Liquid chromatographic study of the interaction between aflatoxins and β-cyclodextrin

Unexpected Encapsulation of Selected Polycyclic Aromatic Hydrocarbons by β-Cyclodextrin Studied Using UV-Vis Spectrophotometry, Micro-Planar Chromatography and Temperature Dependent Inclusion Chromatography

This research communication significantly extends our previous studies focusing on the temperature effects related to the unexpected chromatographic behavior of 1-acenaphthenol in the presence of native β-cyclodextrin (β-CD) additive, working under thin-layer chromatographic (TLC) conditions. We have applied complementary and orthogonal techniques including (i) temperature-controlled ultraviolet-visible (UV-VIS) spectroscopy, (ii) thermostated microplanar high-performance chromatography (micro-HPTLC) and (iii) temperature-dependent inclusion chromatography based on high-performance liquid chromatography (HPLC) to investigate the retention behavior of related host molecules. Particularly, various symmetric and asymmetric molecules were tested, such as: naphthalene and its derivatives including acenaphthylene, acenaphthene and selected dimethynaphthalenes: 1,8-DMN, 1,5-DMN, 2,3-DMN and 2,6-DMN. Reported raw experimental data, particularly performed in liquid phase and detected by UV-Vis spectrophotometry, may suggest that solubility changes of the supramolecular complexes studied and differences in total analysis time between TLC and HPLC separation can trigger strong retention of target components in planar chromatographic systems. This was also supported by principal component analysis (PCA) of the multi-source data obtained. It is hoped that the reported analyses enable the adjustment of phenomenological models describing liquid chromatography retention and the solubility behavior of low-molecular mass guest molecules, controlled by supramolecular interactions with selected macrocycles. It should be noted that the reported phenomenon, specifically supramolecular complexes precipitation, may have a number of practical applications. This can be used to improve the efficiency and selectivity of planar and/or microfluidic systems. On the other hand, precipitation via host-guest interactions may be applied for highly selective water purification technological processes that will be designed for the removal of given organic micropollutants.

Addition of Carbon to the Culture Medium Improves the Detection Efficiency of Aflatoxin Synthetic Fungi

Aflatoxin (AF) is a harmful secondary metabolite that is synthesized by the Aspergillus species. Although AF detection techniques have been developed, techniques for detection of AF synthetic fungi are still required. Techniques such as plate culture methods are continually being modified for this purpose. However, plate culture methods require refinement because they suffer from several issues. In this study, activated charcoal powder (carbon) was added to a culture medium containing cyclodextrin (CD) to enhance the contrast of fluorescence and improve the detection efficiency for AF synthetic fungi. Two culture media, potato dextrose agar and yeast extract sucrose agar, were investigated using both plate and liquid cultures. The final concentrations of CD and carbon in the media were 3 mg/mL and 0.3 mg/mL, respectively. Addition of carbon improved the visibility of fluorescence by attenuating approximately 30% of light scattering. Several fungi that could not be detected with only CD in the medium were detected with carbon addition. The carbon also facilitated fungal growth in the potato dextrose liquid medium. The results suggest that addition of carbon to media can enhance the observation of AF-derived fluorescence.

Development of a simple and high-throughput method for detecting aflatoxins production in culture media

AIMS

To develop a simple, high-throughput and inexpensive procedure to detect and quantify aflatoxins into the culture media of growing mycelia.

METHODS AND RESULTS

Fungal conidia (Aspergillus flavus) were inoculated into the wells of a microplate containing 200 μl of different formulations of coconut-derived liquid medium. Time-dependent production of aflatoxins in the culture media was evaluated by a procedure relying on the UV-induced fluorescence emission by the toxin, using a microplate reader. These data were validated by comparison with the outputs of a conventional HPLC-based procedure. Determinations of aflatoxin concentration, according to the fluorimetric procedure, were performed either by withdrawing samples from the plates or by direct 'in situ' readings, the latter method reinforcing the high-throughput feature of the procedure. Fluorescence enhancers (cyclodextrins) did not ameliorate the sensitivity of the procedure to low concentrations of the toxin into the medium. The efficacy of the procedure was also validated by testing the effect on toxin yield of adding an antioxidant agent (α-lipoic acid) to the medium.

CONCLUSIONS

We give evidence that our improved procedure is reliable and suitable to analyse aflatoxin accumulation time course in coconut-derived culture medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that our procedure may profitably be used to give insights into the mechanisms of regulation of mycotoxin production and, consequently, to implement different strategies for the containment of aflatoxin contamination of food and feed commodities.

Determination of endocrine disrupting compounds using temperature-dependent inclusion chromatography: I. Optimization of separation protocol

In the present work we optimised the separation of battery of key UV non-transparent low-molecular-mass compounds having possible endocrine disrupting compounds (EDCs) activity or which may be used as the endocrine effect biomarkers. Simple optimization strategy was based on strong temperature effect that is driven by electrostatic interactions between macrocyclic mobile phase additives like cyclodextrins and eluted components of interest under C18 stationary phase and acetonitrile/water mobile phase conditions. Particularly, the effect of temperature involving native beta-cyclodextrin and its hydroxypropyl derivative to improve separation of number of natural (d-equilenin, equilin, estetrol, estriol, estrone, 17beta-estradiol, 17alpha-hydroxyprogesterone, 20alpha-hydroxyprogesterone, cortisol, cortisone, progesterone, testosterone, tetrahydrocortisol and tetrahydrocortisone) and artificial steroids (ethynylestradiol, norgestrel isomers, medroxyprogesterone, mestranol, methyltestosterone, norethindrone, 17alpha-estradiol) as well as non-steroidal compounds (diethylstilbesterol, bisphenol A, 4-tert-butylphenol, dimethyl phthalate, dibutyl phthalate and dioctyl phthalate) was investigated. It has been found that successful isocratic separation of 27 chemicals can be achieved using acetonitrile/water eluents modified with beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin at concentration of 10 mM and temperature of 47 degrees C. Separation protocol is simple, reliable, direct and non-radioactive and may be easily adapted for rapid separation and quantification of wide range of given steroids and related EDCs in environmental samples, particularly those that are characterised by unstable biological matrix and components of interest load.

Effect of water molecules on the fluorescence enhancement of Aflatoxin B1 mediated by Aflatoxin B1:β-cyclodextrin complexes. A theoretical study

In order to explain the observed fluorescence enhancement of Aflatoxin B1 (AFB1) when forming AFB1:beta-cyclodextrin (AFB1:beta-CD) inclusion complexes, we have performed a theoretical (quantum chemistry calculations) study of AFB1 and AFB1:beta-CD in vacuum and in the presence of aqueous solvent. The AM1 method was used to calculate the absorption and emission wavelengths of these molecules. With the help of density functional theory (DFT) and time-dependent DFT (TDDFT) vibrational frequencies and related excitation energies of AFB1 and AFB1.(H2O)m = 4,5,6,11 were calculated. On the basis of these calculations we propose a plausible mechanism for the fluorescence enhancement of AFB1 in the presence of beta-CD: (1) before photoexcitation of AFB1 to its S1 excited state, there is a vibrational coupling between the vibrational modes involving the AFB1 carbonyl groups and the bending modes of the nearby water molecules (CG + WM); (2) these interactions allow a thermal relaxation of the excited AFB1 molecules that results in fluorescence quenching; (3) when the AFB1 molecules form inclusion complexes with beta-CD the CG + WM interaction decreases; and (4) this gives rise to a fluorescence enhancement.

Comparison of cultural and analytical methods for determination of aflatoxin production by Mississippi DeltaAspergillusisolates

This study compared cultural and analytical methods for detecting aflatoxin production by Aspergillus species. Aspergillus isolates were obtained from various Mississippi Delta crops (corn, peanut, rice, cotton) and soils. Most of the isolates (99%) were A. flavus and the remainder comprised A. parasiticus and A. nomius. The following three cultural methods were evaluated on potato dextrose agar: fluorescence (FL) on β-cyclodextrin-containing media (CD), yellow pigment (YP) formation in mycelium and medium, and color change after ammonium hydroxide vapor exposure (AV). Aflatoxins in culture extracts were confirmed by thin-layer chromatography (TLC) and quantified by enzyme-linked immunosorbent assay (ELISA). Of the 517 isolates, 314 produced greater than 20 ng/g of total aflatoxin based on ELISA, and 180 produced greater than 10 000 ng/g of aflatoxin in the medium. Almost all the toxigenic isolates (97%) were confirmed by TLC as producers. Of the toxigenic isolates, as determined by ELISA, 93%, 73%, and 70% gave positive FL, YP, and AV responses, respectively. Of the 203 isolates producing less than 20 ng/g of aflatoxin, 20%, 6%, and 0% of respective FL, YP, and AV methods gave false-positive responses. The 9% false-positive results from TLC fall within this range. This study showed good agreement among all tested cultural methods. However, these cultural techniques did not detect aflatoxin in all cultures that were found to produce aflatoxins by ELISA, LC/MS, and TLC. The best results were obtained when the AV color change and CD fluorescence methods were used together, yielding an overall success rate comparable to TLC but without the need for chemical extraction and the time and expense of TLC.Key words: aflatoxins, analytical methods, Aspergillus flavus, Aspergillus parasiticus, cultural methods, β-cyclodextrin, fluorescence enhancers, mycotoxins, yellow pigment.

Chromatographic determination of the association constant between 8-methoxypsoralen and modified β-cyclodextrin: protective effect of hydroxypropyl-β-cyclodextrin on 8-methoxypsoralen toxicity in human keratinocytes

The retention of 8-methoxypsoralen (8-MOP) on an immobilised hydroxypropyl-beta-cyclodextrin (HP-beta-CD) column was analysed in HPLC by the determination of its Langmuir distribution isotherm. A such method was used to confirm the potential drug complexing role of this cyclodextrin. The 8-MOP/HP-beta-CD association constant (K) was equal to 29.5 and 18.7 M-1, respectively, at a temperature equal to 5 and 25 degrees C, respectively. These association constant values were used to determine the cytotoxicity profile of human keratinocyte cell line (HaCaT) in relation to the complex concentration. It was showed through these data that HP-beta-CD had a cytoprotective since a reverse effect of HP-beta-CD on 8-MOP cytotoxicity was observed.

New reversed-phase liquid chromatographic method to detect aflatoxins in food and feed with cyclodextrins as fluorescence enhancers added to the eluent

The effect of succynil-beta-cyclodextrin (beta-CD-Su), dimethyl-beta-cyclodextrin (DIMEB) and beta-cyclodextrin (beta-CD) on the fluorescence of aflatoxins B1, B2, G1, G2 and M1 (AFB1, AFB2, AFG1, AFG2 and AFM1) was studied: beta-CD-Su promoted the largest fluorescence enhancement for AFB1 and AFM1 while DIMEB showed better results for AFG1 . On the basis of the fluorescence enhancement, a new RP-HPLC method for detecting aflatoxins B1, B2, G1, G2 and M1 was developed using cyclodextrins directly dissolved in the LC eluent. Aflatoxins B1, B2, G1 and G2 were resolved using a MICRA NPS ODS-1 column using methanol-water as mobile phase to which 6 x 10(-3) M beta-CD-Su or beta-CD were added. Chromatographic responses of AFB1 and AFG1 achieved using beta-CD dissolved in the mobile phase were enhanced, respectively, 8 and 12 times, and 10 and 15 times with beta-CD-Su. Detection limits lower than 0.3 microg/kg were achieved for all the four aflatoxins. Aflatoxin M1 was analysed using a Spherisorb S3 ODS-2 Narrow Bore column and methanol-water as mobile phase with added 2 x 10(-3) M beta-CD-Su. An area enhancement of 1.5 was detected for the toxin and the detection limit achieved under these analytical conditions was lower than 0.0005 microg/kg. Both methods were statistically validated showing a linear response for all the aflatoxins tested (R2 > or = 0.99), and applied to the analysis of spiked and naturally contaminated food samples.

A capillary electrophoresis study on the influence of beta-cyclodextrin on the critical micelle concentration of sodium dodecyl sulfate

The influence of beta-cyclodextrin (beta-CD) on the critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) was investigated by capillary electrophoresis using anionic chlorophenols as probe molecules at pH 7.0. The variations of the electrophoretic mobility of probe molecules as a function of surfactant concentration in both premicellar and micellar regions in the absence and presence of beta-CD was analyzed. The results indicate that, as a consequence of a strong inclusion complexation between beta-CD and SDS, the encapsulation of beta-CD with probe molecules is greatly diminished, or even vanished, in the presence of SDS. The complexes formed between beta-CD and SDS monomers exist predominantly in the form of a 1:1 stoichiometry, while the complexes with a 2:1 stoichiometry reported previously in the literature as a minor component may exist by less than 10%. The elevation of the CMC value of SDS depends not only on the concentration of beta-CD in the buffer electrolyte but also on methanol content in the sample solution. The binding constants of probe molecules to beta-CD, to surfactant molecules, and to the complexes formed between beta-CD and SDS are reported.

Separation of steroids using temperature-dependent inclusion chromatography

The influence of temperature on retention and separation of estrogens, progesterone derivatives and beta-cyclodextrin in reversed-phase high-performance liquid chromatography has been studied. Steroids were detected using direct UV detection at 240 and 280 nm. Detection of beta-cyclodextrin was achieved using a post-column indirect photometric method. Chromatographic experiments were performed using an acetonitrile-water mobile phase (30%, v/v) and a wide range of column temperatures from 0 to 80 degrees C with 20 degrees C steps. Linear Van't Hoff plots were observed for steroids and beta-cyclodextrin when an unmodified binary mobile phase was applied. The retention of steroids was strongly influenced by temperature when the mobile phase was modified with beta-cyclodextrin at a concentration of 12 mM. Particularly, for 17beta-estradiol and 20alpha-hydroxyprogesterone a strong deviation from the linear Van't Hoff plots and a remarkable affinity for beta-cyclodextrin was observed. Polynomial regression calculations were performed to fit the set of experimental data points. Using third-order polynomial equations, minimum separation factor values (alphamin) were calculated for temperatures from -10 to + 100 degrees C with 1 degrees C steps. The best chromatographic conditions for separation of multicomponent samples were chosen. A possible retention mechanism for solutes in the presence of macrocyclic additives is discussed. The results presented describe the role of temperature in high-performance liquid chromatography systems in which the mobile phase is modified with an inclusion agent.

Comparison of Association Constants of Cyclodextrins and Their tert-Butyl Derivatives With Halogenbenzoic Acids and Acridine Derivatives

Association constants of complexes of a new class of cyclodextrin alkyl derivatives - randomly susbstituted tert-butyl derivatives (TB-CDs) with complete set of mono-halobenzoic acids and acridine derivatives were measured using capillary electrophoresis and compared with association constants of natural cyclodextrins. In most cases the association constants of natural cyclodextrin and its corresponding tert-butyl derivative were comparable. Strong dependence of association constants on actual cyclodextrin concentration was observed for complexes of tert-butyl-α-cyclodextrin with acridine derivatives. Significant increase of the association constant occurred below the critical micelle concentration of the cyclodextrin derivative, while an increase above this value was less significant.

Application of the assay of aflatoxins by liquid chromatography with fluorescence detection in food analysis

HPLC using fluorescence detection has already become the most accepted method for the determination of aflatoxins due to its several advantages over other analytical methods. Both normal- and reversed-phase HPLC can be used. However the reversed-phase HPLC methods are more popular. Liquid chromatographic determination of aflatoxins using fluorescence detection and its application in food analysis is reviewed in this article.

Capillary zone electrophoretic separation of neutral species of chloro-s-triazines in the presence of cationic surfactant monomers

Chloro-s-triazines are difficult to separate by capillary zone electrophoresis (CZE), due to their low pKa values. However, these analytes can be effectively separated by CZE in the presence of cationic surfactant monomers, such as tetradecylammonium bromide (TTAB) and dodecyltrimethylammonium bromide (DTAB). The separation mechanism based on a 1:1 binding of analytes to cationic surfactant monomers is proposed. The binding constants of chloro-s-triazines to cationic surfactant monomers are estimated. The results show that the strength of the interactions of these analytes with TTAB monomers is considerably strong, whereas that of the corresponding analyte with DTAB monomers is about 12- to 14-fold weaker. A linear correlation of binding constants with log P(ow) (the logarithm of the partition coefficient of analytes between 1-octanol and aqueous phases) indicates that the migration order of these chloro-s-triazines depends primarily on their hydrophobicity. Moreover, the skewed peaks of chloro-s-triazines observed may reveal the occurrence of adsolubilization of these analytes in the adsorbed cationic surfactant layer on the capillary surface.

Interaction between cyclodextrins and aflatoxins Q1, M1 and P1. Fluorescence and chromatographic studies

The fluorescence properties of the aflatoxins M1, Q1, P1 in solution and the effect of various cyclodextrins (alpha-, beta-, gamma-, hydroxypropyl-beta- and alpha-beta-heptakis-di-O-methyl-beta-) on their fluorescence emission were studied. Among the aflatoxins, a substantial enhancement of the fluorescence emission of aflatoxin Q1 in the presence of aqueous solutions of alpha-, beta-, hydroxypropyl-beta, and alpha-beta-heptakis-di-O-methyl-beta-cyclodextrin, was observed. On the contrary, gamma-cyclodextrin proved to be inefficient to enhance the fluorescence properties of this compound. No important fluorescence enhancement was found for aflatoxins P1 or M1 for any of the cyclodextrin derivatives tested. The complex formation constant (Kf) of these compounds with beta-cyclodextrin was chromatographically determined, and from the results obtained, we can conclude that Kf cannot be used alone to explain the fluorescence increase. Thermodynamic studies showed that delta-H and delta-S parameters, associated with the partition of aflatoxins in RP-HPLC, increased when beta-cyclodextrin was added to the eluent.

The influence of temperature on the multiple separation of estrogenic steroids using mobile phases modified with beta-cyclodextrin in high-performance liquid chromatography

The effect of temperature on the retention and multiple separation of six estrogenic steroids in reversed-phase liquid chromatography has been studied. Capacity factors (k') of estriol, 17 beta-estradiol, 17 alpha-estradiol, d-equilenin, equilin and estrone were measured using mobile phase modified with different concentrations of beta-cyclodextrin (from 0-16 mM), a fixed solvent composition (acetonitrile-water) and a wide range of column temperatures (from 5 to 80 degrees C). The plots of capacity factors vs. reciprocal of absolute temperature are nonlinear in each case when mobile phase modified with beta-cyclodextrin was used. Particularly strong nonlinearity was observed at lower temperature and at higher beta-cyclodextrin concentration. The complex chromatograms were evaluated using optimization parameters such as capacity factor of the last-eluted peak (k'max), the smallest resolution between adjacent peaks (Rs,min) and relative resolution product (r). The results presented describe precisely the role of temperature in high-performance liquid chromatography systems in which mobile phases modified with cyclodextrin were used. Moreover, the elution order of estrogenic steroids on modified and unmodified mobile phases has been discussed.

The influence of temperature on the high performance liquid chromatographic separation of steroids using mobile phases modified with beta-cyclodextrin

The effect of temperature on the retention and multiple separation of hydrocortisone, testosterone, 17 alpha-methyltestosterone, prednisone, cortisone and 17 alpha-hydroxyprogesterone in reversed-phase liquid chromatography has been studied. Capacity factors (k') of the steroids were measured using a mobile phase modified with different concentrations of beta-cyclodextrin (from 0-16 mM), a fixed solvent composition (acetonitrile-water) and a wide range of column temperatures (from 5-80 degrees C). The plots of capacity factors vs. reciprocal of absolute temperature are nonlinear in every case when mobile phase modified with beta-cyclodextrin was used. Particularly strong nonlinearity was observed at lower temperature and at higher beta-cyclodextrin concentration. The complex chromatograms were evaluated using optimization parameters such as capacity factor of the last-eluted peak (k'max), the smallest resolution between adjacent peaks (Rs; min) and relative resolution product (r). The results presented describe precisely the role of temperature in high performance liquid chromatography systems in which mobile phases modified with cyclodextrin were used.

Postcolumn excitation of aflatoxins using cyclodextrins in liquid chromatography for food analysis

Measurement of fluorescence increase was used for the comparative quantification of the effect that several cyclodextrins (alpha-, beta-, heptakis-2,6-beta-omicron-dimethyl- and gamma-) produce on the fluorescent response of aflatoxins B1 and G1. This constitutes a new chromatographic method with stability of the mobile phase, and shows general improvements in the chromatographic conditions with respect to other methods (especially those using an iodine reservoir as a postcolumn reactor). A C18-type column was used, with methanol-water (60:40, v/v) as the mobile phase. The excitation phase of the natural fluorescence of aflatoxins, a 10(-2) M solution of each cyclodextrin, was introduced postcolumn. The determination of the elution order aflatoxin G2 > G1 > B2 > B1 was performed for each phase in less than 15 min. As expected using an aqueous-alcoholic medium, an increase in the fluorescence response of aflatoxins with an unsaturated furanic ring was found to occur with all the cyclodextrins studied, except gamma-cyclodextrin. The observed increase was larger for heptakis-2,6-beta-omicron-dimethyl- than for beta-cyclodextrin (to our knowledge, the only cyclodextrin previously described in the literature to serve for the determination of aflatoxins). The difference is of the order of 70.1-fold in the case of aflatoxin G1 and 45.2-fold in the case of aflatoxin B1. The detection limit in the mobile phase used was determined (for aflatoxin B1) for beta-cyclodextrin and 2,6-beta-omicron-dimethylcyclodextrin (signal-to-noise ratio 1:3) to be 4 and 9 mg 1(-1), respectively.

Hydroxypropyl cyclodextrins: potential synergism with carcinogens

The solubility of the lipophilic carcinogens benzo[a]pyrene and aflatoxin B1 in water increases linearly and substantially with the concentration of hydroxypropyl beta-cyclodextrin present. Results of a kinetic study of naphthalene, a model for more potent carcinogens, indicate that the increase in the dissolution rate and in the transport through the aqueous phase into a nonpolar phase is on the same order of magnitude as the increase in solubility. Consequently, hydroxypropyl beta-cyclodextrin, when used in pharmaceutical formulations, has the potential to increase the absorption of carcinogens which enter the gastrointestinal tract either as food components or from air pollution through saliva. Only the above mechanism's simple proportionality needs be considered for estimating the increases in carcinogen absorption in the upper gastrointestinal tract and in the colon. In the presence of bile, however, additional factors are involved and the proportionality does not apply. Bile micelles, which themselves are effective solubilizers of lipophilic carcinogens, were disrupted by hydroxypropyl beta-cyclodextrin because of the formation of complexes with bile salts. Thus, in the presence of bile, two systems for delivery of carcinogens may coexist: that of cotransport with lipids and that of delivery through solubilization by hydroxypropyl beta-cyclodextrin.

Effect of temperature on separation of estradiol stereoisomers and equilin by liquid chromatography using mobile phases modified with beta-cyclodextrin

The unusual temperature effect on the retention of 17 alpha-estradiol, 17 beta-estradiol and equilin in reversed-phase liquid chromatography has been observed. Capacity factors (k') of the steroids were measured using mobile phase with different concentrations of beta-cyclodextrin (from 0 to 16 mM) in a fixed mobile phase composition (acetonitrile-water) and wide range of column temperatures (from 5 degrees to 80 degrees C). The plots of capacity factors vs reciprocal of absolute temperature are nonlinear in each case. At subambient temperatures the capacity factors decreased with temperature decrease. This effect is more evident for the natural 17 beta-estradiol than for its 17 alpha-isomer.