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PubMed1999

Use of ANN modelling in structure--retention relationships of diuretics in RP-HPLC.

Structure retention relationship study, conducted by RP HPLC, was used to investigate physical chemical parameters related to the RP retention times of amiloride, hydrochlorothiazide and methyldopa in order to predict the separation of amiloride and methylclothiazide from Lometazid tablets. Retention data were obtained with an ODS column using a mobile phase methanol water (pH adjusted with phosphoric acid). Physical chemical properties were calculated directly from the molecular structure. Artificial neural networks (ANNs) were used to correlate chromatograms retention times with mobile phase composition and pH, and with physical chemical properties of amiloride, hydrochlorothiazide and methyldopa and to predict separation of amiloride and methylclothiazide from Lometazid tablets. Sensitivity analysis was performed to interpret the meaning of the descriptors included in the models. Results confirmed the dominant role of the polar modifier in such chromatographic systems. Within a series of solutes chromatographed under identical conditions, the retention parameters could be approximated by a non-linear combination of logP, logD, pKa, surface tension, parachor, molar volume and to minor extend by polarisability, reetractivity index and density. This study has demonstrated that the use ANNs techniques can result in much more efficient use of experimental information. As HPLC is the most popular analytical technique, improvements in HPLC methods development can yield significant gains in the overall analytical effort. The ANNs extension presented could be the method of choice in some advanced research settings and serves as an indication of the broad potential of neural networks in chromatography analysis.

Chromatography, High Pressure Liquid↗
PubMed2002

Retention of 125I-labeled recombinant human bone morphogenetic protein-2 by biphasic calcium phosphate or a composite sponge in a rabbit posterolateral spine arthrodesis model.

The purpose of this study was to characterize the retention kinetics of recombinant human bone morphogenetic protein-2 (rhBMP-2) applied to two calcium-based delivery matrices. Biphasic calcium phosphate (BCP) and a composite containing BCP in an absorbable collagen sponge (BCP/ACS) were evaluated using a spinal fusion model in rabbits. rhBMP-2 labeled with radioactive iodine (125I) was used as a tracer to assess in vivo retention of rhBMP-2 in the presence of these materials (nine animals per material studied). Over a 36 day study period, animals were assessed for the following: percent administered dose retained at the implant site as measured by scintigraphic imaging (counting) with a gamma camera (all animals), radiography of the implant site (all animals), radioactivity in blood and plasma (all animals), and radioactivity in the urine and feces (three animals for each material). Radioactivity data were corrected for the decay of 125I and the attenuation between the implant in vivo and the gamma camera. Differences observed between the two materials for the area under the retention vs. time profile (AUC; 988%*day for BCP vs. 1070%*day for BCP/ACS, p = 0.57) and the mean residence time (MRT; 10.2 days for BCP vs. 7.6 days for BCP/ACS, p = 0.06) were not statistically significant. Initial retention/incorporation of rhBMP-2 was slightly higher for rhBMP-2/BCP/ACS than for rhBMP-2/BCP (96.8% vs. 86.0%, p < 0.05). Animals receiving rhBMP-2/BCP showed a longer terminal retention half-life (t1/2) than did those receiving rhBMP-2/BCP/ACS (7.5 vs. 4.5 days, p < 0.05). The urinary radioactivity recovery data supported the data obtained by scintigraphy. Over the 36 day collection period, essentially complete recovery of radioactivity (dose) in urine was observed for rhBMP-2/BCP and rhBMP-2/BCP/ACS and the majority of the radioactivity (approximately 95%) was soluble in trichloroacetic acid, suggesting extensive catabolism of rhBMP-2 before renal excretion. Fecal recovery of radioactivity was low, approximately 2-3%. In conclusion, rhBMP-2 was retained at the implant site when delivered with either BCP or BCP/ACS based on mean residence time and area under the retention curve vs. time profile. Use of these matrices resulted in detectable rhBMP-2 levels at the surgical site for over a week in contrast to data reported with several other matrices that lasted less time. Systemic catabolism and elimination of the rhBMP-2 was extensive and systemic presence of the protein was negligible.

Animals↗
PubMedDate not supplied

Drug abuse treatment process components that improve retention.

BACKGROUND: Longer retention has been the most consistent predictor of favorable drug abuse treatment outcomes, but key therapeutic and patient engagement indicators of treatment process need to be more clearly established. METHODS: An integrative model representing treatment dynamics was tested for explaining long-term program retention. It was based on a multisite sample of 527 daily opioid users who remained in methadone maintenance a minimum of 3 months. All had been assigned randomly to a counseling condition at admission (i.e., cognitively enhanced or standard), and information obtained from patient files, as well as periodic assessments completed by patients and their counselors in the first 90 days after admission were the sources of predictors. RESULTS: Counseling enhancements (using node-link mapping, a visual representation tool for improving communication and problem solving) contributed to stronger therapeutic relationships between counselor and patient, which in turn had a positive reciprocal relationship with patient engagement (session attendance). Pretreatment motivation measured at intake was also related to higher engagement. More positive therapeutic relationships (in months 1 and 2) led to lower levels of during-treatment drug use (defined from urinalysis results in months 2 and 3), and better session attendance and therapeutic relationships both predicted longer retention. In addition, lower drug use during treatment was related to longer retention. CONCLUSIONS: Major conceptual domains of drug abuse treatment process were identified in community-based programs and their interrelationships with retention specified. As intermediate (during treatment) criteria, they can help guide functional improvements in program effectiveness as illustrated with our counseling enhancements.

Humans↗
PubMedDate not supplied

Retention of homeless clients in substance abuse treatment. Findings from the National Institute on Alcohol Abuse and Alcoholism Cooperative Agreement Program.

Retaining clients in treatment who are homeless presents a particular challenge for substance abuse treatment providers. A National Institute on Alcohol Abuse and Alcoholism Cooperative Agreement Program offered the first opportunity to systematically study program retention in a multisite study of interventions for homeless persons with alcohol and other drug problems. This article presents results from analyses conducted across 15 interventions implemented at 8 Cooperative Agreement sites. Both qualitative and quantitative data were collected and analyzed. Key findings were that (a) retention problems with homeless clients are as or more pervasive than in the general addicted population; (b) the provision of housing increases retention, but the increases tend to be nullified when the housing is bundled with high-intensity services; (c) homeless clients leave treatment programs for a multitude of reasons; and (d) midcourse corrections to increase retention are frequently successful. The discussion focuses on service components related to retention, the importance of attending to phase transitions, and the importance of being programmatically responsive when serving this population.

Alcoholism↗
PubMed2003

Smooth muscle cells improve endothelial cell retention on polytetrafluoroethylene grafts in vivo.

OBJECTIVE: We investigated the influence of smooth muscle cells (SMC) on endothelial cell (EC) retention on polytetrafluoroethylene (PTFE) grafts and the effect of SMC seeding on intimal hyperplasia in vivo in a rabbit model. METHODS: Fibronectin-coated PTFE grafts (4 mm diameter) were seeded with either EC alone, SMC alone, or SMC followed 24 hours later by EC. The grafts were connected to an extracorporal aortic shunt for 1 hour or were individually implanted for 1, 30, and 100 days into the infrarenal aorta as an end-to-side bypass graft. The number of retained cells was compared at 1 hour and at 1 day after implantation. Neointimal thickness was measured 30 and 100 days after implantation. RESULTS: After 1-hour exposure to blood flow, EC retention rate was greater (P <.005) if seeded on top of SMC (98% +/- 2%; n = 8) versus being seeded alone (65 +/- 11%; n = 8). SMC retention rate was 95 +/- 5% (n = 8) when seeded alone. Similar cell retention was obtained 1 day after implantation. After 30-day implantation the neointima was thicker in grafts seeded with EC and SMC (282 +/- 136 microm; n = 3) than with EC only (52 +/- 45 microm; n = 3; P <.001). However, the neointimal thickness for dual-cell-seeded grafts (126 +/- 60 microm; n = 3) was not significantly different (P =.09) from EC-seeded grafts (79 +/- 48 microm; n = 3) after 100-day implantation. CONCLUSION: EC retention on PTFE grafts in vivo is improved if seeded over a layer of SMC. Further studies are needed to determine whether overlying EC modulate proliferation of underlying SMC.

Animals↗
PubMed2001

The effect of dietary crude protein level on intake, growth, protein retention and utilization of growing male Saanen kids.

The effects of increasing dietary levels of crude protein on growth, feed intake, feed efficiency and composition of gain in male Saanen kids were studied. Four groups of four kids each initially weighing 12.1+/-0.18kg and having a weight gain of 195+/-16g/d were penned individually and fed for 73 to 124 days up to 25kg of BW. They were fed chaffed wheat straw (45g CP/kg DM) which had been molasses sprayed (10%) and pelleted concentrates containing 8.7, 11.7, 14.4 and 17.6% crude protein (CP) on DM basis, the ratio of straw to concentrate being 1:5. Kids were fed controlled to satiation in which case small amounts of the feeds were offered about five times a day as long as the kids wanted to eat. Retention of protein, fat and energy were calculated from their initial and final concentrations in the empty body homogenates of the slaughtered kids. With increasing CP level in the diet, feed intake increased from 448 to 608g DM/d, weight gain from 94 to 181g/d, retention of protein from 9.7 to 27.8g/d, retention of fat from 9.6 to 19.1g/d and feed efficiency improved from 4.79 to 3.39kg DM/kg weight gain. Protein composition of gain increased from 103 to 154g/kg BWG while fat (103-105g/kg BWG) remained constant. Regression analyses showed that BWG can be optimized at 136g CP/kg DM and protein retention at 180g CP/kg DM, whereas, dietary nitrogen was utilized most efficiently at 120g CP/kg DM. Extrapolated from the regression equations, maintenance N requirement of the kids at zero N-retention and at zero BWG were 0.38 and 0.16g N/kg W(0.75), respectively. Recommended dietary CP concentrations and maintenance N requirements depend on the traits desired.

Journal Article↗
PubMed1996

Tunicamycin potentiates drug cytotoxicity and vincristine retention in multidrug resistant cell lines.

Tunicamycin (TM), an inhibitor of glycoprotein processing, was investigated for its potential to reverse the multiple drug resistance (MDR) phenotype. When TM was added in vitro to drug-resistant NIH-3T3-MDR and KB-8-5-11 cells, they developed an increased sensitivity to doxorubicin, epirubicin, vincristine and colchicine. Similarly, the sensitivity of NIH-3T3-MDR cells to cisplatin was also enhanced by TM. In the presence of TM, drug-sensitive NIH-3T3-parental cells exhibited greater susceptibility to the toxic effects of doxorubicin, epirubicin, vincristine (marginally significant), and colchicine, but not to cisplatin. Tunicamycin-treated drug-sensitive KB-3-1 cells showed an increased response to vincristine, but not to the other anticancer drugs. Pretreatment with TM inhibited glycoprotein synthesis in all the cell lines. Neither prior exposure to, nor co-incubation with TM, influenced the uptake of vincristine (VCR) in the various cell lines. However, NIH-3T3-MDR cells accumulated less VCR than their drug-sensitive controls and also exhibited reduced efflux of the drug when treated with TM. There were no significant differences in the levels of intracellular VCR uptake between drug-sensitive KB-3-1 and KB-8-5-11 cells. Tunicamycin increased intracellular VCR retention in KB-8-5-11 and NIH-3T3-MDR cells, but not in NIH-3T3-parental cells. However, drug-sensitive KB-3-1 cells expressed reduced VCR retention in response to TM exposure, indicating that correlations between VCR toxicity and its retention in the presence of TM should be made with caution. The results suggest that the enhancement of intracellular VCR retention in MDR cells lines caused by TM is likely to be the result of inhibition of VCR efflux. Inhibition of glycoprotein synthesis during TM exposure may account for the changes in VCR efflux and retention observed in the MDR cell lines. The enhancement of cisplatin cytotoxicity in NIH-3T3-MDR cells after exposure to TM is an interesting observation, since it is generally believed that agents which modify the MDR phenotype do not show a sensitising effect to cisplatin. These findings may have applications in the reversal of drug resistance.

Antineoplastic Agents↗
PubMed2002

Effect of hydraulic retention time on inorganic nutrient recovery and biodegradable organics removal in a biofilm reactor treating plant biomass leachate.

A fixed-film (biofilm) reactor was designed and its performance was determined at various retention times. The goal was to find the optimal retention time for recycling plant nutrients in an advanced life support system, to minimize the size, mass, and volume (hold-up) of a production model. The prototype reactor was tested with aqueous leachate from wheat crop residue at 24, 12, 6, and 3 h hydraulic retention times (HRTs). Biochemical oxygen demand (BOD), nitrates and other plant nutrients, carbohydrates, total phenolics, and microbial counts were monitored to characterize reactor performance. BOD removal decreased significantly from 92% at the 24 h HRT to 73% at 3 h. Removal of phenolics was 62% at the 24 h retention time, but 37% at 3 h. Dissolved oxygen concentrations, nitric acid consumption, and calcium and magnesium removals were also affected by HRT. Carbohydrate removals, carbon dioxide (CO2) productions, denitrification, potassium concentrations, and microbial counts were not affected by different retention times. A 6 h HRT will be used in future studies to determine the suitability of the bioreactor effluent for hydroponic plant production.

Bacteria, Aerobic↗
PubMed2003

Cellular retention of radioactivity and increased radiation dose. Model experiments with EGF-dextran.

Targeting of tumor cells with radiolabeled biomolecules is a possible approach to inactivate disseminated tumor cells. However, rapid degradation of the biomolecules after cellular internalization and subsequent excretion of the radioactivity is a problem. We studied the possibility of using dextran as a carrier of radionuclides to improve the intracellular retention. An EGF-dextran conjugate, aimed for targeting of tumor cells overexpressing the EGF-receptor, was used as model. Retention tests were performed with (125)I on different parts: [(125)I]-EGF-dextran-[(125)I], [(125)I]-EGF-dextran and EGF-dextran-[(125)I]. Comparisons were made with [(125)I]-EGF. The radiolabeled compounds were incubated with cultured glioma cells for different times. The cellular retention of radioactivity was then measured for up to 24 h. Expected radiation doses at the cellular level were calculated assuming that (131)I, instead of (125)I, was coupled to EGF and EGF-dextran. The results indicated that the EGF-part of the conjugate was degraded and the EGF-attached radioactivity was rapidly excreted, whereas radioactivity on dextran was retained intracellularly to a high degree, i.e. 70-80% of the radioactivity bound to dextran was still cell-associated after 24 h. The retention after 24 h was significantly higher (p < 0.001) when the radioactivity was on the dextran instead of the EGF-part. The radiolabeled EGF-dextran had a notably high specific radioactivity; up to 11 MBq/microg. There was potential for at least hundred times increased radiation dose per receptor interaction when the radioactivity was on the dextran part. The advantage with radioactivity on the dextran part was the high cellular retention and the high specific radioactivity (higher than previously reported for other residualizing labels) without severe loss of receptor specific binding. Thus, dextran seems suitable as a carrier of radionuclides aimed for therapy and gives potential for a highly increased radiation dose.

Animals↗
PubMed1998

In-vivo tissue uptake and retention of Sn-117m(4+)DTPA in a human subject with metastatic bone pain and in normal mice.

Organ and tissue uptake and retention of Sn-117m(4+)DTPA were studied in a human subject treated for metastatic bone pain, and the results were compared with the biodistribution studies in five normal mice. The explanted organs from a patient who received a therapy dose of 18.6 mCi (688.2 MBq) Sn-117m(4+)DTPA and who died 47 days later were imaged with a gamma-camera, and tissue samples were counted and also autoradiographed. Bone, muscle, liver, fat, lungs, kidneys, spleen, heart and pancreas tissue samples were assayed in a well counter for radioactivity. Regions of interest were drawn over bone and major organs to calculate and quantify clearance times using three in vivo Sn-117m(4+)DTPA whole-body scintigrams acquired at 1, 24 and 168 h after injection. Five normal mice injected with the same batch of Sn-117m(4+)DTPA as used for the human subject were sacrificed at 24 h, and tissue samples were collected and assayed for radioactivity for comparison with the human data. For the human subject, whole-body retention at 47 days postinjection was 81% of the injected dose, and the rest (19%) was excreted in urine. Of the whole-body retained activity at 47 days, 82.4% was in bone, 7.8% in the muscle and 1.5% in the liver, and the rest was distributed among other tissues. Gamma-ray scintigrams and electron autoradiographs of coronal slices of the thoracolumbar vertebral body showed heterogeneous metastatic involvement with normal bone between metastatic lesions. There was nonuniform distribution of radioactivity even within a single vertebral body, indicating normal bone between metastatic lesions. Lesion-to-nonlesion ratios ranged from 3 to 5. However, the osteoid-to-marrow cavity deposition ratio, from the microautoradiographs, was 11:1. The peak uptake in the human bone was seen at 137 h with no biological clearance. Soft tissues showed peak uptake at 1 h and exhibited three compartmental clearance components. Whole-body retention in normal mice was 38.7% of the injected dose at 24 h and the rest was excreted. At 24 h postinjection, bone in mice showed 84.2% of the whole-body retention, muscle 1.7% and liver 1.4%, and the rest was distributed in other soft tissues. Percent distribution of the retained dose among bone, muscle, liver and other soft tissues is very similar between mice and a human subject. To calculate precise radiation absorbed doses from bone pain palliation radionuclides, it is necessary to take into account soft-tissue uptake and retention that may not be readily evident from routine external gamma-scintigraphy.

Adenocarcinoma↗
PubMed2003

Recruitment and retention of adolescent participants in HIV research: findings from the REACH (Reaching for Excellence in Adolescent Care and Health) Project.

PURPOSE: To evaluate the importance of 13 items in the recruitment and retention of HIV-positive and HIV-negative adolescent participants in a longitudinal study (REACH study). METHODS: A confidential, self-administered, visual analog, cross-sectional survey was offered to active participants (November 1999-August 2000) with 438 subjects (86%) participating. Sixty-six percent of the cohort were HIV-positive and 34% were HIV-negative with a mean age of 17 years, and 76% were female. Subjects were asked to recall the importance they placed on 13 items in deciding to join the REACH study (recruitment) and to remain on study (retention). Factors that might explain the judgment placed on the items were analyzed using the nonparametric Wilcoxon Rank-Sum test or the Kruskall-Wallis test. RESULTS: The five most important factors for study recruitment were identical to those chosen for retention by participants. The factors were: (a) quality medical care, (b) caring staff, (c) health education, (d) privacy/confidentiality, and (e) altruism. Items judged least important were social activities, compensation, transportation, and food/meals. Subject characteristics (gender, age, HIV status) were not associated with statistically different mean judgment scores at recruitment and retention, although clinical site showed significant variation. Factors that could render subjects vulnerable (health insurance, family finances) were not associated with related items. CONCLUSIONS: Adolescents found quality health care and care team characteristics critically important in considering research participation. Attention to privacy and the opportunity to be altruistic were also important. Compensation for participation was not a significant factor for recruitment and retention of this adolescent cohort. This study demonstrates that adolescents apply sound criteria in evaluating research participation and do not appear to be unduly affected by compensation.

Adolescent↗
PubMed1991

Quantitative retention-biological activity relationship study by micellar liquid chromatography.

In a previous paper, the usefulness of micellar liquid chromatography (MLC) in predicting octanol-water partition coefficients of organic compounds was reported. This paper is the first successful report of a quantitative retention-activity relationship study using the retention factor in MLC for predicting the biological activity of a group of phenolic compounds. Excellent correlation was obtained between the capacity factor in MLC and the bioactivity (measured as log 1/C, where C is the 50% inhibitory growth concentration) of 26 para-substituted phenols. A single MLC retention parameter is capable of describing the bioactivity of phenols, while three conventional molecular descriptors (log P(ow), pKa, and R) are needed to achieve a similar correlation. This indicates that both hydrophobic and electronic interactions are incorporated in a single MLC retention parameter, which is due to the amphiphilic nature of surfactants in the system. In situations like this, QRAR is a suitable alternative to QSAR since measuring MLC retention is much easier than measuring different molecular descriptors needed to build the QSAR model. Addition of 10% 2-propanol to a micellar system (hybrid system) proved to be the best chromatographic system for the best estimation of the phenols bioactivity. Other chromatographic factors such as pH and stationary phase also showed significant effect on the correlation between capacity factor k' and log 1/C.

Chromatography, Liquid↗
PubMed1992

Temperature dependence of retention in reversed-phase liquid chromatography. 2. Mobile-phase considerations.

The retention mechanism in reversed-phase liquid chromatography (RPLC) has been examined over a wide temperature range with emphasis on the role of the mobile phase. van't Hoff plot shapes were used to assess the retention mechanism, and the data showed evidence of the hydrophobic effect when water-rich and/or hydrogen-bonded mobile phases such as methanol/water were used. However, different van't Hoff plot shape was observed with acetonitrile/water mobile phases, indicating a change in the retention mechanism. These data showed that the hydrophobic effect, which had previously been proposed as the driving force for retention, is not a satisfactory explanation for the retention process in all RPLC systems.

Chromatography, High Pressure Liquid↗
PubMed2000

Justification of statistical overlap theory in programmed temperature gas chromatography: thermodynamic origin of random distribution of retention times.

A specific distribution of compounds' standard-state changes of enthalpy and entropy between mobile and stationary phases in programmed temperature gas chromatography (PTGC) is shown to produce the Poisson distribution of retention times often postulated in statistical-overlap theory (SOT). A three-part model is proposed, in which the enthalpy change is Poisson distributed, the average entropy change depends on the enthalpy change, and the actual entropy change varies in a uniformly random manner about the average entropy change. To test the model, the entropy and enthalpy changes of 350 aliphatic and aromatic hydrocarbons in petroleum were calculated with commercial GC software. These changes are shown to follow the three-part model. The model then was used with Monte Carlo methods to mimic the enthalpy and entropy changes. The substitution of the mimicked enthalpy and entropy changes into an equation for the retention temperature in PTGC is shown to produce a Poisson distribution of retention times that is statistically significant. This finding establishes a scientific link between the thermodynamics governing retention in PTGC and the superficially ad hoc assumption of the Poisson retention time distribution in SOT. Similar thermodynamic distributions are found for flavors and fragrances and for tetrachlorodibenzo-p-dioxins and furans, which follow SOT based on the Poisson distribution, but not for polychloronaphthalenes, which do not follow that SOT.

Journal Article↗
PubMed1994

Influence of pH on retention and selectivity in micellar liquid chromatography: consequences of micellar-induced shifts of ionization constants.

The retention and selectivity of ionizable solutes in the two-surfactant-mediated reversed-phase LC techniques (micellar and ion pair) are compared through the use of a general retention equation for mono- and zwitterionic solutes. In RPLC with different mobile-phase modifiers (organic solvents, surfactants), the influence of pH can be quantitatively described by one general equation. The existing theory for the secondary chemical equilibria has been applied to compare selectivity effects in micellar and ion-pair chromatography. In order to predict the influence of different mobile-phase compositions on the selectivity of ionizable compounds, one should determine how a mobile-phase parameter influences the ionization equilibria (i.e., selective shifts of pKa) and self-selectivity (defined as the ratio of retention factors of the acid/conjugate base). For example, the first ionization constants of amino acids and peptides in aqueous mobile phase are between pH 2.3 and 3.4. Consequently, the pH required to maximize the retention of these solutes by ion suppression is less than the operational pH range of silica-based columns. In addition, since the pKa1 values of these solutes are similar, adjustment of pH has little effect on separation selectivity. In contrast, ion-pairing and micellar mobile phases with SDS surfactant increase the magnitude and range of the ionization constants. These trends are more pronounced with micellar mobile phase. The displacement of solute ionization constants to higher PH with micellar mobile phase allows the maximal, limiting, retention of zwitterionic solutes to be observed within the pH limits of silica-based columns.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromatography, High Pressure Liquid↗
PubMed2001

Retention and selectivity of teicoplanin stationary phases after copper complexation and isotopic exchange.

Teicoplanin is a macrocyclic glycopeptide that is highly effective as a chiral selector for LC enantiomeric separations. Two possible interaction paths were investigated and related to solute retention and selectivity: (1) interactions with the only teicoplanin amine group and (2) role of hydrogen bonding interactions. Mobile phases containing 0.5 and 5 mM copper ions were used to try to block the amine group. In the presence of copper ions, it was found that the teicoplanin stationary phase has a decreased ability to separate most underivatized racemic amino acids. However, it maintained its ability to separate enantiomers that were not alpha-amino acids. It is established that there is little copper-teicoplanin complex formation. The effect of Cu2+ on the enantioseparation of some alpha-amino acids appears to be due to the fact that these solutes are good bidentate ligands and form complexes with copper ions in the mobile phase. Isotopic exchange with deuterium oxide was performed using acetonitrile-heavy water mobile phases. It was found that the retention times of all amino acids were lower with deuterated mobile phases. The retention times of polar or apolar molecules without amine groups were higher with deuterated mobiles phases. In all cases, the enantioselectivity factors were unaffected by the deuterium exchange. It is proposed that the electrostatic interactions are decreased in the deuterated mobile phases and the solute-accessible stationary-phase volume is somewhat swollen by deuterium oxide. The balance of these effects is a decrease in the amino acid retention times and an increase in the apolar solute retention time. The enantioselectivity factors of all of the molecules remain unchanged because all of the interactions are changed equally. We propose a new global quality criterion (the E factor) for comparing and evaluating enantiomeric separations.

Amino Acids↗
PubMed2002

Thermodynamic interpretation of retention equilibrium in reversed-phase liquid chromatography based on enthalpy-entropy compensation.

The fundamentals of the retention equilibrium in reversed-phase liquid chromatography (RPLC) are studied on the basis of enthalpy-entropy compensation (EEC). First, retention data were acquired and the influence of the nature of the compounds, organic solvent modifier, and temperature on these data was assessed. Then, the data were analyzed according to the four different methods proposed by Krug et al., and an EEC was formally established. Linear correlations were observed between the logarithm of the adsorption equilibrium constants under the different RPLC conditions, suggesting linear free energy relationships (LFERs). Finally, the variations of the retentions with the experimental conditions are shown to be quantitatively explained by a new model based on EEC. This model affords a comprehensive interpretation of the variations of retention originating from changes of either one parameter alone or several simultaneously. The slope and intercept of the LFER that relates two equilibrium systems are accounted for by the new model. The parameters of this model are the changes of enthalpy and entropy associated with the retention, the compensation temperatures, and the experimental conditions.

Journal Article↗
PubMed2005

Electrostatic contributions to protein retention in ion-exchange chromatography. 2. Proteins with various degrees of structural differences.

The relation of protein structure to retention provides a framework within which to investigate chromatographic adsorption mechanisms. Protein sets with varying degrees of structural differences were studied to relate variations in protein properties to retention behavior. To explore molecular contributions to protein adsorption in ion-exchange chromatography, protein-adsorbent electrostatic interactions were modeled using a continuum approach. The calculations qualitatively capture the chromatographic differentiation of closely related subtilisin variants. Descriptions of the electrostatic interactions of FGF-1 vs FGF-2 with cation exchangers were obtained, and aid in rationalizing differences in experimental retention trends across a set of adsorbents based on different adsorption mechanisms linked to the adsorbent structure. Comparative calculations for proteins with differences in local or overall arginine-lysine composition, including subtilisin variants G166R/G166K and lysozyme/cytochrome c, suggest that continuum electrostatics is not adequate to capture the full quantitative characteristics of the chromatographic retention of proteins. To allow more accurate description of retention, additional molecular interactions, specifically hydration effects, must be incorporated in the model.

Arginine↗