Dealing with Confounding pH-Dependent Surface Charges in Immobilized Artificial Membrane HPLC Columns

The retention capacity factor (<i>k</i>_IAM) on immobilized artificial membrane chromatography columns (IAM-HPLC) is widely used as experimental descriptor of lipophilicity. For predominantly ionized compounds, however, unexpected and significant effects of pH, buffers, and salinity on <i>k</i>_IAM have been reported. Besides zwitterionic phospholipids, IAM particles contain acidic silanol moieties and positively charged propylamine groups. The electrostatic model and experimental <i>k</i>_IAM values presented in this study for organic cations show that the net IAM surface charge is positive below pH 5 and negative above pH 5. The resulting confounding electrostatic repulsion/attraction is strongly influenced by eluent salinity: <i>k</i>_IAM values for cations differ by more than 2 orders of magnitude over the tested range of aqueous eluents. In phosphate buffered saline medium the actual lipophilicity of cationic drugs (<i>K</i>_PLIPW,cation) is overestimated by at least a factor of 2. The <i>K</i>_PLIPW,cation can be readily determined by IAM-HPLC in any 10 mM buffered eluent at pH 5. Accounting for, or avoiding, confounding electrostatic effects in IAM-HPLC considerably advances assessments of (phospho)­lipophilicity for drug discovery and for environmental risk assessment of organic cations

Ion-exchange affinity of organic cations to natural organic matter : Influence of amine type and nonionic interactions at two different pHs

Sorption to standard soil organic matter (SOM) has been studied for a wide variety of organic cations using a flow through method with fully aqueous medium as eluent. SOM sorption for weak bases (pKa 4.5-7) was stronger at pH 4.5 than at pH 7, indicating that the ion-exchange affinity of the cationic species to SOM was higher than the bulk partition coefficient of corresponding neutral species to SOM. In the range of pH 4.5-7, the effect of pH on the sorption coefficients for strong bases with pKa > 7 was small, within 0.3 log units. For cations with the molecular formula CxH yN, sorption was accurately predicted by a model accounting for size (increase with alkyl chain length) and type of charged group (1 amine >4 ammonium of equal size). In addition to the CxHyN-model, several empirical correction factors were derived from the data for organic cations with polar functional groups. Models based on KOW or pK a fail to explain differences in sorption affinity of the ionic species. Our data on ion-exchange affinities for 80 organic cations show many examples where specific chemical moieties, for example, CH2-units, aromatic rings or hydroxyl groups, contribute differently to the sorption coefficient as compared to bulk partitioning data of neutral compounds. Other sorption models that were evaluated to explain variation between compounds suffered from outliers of more than one log unit and did not reduce relative log mean standard errors below 0.5. A wider range of sorption coefficients and more sorption data in general are required to improve modeling efforts further

Ion-exchange affinity of organic cations to natural organic matter : Influence of amine type and nonionic interactions at two different pHs

Sorption to standard soil organic matter (SOM) has been studied for a wide variety of organic cations using a flow through method with fully aqueous medium as eluent. SOM sorption for weak bases (pKa 4.5-7) was stronger at pH 4.5 than at pH 7, indicating that the ion-exchange affinity of the cationic species to SOM was higher than the bulk partition coefficient of corresponding neutral species to SOM. In the range of pH 4.5-7, the effect of pH on the sorption coefficients for strong bases with pKa > 7 was small, within 0.3 log units. For cations with the molecular formula CxH yN, sorption was accurately predicted by a model accounting for size (increase with alkyl chain length) and type of charged group (1 amine >4 ammonium of equal size). In addition to the CxHyN-model, several empirical correction factors were derived from the data for organic cations with polar functional groups. Models based on KOW or pK a fail to explain differences in sorption affinity of the ionic species. Our data on ion-exchange affinities for 80 organic cations show many examples where specific chemical moieties, for example, CH2-units, aromatic rings or hydroxyl groups, contribute differently to the sorption coefficient as compared to bulk partitioning data of neutral compounds. Other sorption models that were evaluated to explain variation between compounds suffered from outliers of more than one log unit and did not reduce relative log mean standard errors below 0.5. A wider range of sorption coefficients and more sorption data in general are required to improve modeling efforts further

Sorption of Cationic Surfactants to Artificial Cell Membranes: Comparing Phospholipid Bilayers with Monolayer Coatings and Molecular Simulations

This study reports the distribution coefficient between phospholipid bilayer membranes and phosphate buffered saline (PBS) medium (<i>D</i>_MW,PBS) for 19 cationic surfactants. The method used a sorbent dilution series with solid supported lipid membranes (SSLMs). The existing SSLM protocol, applying a 96 well plate setup, was adapted to use 1.5 mL glass autosampler vials instead, which facilitated sampling and circumvented several confounding loss processes for some of the cationic surfactants. About 1% of the phospholipids were found to be detached from the SSLM beads, resulting in nonlinear sorption isotherms for compounds with log <i>D</i>_MW values above 4. Renewal of the medium resulted in linear sorption isotherms. <i>D</i>_MW values determined at pH 5.4 demonstrated that cationic surfactant species account for the observed <i>D</i>_MW,PBS. Log <i>D</i>_MW,PBS values above 5.5 are only experimentally feasible with lower LC-MS/MS detection limits and/or concentrated extracts of the aqueous samples. Based on the number of carbon atoms, dialkylamines showed a considerably lower sorption affinity than linear alkylamine analogues. These SSLM results closely overlapped with measurements on a chromatographic tool based on immobilized artificial membranes (IAM-HPLC) and with quantum-chemistry based calculations with COSMOmic. The SSLM data suggest that IAM-HPLC underestimates the <i>D</i>_MW of ionized primary and secondary alkylamines by 0.8 and 0.5 log units, respectively

Development and Evaluation of a New Sorption Model for Organic Cations in Soil: Contributions from Organic Matter and Clay Minerals

This study evaluates a newly proposed cation-exchange model that defines the sorption of organic cations to soil as a summed contribution of sorption to organic matter (OM) and sorption to phyllosilicate clay minerals. Sorption to OM is normalized to the fraction organic carbon (<i>f</i>_OC), and sorption to clay is normalized to the estimated cation-exchange capacity attributed to clay minerals (CEC_CLAY). Sorption affinity is specified to a fixed medium composition, with correction factors for other electrolyte concentrations. The model applies measured sorption coefficients to one reference OM material and one clay mineral. If measured values are absent, then empirical relationships are available on the basis of molecular volume and amine type in combination with corrective increments for specific polar moieties. The model is tested using new sorption data generated at pH 6 for two Eurosoils, one enriched in clay and the other, OM, using 29 strong bases (p<i>K</i>_a > 8). Using experimental data on reference materials for all tested compounds, model predictions for the two soils differed on average by only −0.1 ± 0.4 log units from measured sorption affinities. Within the chemical applicability domain, the model can also be applied successfully to various reported soil sorption data for organic cations. Particularly for clayish soils, the model shows that sorption of organic cations to clay minerals accounts for more than 90% of the overall affinity

Sorption of Organic Cations to Phyllosilicate Clay Minerals: CEC-Normalization, Salt Dependency, and the Role of Electrostatic and Hydrophobic Effects

Sorption to the phyllosilicate clay minerals Illite, kaolinite, and bentonite has been studied for a wide variety of organic cations using a flow-through method with fully aqueous medium as the eluent. Linear isotherms were observed at concentrations below 10% of the cation-exchange capacity (CEC) for Illite and kaolinite and below 1 mmol/kg (<1% CEC) for bentonite. Sorption to clays was strongly influenced by the electrolyte composition of the eluent but with a consistent trend for a diverse set of compounds on all clays, thus allowing for empirical correction factors. When sorption affinities for a given compound to a given clay are normalized to the CEC of the clay, the differences in sorption affinities between clays are reduced to less than 0.5 log units for most compounds. Although CEC-normalized sorption of quaternary ammonium compounds to clay was up to 10-fold higher than CEC-normalized sorption to soil organic matter, CEC-normalized sorption for most compounds was comparable between clays and soil organic matter. The clay fraction is thus a potentially relevant sorption phase for organic cations in many soils. The sorption data for organic cations to clay showed several regular trends with molecular structure but also showed quite a few systematic effects that we cannot explain. A model on the basis of the molecular size and charge density at the ionized nitrogen is used here as a tool to obtain benchmark values that elucidate the effect of specific polar moieties on the sorption affinity

Ion-Exchange Affinity of Organic Cations to Natural Organic Matter: Influence of Amine Type and Nonionic Interactions at Two Different pHs

Sorption to standard soil organic matter (SOM) has been studied for a wide variety of organic cations using a flow through method with fully aqueous medium as eluent. SOM sorption for weak bases (p<i>K</i>_a 4.5–7) was stronger at pH 4.5 than at pH 7, indicating that the ion-exchange affinity of the cationic species to SOM was higher than the bulk partition coefficient of corresponding neutral species to SOM. In the range of pH 4.5–7, the effect of pH on the sorption coefficients for strong bases with p<i>K</i>_a > 7 was small, within 0.3 log units. For cations with the molecular formula C_<i>x</i>H_<i>y</i>N, sorption was accurately predicted by a model accounting for size (increase with alkyl chain length) and type of charged group (1° amine >4° ammonium of equal size). In addition to the C_<i>x</i>H_<i>y</i>N-model, several empirical correction factors were derived from the data for organic cations with polar functional groups. Models based on <i>K</i>_OW or p<i>K</i>_a fail to explain differences in sorption affinity of the ionic species. Our data on ion-exchange affinities for 80 organic cations show many examples where specific chemical moieties, for example, CH₂-units, aromatic rings or hydroxyl groups, contribute differently to the sorption coefficient as compared to bulk partitioning data of neutral compounds. Other sorption models that were evaluated to explain variation between compounds suffered from outliers of more than one log unit and did not reduce relative log mean standard errors below 0.5. A wider range of sorption coefficients and more sorption data in general are required to improve modeling efforts further

Dealing with Confounding pH-Dependent Surface Charges in Immobilized Artificial Membrane HPLC Columns

The retention capacity factor (kIAM) on immobilized artificial membrane chromatography columns (IAM-HPLC) is widely used as experimental descriptor of lipophilicity. For predominantly ionized compounds, however, unexpected and significant effects of pH, buffers, and salinity on kIAM have been reported. Besides zwitterionic phospholipids, IAM particles contain acidic silanol moieties and positively charged propylamine groups. The electrostatic model and experimental kIAM values presented in this study for organic cations show that the net IAM surface charge is positive below pH 5 and negative above pH 5. The resulting confounding electrostatic repulsion/attraction is strongly influenced by eluent salinity: kIAM values for cations differ by more than 2 orders of magnitude over the tested range of aqueous eluents. In phosphate buffered saline medium the actual lipophilicity of cationic drugs (KPLIPW,cation) is overestimated by at least a factor of 2. The KPLIPW,cation can be readily determined by IAM-HPLC in any 10 mM buffered eluent at pH 5. Accounting for, or avoiding, confounding electrostatic effects in IAM-HPLC considerably advances assessments of (phospho)lipophilicity for drug discovery and for environmental risk assessment of organic cations