Characterization of lipophilicity scales using vectors from solvation energy descriptors

Ability of biomimetic chromatography and physicochemical systems to predict the skin permeation of neutral compounds. A comparison study

Five in vitro physicochemical systems have been evaluated in terms of its ability to emulate the skin permeation of neutral compounds: the permeation in two different PAMPA membranes, the classical octanol-water partition coefficient, and two biomimetic chromatography systems, one based in cerasome electrokinetic chromatography and another based in reversed-phase liquid chromatography measurements. The coefficients of the solvation parameter model equation of the mentioned systems have been compared to the ones of the skin permeation process through different comparison parameters. Moreover, a method to predict whether a physicochemical system is able to emulate satisfactorily a biological one, just by the analysis of the equation coefficients has been developed. Results reveal that the two PAMPA systems are a good choice to emulate directly the skin permeation of neutral compounds. Instead, the other three systems need a volume correction term to provide a satisfactory emulation. However, after the correction, all the evaluated systems show a similar ability to emulate well skin permeation, as predicted.

Recent advances for estimating environmental properties for small molecules from chromatographic measurements and the solvation parameter model

The transfer of neutral compounds between immiscible phases in chromatographic or environmental systems can be described by six solute properties (solute descriptors) using the solvation parameter model. The solute descriptors are size (McGowan's characteristic volume), V, excess molar refraction, E, dipolarity/polarizability, S, hydrogen-bond acidity and basicity, A and B, and the gas-liquid partition constant on n-hexadecane at 298.15 K, L. V and E for liquids are accessible by calculation but the other descriptors and E for solids are determined experimentally by chromatographic, liquid-liquid partition, and solubility measurements. These solute descriptors are available for several thousand compounds in the Abraham solute descriptor databases and for several hundred compounds in the WSU experimental solute descriptor database. In the first part of this review, we highlight features important in defining each descriptor, their experimental determination, compare descriptor quality for the two organized descriptor databases, and methods for estimating Abraham solute descriptors. In the second part we focus on recent applications of the solvation parameter model to characterize environmental systems and its use for the identification of surrogate chromatographic models for estimating environmental properties.

Evaluation of fatty tissue representative solvents in extraction of medical devices for chromatographic analysis of devices' extractables and leachables based on Abraham general solvation model

Extraction solvents used in chemical characterization (i.e., extractables and leachables testing, E&L) of fatty tissue-contacting medical devices for biocompatibility assessment per ISO 10993 have been studied by Abraham general solvation models. Chemically suitable alternative solvents to fatty tissues in solvation properties (solubility, partition, extraction, etc.) have been proposed based on Abraham's organic solvent system coefficients for water and air to condensed organic solvent phases. This evaluation is built upon the conclusion by Abraham, Acree Jr and Cometto-Muñiz that olive oil is chemically corresponding to fatty tissues. However, olive oil, if used as an extraction solvent to simulate fatty tissues, is in general not analytically expedient (realistic) per ISO 10993-18 (2020) for chromatographic analysis, and it is critical to seek alternative solvents to olive oil to perform the extraction. Although nonpolar solvents such as alkanes have been proposed and used as alternative solvents to vegetable oils, they are not equivalent to olive oil in solvation properties. Due to the practical challenge in chromatographic analysis of oil samples and the difference in migration kinetics of E&L between oil and organic solvents, the computational approach is the only realistic option to evaluate chemically alternative solvents to olive oil to simulate fatty tissue extraction. By comparing Abraham solvent system coefficients for water and air to condensed organic solvent phases distribution, a five-dimensional space distance (D) between solvents and olive oil as a reference solvent is calculated using Abraham and Martin equation to predict alternative or similar solvents to olive oil. The results of the calculation are further evaluated using E&L solubility ratio between solvents and olive oil, taking into consideration of solvent safety and physical properties. It is concluded from the study that butanone and dioxane are chemically the most suitable alternative or representative solvents to olive oil. They can be used as fatty tissue representative solvents in chemical characterization study of medical device. As Abraham solvation model is solvent system specific, not solute specific, the conclusions from this study are considered as universal.

Characterization of stationary phases in supercritical fluid chromatography including exploration of shape selectivity

Achiral packed column supercritical fluid chromatography (SFC) has shown an important regain of interest in academic and industrial laboratories in the recent years. In relation to this increased concern, major instrument manufacturers have designed some stationary phases specifically for SFC use. SFC stationary phases have been widely examined over the last two decades, based on the use of linear solvation energy relationships (LSER), which relate analyte retention to its properties and to the interaction capabilities of the chromatographic system. The method provides some understanding on retention mechanisms (normal phase, reversed phase or mixed-mode) and the possibility to compare stationary phases on a rational basis, especially through a spider diagram providing a visual classification. The latter can be used as a primary tool to select complementary stationary phases to be screened for any separation at early stages of method development, before optimization steps. In this context, the characterization of the 14 columns from the Shim-pack UC series (Shimadzu Corporation, Kyoto, Japan), which are dedicated to SFC and more broadly to unified chromatography (UC), was performed, using the LSER methodology. As in previous works, seven descriptors, including five Abraham descriptors (E, S, A, B, V) and two descriptors describing positive and negative charges (D^- and D⁺) were first employed to describe interactions with neutral and charged analytes. Secondly, two more descriptors were introduced, which were previously employed solely for the characterization of enantioselective systems and expressing shape features of the analytes (flexibility F and globularity G). They brought additional insight into the retention mechanisms, showing how spatial insertion of the analytes in some stationary phases is contributing to shape separation capabilities and how folding possibilities in flexible molecules is unfavorable to retention in other stationary phases.

Reversed phase HPLC with high temperature ethanol/water mobile phases as a green alternative method for the estimation of octanol/water partition coefficients

High temperature ethanol/water was explored as a green eluent in the reversed-phase liquid chromatographic approximation of pure water retention (log k_w) and subsequent estimation of the octanol/water partition coefficient (log P) via the Collander equation and the Leave-One-Out method. As part of this work, linear solvation energy relationships were employed to compare the log k_w extrapolated systems based on high temperature ethanol/water, ambient acetonitrile/water, and ambient methanol/water mobile phases. Based on the comparisons of the three organic modifiers, high temperature ethanol/water mobile phases were observed to provide the best estimation of log P. This conclusion is based on a high log P correlation of 0.968 R² and a near unity cos θ value of 0.997 between LSER coefficient vectors of ethanol/water estimated log P and octanol/water log P systems. The method employed in this work, further, provided high correlation for the hydrogen-bonding basicity term between the two systems.

RAFT polymerized nanoparticles: influences of shell and core chemistries on performance for electrokinetic chromatography

The performance and solvation characteristics of two novel latex nanoparticle (NP) pseudo-stationary phases (PSPs) for EKC are determined and compared to those of previously reported micellar, polymeric, and NP materials. The new NPs have shells composed of strongly acidic poly(AMPS) as opposed to the poly(acrylic acid) shell of the prior NP, and have varied hydrophobic core chemistry of either poly(butyl acrylate) or poly(ethyl acrylate). The NPs poly(AMPS) shell shows only minor changes in mobility and selectivity between pH 4.9 and 9.4, allowing adjustment of pH to influence and optimize separation performance. All of the NP phases have significantly different solvation characteristics and selectivity relative to SDS micelles. The selectivity and solvent character are similar for NPs with poly(butyl acrylate) cores and different shells, but vary significantly between NPs with poly(butyl acrylate) versus poly(ethyl acrylate) cores. NPs with poly(butyl acrylate) cores are among the least cohesive PSPs reported to date, while the NP with poly(ethyl acrylate) core is among the most cohesive. The results demonstrate that PSPs with unique selectivity can be generated by altering the chemistry of the hydrophobic core.

Retention mechanism of a cholesterol-coated C18 stationary phase: van't Hoff and Linear Solvation Energy Relationships (LSER) approaches

This study examines the effect of temperature on the dynamic cholesterol coating of a C18 stationary phase and the effect of this coating on the retention mechanism. In general, an increase in temperature results in a decrease in the mass of cholesterol coated on the stationary phase. Typically, an increase in temperature from 25°C to 55°C results in a nearly 60% reduction in the mass of cholesterol loaded. The inclusion of temperature, along with loading solvent composition and cholesterol concentration in the loading solvent, allows for loading a targeted amount of cholesterol on the stationary phase over an order-of-magnitude range. In addition to loading studies, the retention mechanism of small non-ionizable solutes was examined on cholesterol-coated stationary phases. A van't Hoff analysis was performed to assess retention thermodynamics, while a LSER approach was used to examine retention mechanism. With 50/50 water/organic mobile phases, the addition of cholesterol results in an increase in the entropic contribution to retention, with a decrease in the enthalpic contribution. The opposite trend is seen with 40/60 water/organic mobile phases. LSER system constants are also affected by a cholesterol coating on the stationary phase, with some changing to favor elution and others changing to favor retention.

Water-solvent partition coefficients and Delta Log P values as predictors for blood-brain distribution; application of the Akaike information criterion

It is shown that log P values for water-alkane or water-cyclohexane partitions, and the corresponding Delta log P values when used as descriptors for blood-brain distribution, as log BB, yield equations with very poor correlation coefficients but very good standard deviations, S from 0.25 to 0.33 log units. Using quite large data sets, we have verified that similar S-values apply to predictions of log BB. A suggested model, based on log P for water-dodecane and water-hexadecane partition coefficients, has 109 data points and a fitted S = 0.254 log units. It is essential to include in the model an indicator variable for volatile compounds, and an indicator variable for drugs that contain the carboxylic group. A similar equation based on water-chloroform partition coefficients has 83 data points and a fitted S = 0.287 log units. We can find no causal connection between these log P values and log BB in terms of correlation or in terms of chemical similarity, but conclude that the log P descriptor will yield excellent predictions of log BB provided that predictions are within the chemical space of the compounds used to set up the model. We also show that model based on log P(octanol) and an Abraham descriptor provides a simple and easy method of predicting log BB with an error of no more than 0.31 log units. We have used the Akaike information criterion to investigate the most economic models for log BB.

Effect of anesthetic structure on inhalation anesthesia: implications for the mechanism

Many previous attempts (e.g., the Meyer-Overton hypothesis) to provide a single set of physical or chemical characteristics that accurately predict anesthetic potency have failed. A finding of a general predictive correlation would support the notion of a unitary theory of narcosis. Using the Abraham solvation parameter model, the minimum alveolar concentration, MAC, of 148 varied anesthetic agents can be fitted to a linear equation in log (1/MAC) with R(2) = 0.985 and a standard deviation, SD = 0.192 log units. Division of the 148 compounds into a training set and a test set shows that log (1/MAC) values can be predicted with no bias and with SD = 0.20 log units. The two main factors that determine MAC values are compound size and compound hydrogen bond acidity, both of which increase anesthetic activity. Shape has little or no effect on anesthetic activity. Our observations support a unitary theory of narcosis by inhalation anesthetics. A two-stage mechanism for inhalation anesthesia accounts for the observed structural effects of anesthetics. In this mechanism, the first main step is transfer of the anesthetic to the site of action, and the second step is interaction of the anesthetic with a receptor(s).

Cationic surfactants for micellar electrokinetic chromatography: 1. Characterization of selectivity using the linear solvation energy relationships model

MEKC and the linear solvation energy relationship (LSER) model have been applied to two series of cationic surfactants. The synthetic flexibility of the quaternary ammonium group is exploited to generate the two series, one consisting of linear substitutions and the other incorporating the ammonium into ring structures of varying size. The effects of the head group structure on the CMC, aggregation number, and electrophoretic properties of the surfactants were determined. These surfactants were also characterized with the LSER model, which allowed the contributions of five chemical factors to the interactions between solutes and the micelles to be evaluated. Trends were observed in the cohesivity and polarity of the linear surfactant series, with both increasing with the size of the head group. No trends in the LSER parameters were observed in the cyclic series, but the LSER results do show that the surfactants with cyclic head groups provide a significantly different solvation environment from the linear series. Additional trends were observed in the aggregation behavior and chromatographic properties of the surfactants. These included changes in the CMCs, aggregation numbers, EOF, and electrophoretic mobility of the micelles that correlate to changes in head group size.