Study Of The Relationships Between Lipophilicity Of Solutes And Their Retention Behaviors In High Performance Liquid Chromatography

Lipophilicity of a compound is generally expressed by the logarithm of its n-octanol/water partition coefficient(P)or apparent n-octanol/water partition coefficient(also called distribution coefficient,D).In research of environmental pollutants,pharmaceuticals and natural products,logP and logD are often used to evaluate the environmental fate and transport of pollutants,the absorption,distribution,metabolism and excretion characteristics of drugs,as well as bioavailability and toxicity of these substances in organism.logP and logD of a solute closely related to its retention behaviors in reversed-phase liquid chromatography(RPLC),ion-suppression reversed-phase liquid chromatography(IS-RPLC),ion-pair reversed-phase liquid chromatography(IP-RPLC)and hydrophilic interaction liquid chromatography(HILIC).In this paper,based on some classic retention theories such as the linear solvent strength(LSS)model and Collander equation,relationships between lipophilicity of neutral,weakly ionizable and strong ionizable solutes and their retention behaviors in RPLC,IS-RPLC,IP-RPLC and HILIC were systematically studied.Reliable quantitative structure-retention relationship(QSRR)models were established and used for logP or logD determination,retention prediction of compounds and retention mechanism exploration of emerging stationary phases.The major contents of this paper are described as follows:(1)A reliable QSRR model between logP and the logarithm of the retention factor extrapolated to pure water(logkw)was established by employing RPLC with a dual-point retention time correction method(DP-RTC-RPLC).This QSRR model was then used to determine logP values of 29 highly focused persistent organic pollutants(POPs)including polycyclic aromatic hydrocarbons(PAHs),polychlorinated dibenzo-p-dioxins(PCDDs),polychlorinated dibenzofurans(PCDFs)and decabromodiphenylethane(DBDPE).(2)A strategy to utilize neutral model compounds for establishment of logD-logkw model for logD prediction of ionizable basic compounds by IS-RPLC was proposed.The availability of the novel protocol was justified by theoretical derivation and experimental results.Afterwards,the established logD-logkw model adopted to determine logP of harmaline,a weakly ionizable alkaloid.Using neutral compounds as model compounds are recommended when lacking suitable structure-related ionizable basic model compounds in RP-HPLC lipophilicity measurement of ionizable basic compounds.In addition,no need for rigorous pH control of the mobile phase makes the modified method more convenient.(3)The influence of n-octanol as mobile phase additive on retention behaviors of weakly ionizable acidic and basic compounds in IS-RPLC was systematically discussed.The roles of n-octanol were different in various systems.For weak acids,if strong acid acts as the ion-suppressor,n-octanol acts only as organic modifier;if weak acid or buffer salt was used as the ion-suppressor,n-octanol is not only organic modifier,but also masking agent of free silanols.For weak bases,if buffer salt was employed as the ion-suppressor,the addition of n-octanol will make QSRR model significantly worse.(4)Retention behaviors of neutral and weak acidic compounds on a cholesterol bonded stationary phase in RPLC and IS-RPLC were systematically studied.logP-logk and logD-logk models at arbitrary organic modifier composition(?)was established and evaluated.On this kind of stationary phase,when methanol is used as organic modifier,LSS model is valid in a wider logk range(-0.5<logk<1.5),thus logkw can be extrapolated with logk obtained from mobile phases containing high proportion of organic modifier.For neutral compounds and acidic compounds in neutral forms,results showed that logP could be predicted with isocratic logk at arbitrary ? with methanol as the organic modifier.(5)QSRR models for logP or logD determination by isocratic elution or gradient elution methods in RPLC and IS-RPLC were established,compared and evaluated objectively with same model compounds(including neutral,weakly ionizable acidic and basic ones)under a same chromatographic condition(column and mobile phase).Compared with the chromatographic hydrophobicity index(CHI),the apparent gradient retention factor(kg')and the gradient retention time(tg),logkw correlated better with logP or logD.To obtain logkw from retention data of several gradient elution runs is recommended for fast determination of logP or logD.(6)An IP-RPLC method was developed for logD determination of strong ionizable compounds.A QSRR model including logD,logkw,net charge(ne)and hydrogen-bond parameter was established with mixed model compounds including neutral,weak acidic and strong acidic ones.Experimental logD values of 14 benzenesulfonic acids and naphthalenesulfonic acids were determined by for the first times.(7)Retention behaviors of oligonucleotides and short double-strand DNAs under isocratic and gradient elution in IP-RPLC were discussed in detail.LSS model is applicable for describing isocratic retention of oligonucleotides and dsDNAs,which indicated that nucleic acids share the similar retention mechanism as other common small molecules in IP-RPLC.We define the parameter logkw/S obtained from LSS model in IP-RPLC the chromatographic hydrophobic and electrostatic interaction index(CHEI).CHEI has been revealed to correlate well with gradient retention time.Then a CHEI prediction model based on sequence information of nucleic acids was developed with support vector regression(SVR)algorithm.Combined with the CHEI-tg model,a novel protocol for retention prediction of nucleic acids in IP-RPLC was proposed.This protocol is simple and easy to be popularized.(8)Retention behaviors of hydrophilic neutral and ionziable compounds on three different kinds of HILIC columns.QSRR models based on logP and logD of these compounds were established.The retention mechanisms of hydrophilic compounds on different HILIC columns were speculated.HILIC columns with zwitterion-bonded phase and glycol-bonded phased is potential for logP and logD study of hydrophilic compounds.Retention behaviors of hydrophilic compounds on the triazole-bonded HILIC column is very complex and need further study.The elucidation of relationships between lipophilicity and retention behaviors contributes to our understanding of the retention mechanism of RPLC,IS-RPLC,IP-RPLC and HILIC.The developed IP-RPLC and HILIC methods for lipophilicity measurement of hydrophilic compounds complement the RPLC and IS-RPLC methods,expand the determinable lipophilicity range of HPLC and provide new experimental approaches for lipophilicity determination.In this work,reliable lipophilicity data of many compounds were determined,which offers powerful data in predicting the environmental fate,the bioconcentration and biomagnification in organism,and activity/poisoness mechanism of these compounds.The studies of mechanism of mobile phase additives and retention behaviors on new stationary phases have instructional significance for optimizing chromatographic conditions.To apply classic retention equations developed for small molecules to macromolecules provides a new idea for retention study of macromolecules.