions; it was sorbed irreversibly on PGC with acetonitrile content <60%. Reference (57), on the contrary, pointed out that positively charged analytes are weakly retained on PGC. The chromatographic behavior of Compound 16 was unusual since its retention was very similar on ODS and PGC. This may be connected with the spatial structure of this analyte: bulky adamantyl substituent prevent close approach of heterocyclic part to the flat surface of PGC. Compounds 3, 10 and 15 have the maximal retention factors among tetrahydroquinolines due to presence of phenyl substituent in their structures and thereby high values of their molecular volume, lipophilicity and polarizability. It is interesting that elution order of Compounds 3 and 10 was changed from ODS to PGC. Apparently, it is connected with the spatial structure of this sorbates (Figure 4). Both of these molecules have non-planar geometry, but maximal projection area of Compound 3 is significantly larger than that of Compound 10. It leads to corresponding differences in its contact area with planar surface of PGC and accordingly to stronger dispersion and π–π interactions “Compound 3—PGC surface”. At the same time, ODS is not so sensitive to sorbates geometry and Compound 10 was stronger retained on it due to higher value of its lipophilicity and lower solvation energy. Figure 4. View largeDownload slide Spatial structure of Compounds 3 (а) and 10 (b). Figure 4. View largeDownload slide Spatial structure of Compounds 3 (а) and 10 (b). As it was mentioned before, most of the investigated sorbates exhibit stronger retention on HCLP when compared with both ODS and PGC. It should be noted that it could be partially caused by significantly larger surface area of HCLP as compared with the rest of the sorbents. Retention on HCLP and ODS was the most different for phenyl-containing compounds as well as for unsaturated quinolines (Compounds 3, 10 and 13–16) due to sorbate–sorbent π interactions. Compound 14, unlike other phenyl-containing sorbates, has a planar structure (Figure 5); therefore, it was retained much stronger on a flat surface of PGC than on HCLP. Figure 5. View largeDownload slide Spatial structure of Compounds 14 (а) and 15 (b). Figure 5. View largeDownload slide Spatial structure of Compounds 14 (а) and 15 (b). Carboxyl-containing Compound 1 was poorly retained on HCLP in contrast to PGC. Retention of one of the most polar sorbates—Compound 11—was greater on ODS than on HCLP. These differences in relation of PGC and HCLP to polar solutes suggest that for HCLP inductive sorbate–sorbent interactions play a minor role. Amino group of Compound 16 is conjugated with quinoline fragment, and that leads to quite high value of molecular dipole moment. This solute also contains voluminous adamantyl substituent, which causes the maximal values of its geometrical parameters and polarizability. It is interesting to note that retention of this compound was slightly changed from ODS to PGC (~1.3% at 40% of acetonitrile in the eluent). It was apparently caused by impossibility of its sufficient approximation to the planar surface of the PGC for the realization of specific interactions with quinoline frame. On the contrary, this compound interacted so strongly with HCLP that it was irreversible sorbed at whole studied range of the mobile phase compositions. Alkyl radical in ester group leads to certain increase in polarizability and lipophilicity of the compounds. It promotes the strengthening of dispersion interactions with sorbent. Accordingly, ethyl esters (Compounds 8 and 9) were retained strongly than corresponding methyl esters (Compounds 7 and 5). The relationship between retention factors and lipophilicity of the sorbates is given in Figure 6. It is nearly linear for all studied sorbents, that is generally typical for reversed-phase mode (1). The point corresponding to Compound 1 significantly deviates from the fitting line on ODS and HCLP. The reason for this may be in the presence of carboxylic group in its structure: it strongly interacts with the mobile phase solution on account of hydrogen bonds creation. However, on PGC, this sorbate was retained much stronger because of inductive interactions with the sorbent, so corresponding point fits well to the main trend. Among the sorbents studied, the points corresponding retention on PGC are more scattered on this fit, which is consistent with the results of previous works of other authors (21, 23, 25, 26) and suggests that the lipophilicity of sorbates plays a significant role in sorption on this sorbent; however, it does not determine the retention completely. Figure 6. View largeDownload slide Logarithm of retention factors—sorbates lipophilicity (log P) dependencies: (a) sorbent—PGC, eluent—acetonitrile:water 2:3; (b) sorbent—ODS, eluent—acetonitrile:water 2:3 and (c) sorbent—HCLP, eluent—acetonitrile:water 3:2. Figure 6. View largeDownload slide Logarithm of retention factors—sorbates lipophilicity (log P) dependencies: (a) sorbent—PGC, eluent—acetonitrile:water 2:3; (b) sorbent—ODS, eluent—acetonitrile:water 2:3 and (c) sorbent—HCLP, eluent—acetonitrile:water 3:2. Polarizability—retention dependencies have similar trends for the studied sorbents—sorption increases with the increasing of molecular polarizability (Figure 7). It is typical trend for ODS and HCLP. However, for PGC, it was found by Polyakova and co-author that polarizability did not influence the retention of nitrogen-containing heterocycles from water–acetonitrile mixtures due to competing specific interaction with eluent (26). Probably, the reason for the fact that this dependence manifests for the studied compounds is more uniform structure of the molecules. Figure 7. View largeDownload slide Logarithm of retention factors—sorbates polarizability (α) dependencies: (a) sorbent—PGC, eluent—acetonitrile:water 2:3; (b) sorbent—ODS, eluent—acetonitrile:water 2:3 and (c) sorbent—HCLP, eluent—acetonitrile:water 3:2. Figure 7. View largeDownload slide Logarithm of retention factors—sorbates polarizability (α) dependencies: (a) sorbent—PGC, eluent—acetonitrile:water 2:3; (b) sorbent—ODS, eluent—acetonitrile:water 2:3 and (c) sorbent—HCLP, eluent—acetonitrile:water 3:2. The obtained dependencies differ by outliers points. The point corresponding to Compound 1 is not included in the fit for ODS and HCLP, like in the case of lipophilicity–retention relationships. Compounds 10 and 12 were retained slightly stronger than it was expected from the trend for ODS and HCLP, respectively. The point corresponding to Compound 16 significantly deviates on PGC; it was retained less than it could be expected from the main trend. It can be explained by the presence of adamantyl radical in structure of the compound that prevents its close approach to the sorbent surface. Retention fits best with polarizability in the case of PGC, which can be explained by high polarizability of its surface and ability to participate in strong dispersion interactions with the solutes. Similar trend with close values of regression coefficients has dependence retention–polarizability on HCLP. Apparently, it indicates the analogous role of dispersion interactions in the sorption process on PGC and HCLP. The probable reason for the deviation of some points from the abovementioned dependencies is the difference in the strength of the orientational interactions with the polar eluent: the sorption behavior of compounds with lower dipole moment values than the main group strongly deviates from the trends. Linearity is satisfied for compounds whose dipole moment values lie in the range 3.5–7 D (Figure 8). Figure 8. View largeDownload slide Dipole moments of the studied compounds. The blackout region covers the compounds for which the “retention–polarizability” relationship on ODS and HCLP is linear. Figure 8. View largeDownload slide Dipole moments of the studied compounds. The blackout region covers the compounds for which the “retention–polarizability” relationship on ODS and HCLP is linear. Dispersion interactions, besides the polarizability, are determined by the size of the molecules. Generally, retention increases with increasing of geometric characteristics of the sorbates (Figure 9) and with rising in values of molecular shape indices, which also encode information about molecular size (59) (Figure 10). For PGC relationships, “retention–molecular volume” and “retention–molecular shape indices” have a linear form with high determination coefficient value. However, for ODS and HCLP, these correlations are mostly non-linear. A possible explanation for this lies in differences in the surface geometry of the sorbents: these correlations are close to linear for PGC, which has a planar surface, in contrast to the other stationary phases. It confirms the selectivity of PGC to the spatial structure of sorbates, noted in many studies (19). Figure 9. View largeDownload slide Logarithm of retention factors—sorbates molecular volume (V) dependencies: (a) sorbent—PGC, eluent—acetonitrile:water 2:3; (b) sorbent—ODS, eluent—acetonitrile:water 2:3 and (c) sorbent—HCLP, eluent—acetonitrile:water 3:2. Figure 9. View largeDownload slide Logarithm of retention factors—sorbates molecular volume (V) dependencies: (a) sorbent—PGC, eluent—acetonitrile:water 2:3; (b) sorbent—ODS, eluent—acetonitrile:water 2:3 and (c) sorbent—HCLP, eluent—acetonitrile:water 3:2. Figure 10. View largeDownload slide Logarithm of retention factors—sorbates first-order molecular shape indices (æ1) dependencies: (a) sorbent—PGC, eluent—acetonitrile:water 2:3; (b) sorbent—ODS, eluent—acetonitrile:water 2:3 and (c) sorbent—HCLP, eluent—acetonitrile:water 3:2. Figure 10. View largeDownload slide Logarithm of retention factors—sorbates first-order molecular shape indices (æ1) dependencies: (a) sorbent—PGC, eluent—acetonitrile:water 2:3; (b) sorbent—ODS, eluent—acetonitrile:water 2:3 and (c) sorbent—HCLP, eluent—acetonitrile:water 3:2. An important feature of the regularities of sorption on ODS is the linear correlations of retention factors with indices of maximum electrotopological state of atoms and with Balaban index (Figure 11). The index of the maximum electrotopological state of atom in the molecule characterizes its ability to specific, particularly to donor–acceptor interactions. With an increase in the value of the index, the tendency of sorbates to form donor–acceptor and hydrogen bonds with polar eluent increases and retention decreases. The Balaban index takes into account the electronegativity of the atoms, and accordingly, it considers ability of the molecule to participate in specific interactions with the mobile phase. Apparently, the absence of such correlations in the case of retention on PGC is the result of competing specific interactions of sorbates with the electron density of a graphite-like adsorbent. Figure 11. View largeDownload slide Dependences of the retention on ODS (mobile phase is acetonitrile–water 2:3) on the maximum electrotopological state of the atom in the molecule (a) and on the Balaban index (b). Figure 11. View largeDownload slide Dependences of the retention on ODS (mobile phase is acetonitrile–water 2:3) on the maximum electrotopological state of the atom in the molecule (a) and on the Balaban index (b). The correlations of retention with the rest of the calculated topological indices have the similar form. The relationship between retention on PGC and dipole moment of the analytes, shown in Figure 12, indicates that the polar retention effect manifests itself for two groups of the sorbates: for tetrahydroquinolines with hydrazide groups—Compounds 2 and 11—and for tetrahydroquinolines with ester group and carbonyl substituent—Compounds 5 and 7–9. Their retention order was changed from ODS to PGC and begins to increase with increasing of dipole moment of sorbates. For hydroxymethyl-containing quinolines and tetrahydroquinolines (Compounds 6 and 12–14) and for the rest of sorbates (Compounds 3, 4, 10 and 16), this effect is not so evident. This may be connected with the presence of functional groups in the structure of these molecules capable to strong competing interactions with the polar components of the mobile phase. Figure 12. View largeDownload slide Dependencies of logarithm of retention factors on dipole moment of the analytes: (a) sorbent—ODS, eluent—acetonitrile:water 2:3 and (b) sorbent—PGC, eluent—acetonitrile:water 2:3. Figure 12. View largeDownload slide Dependencies of logarithm of retention factors on dipole moment of the analytes: (a) sorbent—ODS, eluent—acetonitrile:water 2:3 and (b) sorbent—PGC, eluent—acetonitrile:water 2:3. The sorption mechanisms on two sorbents can be estimated using the following equation (60): lgk′A=a+blgk′B, where k′A and k′B are retention factors on the sorbents A and B, respectively. The value of the angular coefficient (b) close to 1 indicates the identity of the sorption mechanisms on the selected stationary phases. Deviations from this dependence are manifested when the mechanisms of retention on these sorbents differ. The dependencies between retention factors for ODS and PGC (Figure 13) and for ODS and HCLP (Figure 13b) were characterized by low values of the correlation coefficients. It confirms the possibility of participation of HCLP and PGC in specific interactions that are not realized on ODS. At the same time, inspection of the graph presented in Figure 13c showed that the dependence between retention factors on HCLP and PGC is close to linear (correlation coefficient 0.93) and its angular coefficient b = 1.13 can be considered close to 1. It indicates the similarity of the sorption mechanisms of the quinoline and tetrahydroquinoline derivatives on PGC and HCLP. Figure 13. View largeDownload slide Correlations between retention factors on (a) ODS and PGC (eluent—acetonitrile:water 2:3); (b) ODS and HCLP (eluent—acetonitrile:water 2:3) and (c) PGC and HCLP (eluent—acetonitrile:water 3:2). Figure 13. View largeDownload slide Correlations between retention factors on (a) ODS and PGC (eluent—acetonitrile:water 2:3); (b) ODS and HCLP (eluent—acetonitrile:water 2:3) and (c) PGC and HCLP (eluent—acetonitrile:water 3:2). Conclusions We have examined regularities of the sorption process of quinoline and tetrahydroquinoline derivatives under reversed-phase liquid chromatography conditions. The relationship between retention and acetonitrile content in the eluent was non-linear for compounds containing carboxy, hydrazo and methoxy groups, which could be explained by the strong specific interactions of these groups with components of the mobile phase. Although the same reversed-phase mode was implemented in all the used sorption systems, retention on different sorbents was determined by different descriptors. The values of logarithm of retention factors (log k) on ODS were highly correlated with lipophilicity only while log k on HCLP depended on both lipophilicity and polarizability values. These findings could be explained by the presence of aromatic rings in the HCLP structure, whereby analytes—sorbent π–π and strong dispersion interactions arise in this system. Retention on PGC was well described by topological and geometric parameters—particularly, molecular volume and molecular shape indices. The reason of that is the sorbent planar structure. Under all studied conditions, the increase in these parameters—lipophilicity, polarizability, volume and projection area—leads to increase in retention factors due to the strengthening of sorbates dispersion attraction to the sorbent. Additionally, ability of PGC to form OH…π bonds with hydroxymethylquinolines was observed—it was pointed by irreversible sorption of these compounds and confirmed by theoretical calculations. Acknowledgments Calculations were carried out at the Zeolite cluster in Samara Center for Theoretical Materials Science. We would like to thank Dr M.N. Zemtsova for providing the substances for the study and P.N. Zolotarev for providing program code for calculating topological indices. Funding The work was supported by the Russian Government (grant numbers 14.B25.31.0005 and 4.5883.2017/8.9). References 1 Dorsey , J.G. , Dill , K.A. ; The molecular mechanism of retention in reversed-phase liquid chromatography ; Chemical Reviews , ( 1989 ); 89 : 331 – 346 . Google Scholar Crossref Search ADS 2 Baczek , T. , Kaliszan , R. , Novotná , K. , Jandera , P. ; Comparative characteristics of HPLC columns based on quantitative structure-retention relationships (QSRR) and hydrophobic-subtraction model ; Journal of Chromatography A , ( 2005 ); 1075 : 109 – 115 . 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Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - Comparative Study of Quinolines and Tetrahydroquinolines Sorption on Various Sorbents from Water–Acetonitrile Solutions
JF - Journal of Chromatographic Science
DO - 10.1093/chromsci/bmy111
DA - 2019-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/comparative-study-of-quinolines-and-tetrahydroquinolines-sorption-on-f6y5i6UdLc
SP - 369
VL - 57
IS - 4
DP - DeepDyve
ER -