Determination of octanol-water partition coefficient for terpenoids using reversed-phase high-performance liquid chromatography

Octanol-water partition coefficients (Kow) for 57 terpenoids were measured using a RP-HPLC method. Sample detection was achieved with standard UV and refractive index detectors and required no special column treatment. Measured log Kow values for the terpenoids ranged from 1.81 to 4.48 with a standard error of between 0.03 and 0.08 over the entire range. Partition coefficients determined by the RP-HPLC method were compared against shake flask, atom/fragment contribution, fragment and atomistic methods. The HPLC values were found to give the best correlation with shake flask results. Log Kow values calculated by the atom/fragment contribution method gave the best correlation with the HPLC values when compared to fragment and atomistic methods.     

A structural analogue approach to the prediction of the octanol-water partition coefficient

A new strategy for the calculation of n-octanol/water partition coefficients is presented. Log P calculations of unknown chemicals are based on their closest structural analogues from a database of molecules with known experimental log P values. The contribution of the differing molecular parts is then estimated from a compilation of fragment contributions. Such a strategy is found to be superior to conventional group contribution methods and promises an overall enhancement of the prediction's accuracy.     

Relationships between LC retention, octanol-water partition coefficient, and fungistatic properties of 2-(2,4-dihydroxyphenyl)benzothiazoles

The retention behavior of newly synthesized compounds with antimycotic activity from the 2-(2,4-dihydroxyphenyl)benzothiazole group by high-performance liquid chromatography has been investigated. RP-18 stationary phase and methanol-acetate buffer aqueous mobile phases at pH 4 and 7.4 have been used. In the case of the mobile phase at pH 7.4, higher concentrations of water can be applied than at pH 4. The studied compounds showed regular retention behavior, their log k values decreasing linearly with an increasing concentration of methanol in the mobile phase. On the basis of these relationships, the lipophilicity (log kw), specific hydrophobic surface area (S), and isocratic chromatographic hydrophobicity index (psi0) were determined. Similar log kw values and sensitivity to changes in the structure of compounds studied for both mobile phases have been found. Moderate correlations between the chromatographic parameters and the calculated octanol-water log P values were found. Finally, the lipophilicity parameters were compared with the fungistatic properties of compounds expressed by log MIC (minimum inhibitory concentration) values to find quantitative structure activity relationship equations.     

Prediction of the environmental distribution of alkyl dinitrates – Chromatographic determination of vapor pressure p0, water solubility SH2O, gas-water partition coefficient KGW (Henry’s law constant) and octanol-water partition coefficient KOW

Alkyl nitrates are complex mixtures of homologues and isomers and are part of the large NO_y pool of important atmospheric trace compounds. The knowledge of their physico-chemical parameters allows to forecast their partitioning into the environment. Water solubility, octanol-water partition coefficient, vapor pressure, and gas-water partition coefficient have been determined for 26 alkyl dinitrates by RP-HPLC and HRGC, respectively. Octanol-water partition coefficient (K_OW) and water solubility (S_{H 2 O}) were determined by C18 reversed-phase liquid chromatography using phenylalkanes as reference standards. Retention data obtained with methanol-water mixtures as eluents were extrapolated to 100% water. The vapor pressures were determined by capillary gas chromatography on a non polar stationary phase (CP-Sil 2, Chrompack). The octanol-water partition coefficients ranges from log K_OW = 1.4 for 1,2-ethyl dinitrate up to log K_OW = 5.9 for 1,2-decyl dinitrate, respectively. The experimental results are within ± 0.15 log units in agreement with calculations of log K_OW by the increment method according to Hansch and Leo. Water solubilities are in the range of 154 mol/m³ (23.1 g/L) for 1,2-ethyl dinitrate and 0.002 mol/m³ (0.5 mg/L) for 1,2-decyl dinitrate, respectively. The vapor pressures p⁰ _L ranges from 196 Pa for 1,2-ethyl dinitrate to 0.01 Pa for 1,10-decyl dinitrate (sub-cooled liquid value). The gas-water partition constants (dimensionless) or Henry’s law constants ranges from K_GW = 0.0005 (H = 1.28 Pa m³/mol) for 1,2-ethyl dinitrate to K_GW = 0.020 (H = 51.1 Pa m³/mol) for 1,2-decyl dinitrate.     

Distribution ratio, distribution constant and partition coefficient. Countercurrent chromatography retention of benzoic acid

There is some confusion in chromatography between terms such as solute distribution ratio, distribution constant and partition coefficient. These terms are very precisely defined in the field of liquid-liquid systems and liquid-liquid extraction as well as in the field of chromatography with sometimes conflicting definitions. Countercurrent chromatography (CCC) is a chromatographic technique in which the stationary phase is a support-free liquid. Since the mobile phase is also liquid, biphasic liquid systems are used. This work focuses on the exact meaning of the terms since there are consequences on experimental results. The retention volumes of solutes in CCC are linearly related to their distribution ratios. The partition coefficient that should be termed (IUPAC recommendation) distribution constant is linked to a single definite species. Using benzoic acid that can dimerize in heptane and ionize in aqueous phase and an 18 mL hydrodynamic CCC column, the role and relationships between parameters and the consequences on experimental peak position and shape are discussed. If the heptane/water distribution constant (marginally accepted to be called partition coefficient) of benzoic acid is 0.2 at 20 °C and can be tabulated in books, its CCC measured distribution ratio or distribution coefficient can change between zero (basic aqueous mobile phase) and more than 25 (acidic aqueous mobile phase and elevated concentration). Benzoic acid distribution ratio and partition coefficient coincide only when both dimerization and ionization are quenched, i.e. at very low concentration and pH 2. It is possible to quench dimerization adding butanol in the heptane/water system. However, butanol additions also affect the partition coefficient of benzoic acid greatly by increasing it.     

Method for measuring the logarithm of the octanol-water partition coefficient by using short octadecyl-poly(vinyl alcohol) high-performance liquid chromatography columns

A simple, quick, versatile and inexpensive HPLC method to estimate the logarithm of the octanol-water partition coefficient (log Pow) employing a methanol-water gradient and a short octadecyl-poly(vinyl alcohol) (ODP) column is described. This method is different from published HPLC-based log Pow methods because it uses retention times from a rapid methanol-water gradient to directly generate log Pow estimates, rather than from a series of isocratic mixtures extrapolated to 100% water. These HPLC log Pow values have good precision and correlate well with traditional shake-flask log Pow values. If necessary, the log Pow determination (including replications) can easily be carried out using only a milligram of sample. By suppressing ionization of acids and bases by the use of a buffer in the aqueous phase, the method can measure the log Pow of neutral organic molecules at any pH between 2 and 13. The method can be used with impure material and is rapid, 7 min per run and 4 min equilibration; it lends itself to and has been utilized for high-throughput hydrophobicity determinations (we have now carried out thousands of HPLC log Pow measurements by this method).     

Determination of the protonation constant and partition coefficient of 4-methylpyridine in KNO3-toluene at 25°C

The protonation constant in 1.0 mol-dm^–3 KNO₃, and partition coefficient of 4-methylpyridine (B) between 1.0 mol-dm^–3 KNO₃, and toluene have been determined at 25°C by measuring the H₃O⁺ ion concentration with a glass electrode. The experimental work consisted of two sets of potentiometric titrations. In the first case, the titrations were carried out in an aqueous system of constant ionic strength and in the second, two-phase titrations using toluene as organic phase were performed. All the titrations were conducted automatically by the use of a computer controlled potentiometric system. The results were treated using graphical and numerical methods and the systematic erros were also considered. The formation constant of the protonated 4-methylpyridine was correlated with the composition of the aqueous phase by means of Bromley's theory.Recently deceased. This paper is dedicated to the memory of Dr. José Maria Castresana.     

Determination of octanol-water partition coefficients for carbonate esters and other small organic molecules by microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) was assessed as a tool for determination of octanol-water partition coefficients using 34 solutes encompassing 8 carbonate esters. It was confirmed that microemulsions containing 1.44-2.88% w/w SDS, 6.49% w/w 1-butanol, and 0.82% w/w n-heptane constitute a good model of octanol-water partitioning in the pH range of 1.4-7.4. Use of the migration index concept led to improved repeatability of the MEEKC method compared to the use of retention factors. Using a dynamical coating, a high electroosmotic flow at pH 1.4 and 4.75 was achieved expanding the practical pH working range of the MEEKC system. The correlation obtained between the migration index and log P was unaffected by pH indicating that the properties of the microemulsion droplets and, thus, partitioning are independent of pH. No evidence for congeneric behavior was found for the sample set comprising solutes with different hydrogen bonding properties suggesting that simple reference compounds can be used as calibrators. Lipophilicity estimates for the series of carbonate esters were obtained. The increase in lipophilicity with chain length was smaller than expected from the Hansch substituent constant, pi.     

Temperature effect on the partition and dimerization constant of benzoic acid

Making use of the regular solution theory with binary coefficients, the partition of benzole acid between aqueous benzene phases was investigated. ΔH, ΔS and ΔG thermodynamic functions associated with the hydration-corrected partition and dimerization were calculated. It was found that the partition was spontaneous at high temperatures and of the hydrates of benzoic acid, the monohydrate monomer was dominant at high temperatures.     

Chromatographic retention prediction and octanol-water partition coefficient determination of monobasic weak acidic compounds in ion-suppression reversed-phase liquid chromatography using acids as ion-suppressors

Although simple acids, replacing buffers, have been widely applied to suppress the ionization of weakly ionizable acidic analytes in reversed-phase liquid chromatography (RPLC), none of the previously reported works focused on the systematic studies about the retention behavior of the acidic solutes in this ion-suppression RPLC mode. The subject of this paper was therefore to investigate the retention behavior of monobasic weak acidic compounds using acetic, perchloric and phosphoric acids as the ion-suppressors. The apparent octanol-water partition coefficient (K^″ow) was proposed to calibrate the octanol-water partition coefficient (Kow) of these weak acidic compounds, which resulted in a better linear correlation with log kw, the logarithm of the hypothetical retention factor corresponding to neat aqueous fraction of hydroorganic mobile phase. This log K^″ow−log kw linear correlation was successfully validated by the results of monocarboxylic acids and monohydrating phenols, and moreover by the results under diverse experimental conditions for the same solutes. This straightforward relationship not only can be used to effectively predict the retention values of weak acidic solutes combined with Snyder-Soczewinski equation, but also can offer a promising medium for directly measuring Kow data of these compounds via Collander equation. In addition, the influence of the different ion-suppressors on the retention of weak acidic compounds was also compared in this RPLC mode.     

Determination of the second ionization constant of 3,5-dinitrosalicylic acid (DNSH2) and the instability constant of [DNSFe]+ and titrimetric determination of thiourea with a 3,5-dinitrosalicylate-selective electrode

Measurements made with a combination of glass and 3,5-dinitrosalicylate-selective membrane electrode in DNSHNa solutions varying in pH and ionic strength have been used to determine the second ionization constant of 3,5-dinitrosalicylic acid. Also the instability constant of the complex [DNSFe]⁺ has been determined by titrating known amounts of iron(III) with standard DNSHNa solution. A method has been developed for the semiautomatic potentiometric titration of thiourea with silver nitrate in the presence of 3,5-dinitrosalicylate ions, using the aforementioned selective electrode. Thiourea in the range 1.5–15 mg was determined with average relative error and relative standard deviation of about 1%.     

Evaluation of a reversed-phase column (supelcosil LC-ABZ) under isocratic and gradient elution conditions for estimating octanol-water partition coefficients

The solvation parameter model is used to identify suitable chromatographic models for estimating the octanol-water partition coefficient for neutral compounds of varied structure by reversed-phase liquid chromatography. The stationary phase Supelcosil LC-ABZ with methanol-water mobile phases affords a series of suitable correlation models for estimating the octanol-water partition coefficient (log KOW) under isocratic and gradient elution conditions. Isocratic separations with mobile phase compositions containing from about 25 to 40% (v/v) methanol provide the most accurate results for log KOW values in the range -0.1 to 4.0. Gradient separations programmed from 5 to 100% (v/v) methanol are suitable for faster separations of compounds with large log KOW values. The standard error in the estimate for the regression models of the predicted log KOW values against literature values are 0.135 log units for the 30% (v/v) methanol-water isocratic system and 0.263 log units for the methanol-water gradient system. Isocratic retention factors predicted from two gradient separations with gradient times of 15 and 45 min afford a poorer fit for the correlation models between log KOW and the estimated retention factors than that of either the above isocratic and gradient models. Plots of the retention factor (log k) as a function of mobile phase composition are generally non-linear. Values of log kw obtained by non-linear extrapolation to a volume fraction of 0% (v/v) methanol do not afford a useful model for estimating log KOW.     

Potentiometric determination of octanol-water and liposome-water partition coefficients (log P) of ionizable organic compounds

The logarithm of the octanol-water partition coefficient, log P, is a key physicochemical property for both pharmaceutical drugs and agrochemicals. It is also required by legislation as part of the physicochemical properties profile for high volume production chemicals. This Letter describes a simple method for determining log P values (over a wide range from −0.8 to 5.3) for 12 organic weak acids and bases using potentiometric titrations, with octanol or phosphatidyl choline liposomes as the partitioning medium. Such titrations take comparatively little time (about 30-45 min per titration), are easy to implement, and can be carried out with an inexpensive laboratory titrator.     

Determination of partition coefficient of spin probe between different lipid membrane phases

Model lipid membranes made from binary mixtures of dimyristoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DMPC/DPPC) and dimyristoylphosphatidylcholine/cholesterol (DMPC/Chol) exhibit coexistence of diverse lipid phases at appropriate temperature and composition. Since lipids in different phases show different structural and motional properties, it is expected that the corresponding spin probe electron paramagnetic resonance (EPR) spectra will be superposition of several spectral components. From comparison of proportions of spectral components of the EPR spectrum with the fractions of the corresponding lipid phases obtained from known phase diagrams the partition coefficient of spin probe methyl ester of 5-doxyl palmitate between different lipid phases was determined. The results indicate that the used spin probe partitions approximately equally between different phases.