Determination of endocrine-disrupting compounds in drinking waters by fast liquid chromatography-tandem mass spectrometry

GC-MS/MS analysis of synthetic cannabinoids 5F-MDMB-PICA and 5F-CUMYL-PICA in forensic cases

Aim: Validate a method to quantify 1-(5-fluoropentyl)-N-(2-phenylpropan-2-yl)-1H-indole-3-carboxamide (5F-CUMYL-PICA) and methyl 2-[[1-(5-fluoropentyl) indole-3-carbonyl] amino]-3,3-dimethyl-butanoate (5F-MDMB-PICA) in blood samples using GC-MS/MS. Materials & methods: A solid-phase extraction (SPE) method has been developed to quantify 5F-MDMB-PICA and 5F-CUMYL-PICA in authentic human blood samples. Results & conclusion: The limit of detection (LOD) was 0.1 and 0.11 ng/ml for 5F-CUMYL-PICA and 5F-MDMB-PICA, respectively, while the limit of quantification (LOQ) was 0.50 ng/ml for both two compounds. Recovery was 91.40, 82.54 and 85.10% for SPE, supported liquid extraction (SLE) and ISOLUTE C18; matrix effects 15, 24 and 22.5% for SPE, SLE and ISOLUTE C18; accuracy was 2.4-5.5 and 3.9-7.3% for SPE, SLE and ISOLUTE C18, while precision was 4.6-7.7 and 6.4-8.3% for SPE, SLE and ISOLUTE C18, respectively. The concentrations of 5F-CUMYL-PICA and 5F-MDMB-PICA in the authentic human blood samples were 2.18 and 3.07 ng/ml, respectively. The validated method was successfully used in supporting the quantification of analytes in blood.

Endophytic fungus from Handroanthus impetiginosus immobilized on electrospun nanofibrous membrane for bioremoval of bisphenol A

The current industrial and human activities scenario has accelerated the widespread use of endocrine-disrupting compounds (EDCs), which can be found in everyday products, including plastic containers, bottles, toys, cosmetics, etc., but can pose a severe risk to human health and the environment. In this regard, fungal bioremediation appears as a green and cost-effective approach to removing pollutants from water resources. Besides, immobilizing fungal cells onto nanofibrous membranes appears as an innovative strategy to improve remediation performance by allowing the adsorption and degradation to occur simultaneously. Herein, we developed a novel nanostructured bioremediation platform based on polyacrylonitrile nanofibrous membrane (PAN NFM) as supporting material for immobilizing an endophytic fungus to remove bisphenol A (BPA), a typical EDC. The endophytic strain was isolated from Handroanthus impetiginosus leaves and identified as Phanerochaete sp. H2 by molecular methods. The successful assembly of fungus onto the PAN NFM surface was confirmed by scanning electron microscopy (SEM). Compared with free fungus cells, the PAN@H2 NFM displayed a high BPA removal efficiency (above 85%) at an initial concentration of 5 ppm, suggesting synergistic removal by simultaneous adsorption and biotransformation. Moreover, the biotransformation pathway was investigated, and the chemical structures of fungal metabolites of BPA were identified by ultra-high performance liquid chromatography - high-resolution mass (UHPLC-HRMS) analysis. In general, our results suggest that by combining the advantages of enzymatic activity and nanofibrous structure, the novel platform has the potential to be applied in the bioremediation of varied EDCs or even other pollutants found in water resources.

Adaptation of the o-DGT for the sampling of 12 hormones: calibration, performance evaluation, and recommendation

This work highlights the methodology for the development of diffusive gradients in thin films (o-DGT) through its adaptation for 12 natural and synthetic hormones belonging to three different families (estrogens, progestins, and androgens). A reliable strategy must be applied during o-DGT lab adaptation to avoid issues related to the analysis (i.e., presence of matrix effects in grab or passive samples) but also to the o-DGT configuration (i.e., undesirable sorption or desorption, lack of performance with insufficient elution or unreliable diffusion coefficient). To avoid analytical issues due to the presence of salts in grab samples, CaCl₂ exposure solutions must be used on a lab-scale development to monitor the hormone concentration. The selected o-DGT was composed of an Oasis® HLB binding gel and a diffusive gel in agarose because they provided better performance than polyacrylamide gels (i.e., higher elution factors and more repeatable diffusion coefficients). The elution factors of the binding gel were then from 0.79 ± 0.13 to 1.04 ± 0.13 (RSD < 15%) and the diffusion coefficients at 25 °C were from 4.07 ± 0.24 to 5.49 ± 0.28 × 10^-6 cm² s^-1 (RSD < 9%). A laboratory exposure to a synthetic solution was performed to check the consistency with the DGT quantification model validating the calibration parameters for all hormones (except 17α-ethinylestradiol with a bias of 40%). Therefore, the o-DGT configuration is suitable for sampling hormones in the natural environment with LOQ_DGT ranging from 0.3 to 6.6 ng L^-1.

A "dilute-and-shoot" column-switching UHPLC-MS/MS procedure for the rapid determination of branched nonylphenol in human urine: method optimisation and some fundamental aspects of nonylphenol analysis

Technical grade branched nonylphenol (NP) was determined in human urine by online solid phase extraction-ultra high-performance liquid chromatography-tandem mass spectrometry (SPE-UHPLC-MS/MS). Prior to analysis, urine specimens were simply diluted and enzymatically deconjugated. The run time of the chromatography, including SPE and re-equilibration, was 9 min per injection. The enzymatic cleavage of NP conjugates was optimised with incurred sample material from a human metabolism study: the highest recoveries were obtained with β-glucuronidase from E. coli K 12 in 0.1 M ammonium acetate at pH 6.5, within a minimal hydrolysis time of 30 to 60 min. Using sodium acetate instead of ammonium acetate led to systematically decreased recovery rates. The analytical method was validated regarding its precision (coefficients of variation: 2.9-7.4%), accuracy (relative recovery rates: 93-105%), robustness (relative recovery rates in individual urine matrices: 92-117%), selectivity, and limit of quantification (1.0 μg L^-1). Fundamental aspects in the analysis of technical product mixtures such as NP, comprising various isomers and homologues, were considered. Validation results, an exposure scenario and the application of the procedure to real samples, show that it enables a rugged monitoring of NP exposures above, at, and significantly below health-based guidance values, corresponding to daily NP intakes in the low μg kg^-1 d^-1 range. On the other hand, background levels in non-specifically exposed populations cannot be detected with this method. Hence, while alternative approaches should be pursued for NP analysis at environmental trace level, the speed and simplicity of our method are ideal for high-throughput human biomonitoring in occupational medicine.

A Review on Polyethersulfone Membranes in Polar Organic Chemical Integrative Samplers: Preparation, Characterization and Innovation

The membranes in polar organic chemical integrative samplers (POCIS) enclose the receiving sorbent and protect it from coming into direct contact with the environmental matrix. They have a crucial role in extending the kinetic regime of contaminant uptake, by slowing down their diffusion between the water phase and the receiving phase. The drive to improve passive sampling requires membranes with better design and enhanced performances. In this review, the preparation of standard polyethersulfone (PES) membranes for POCIS is presented, as well as methods to evaluate their composition, morphology, structure, and performance. Generally, only supplier-related morphological and structural data are provided, such as membrane type, thickness, surface area, and pore diameter. The issues related to the use of PES membranes in POCIS applications are exposed. Finally, alternative membranes to PES in POCIS are also discussed, although no better membrane has yet been developed. This review highlights the urge for more membrane characterization details and a better comprehension of the mechanisms which underlay their behavior and performance, to improve membrane selection and optimize passive sampler development.

Multiple reaction monitoring for identification and quantification of oligosaccharides in legumes using a triple quadrupole mass spectrometer

Raffinose family oligosaccharides are non-digestible compounds considered as dietary prebiotics with health-related properties. Hence, it is important to develop highly specific methods for their determination. An analytical method is developed in this study for oligosaccharide identification and quantification using liquid chromatography-tandem mass spectrometry equipped with a triple quadrupole analyser operating in Multiple Reaction Monitoring mode. Raffinose, stachyose and verbascose are separated in a 10-minute run and the method is validated over a broad concentration range, showing good linearity, accuracy, precision and high sensitivity. A low-cost, short eco-friendly procedure for oligosaccharide extraction from legumes, with a high recovery rate extraction, good repeatability and reproducibility is also proposed. No plant-matrix effects were demonstrated. The method applied to the screening of 28 different legumes revealed species-related traits for oligosaccharide distribution, highlighting Pisum sativum (9.22 g/100 g) as the richest source of these prebiotics and its suitability as a functional food ingredient.

Capillary electrophoresis and liquid chromatography for determining steroids in concentrates of purified water from Päijänne Lake

The research was done with partial filling micellar electrokinetic chromatography, microemulsion electrokinetic chromatography, and ultra-high performance liquid chromatography. The study focuses on determination of male and female steroids from cold and hot tap water of households in Helsinki City. The district´s raw water is made run from Päijänne Lake through a water tunnel to the purification plants in Helsinki area. The effluents delivered from the plants to households as tap water were sampled and used for the study. They were concentrated with solid phase extraction to exceed the detection limits of the three methods. With partial filling method the limits were 0.50, 0.48, 0.33, and 0.50 mg/L for androsterone, testosterone, progesterone, and testosterone-glucuronide, respectively. In microemulsion method the limit values were 1.33, 1.11, and 0.40 mg/L for androsterone, testosterone, and progesterone, respectively, and 0.83, 0.45, and 0.50 mg/L for hydrocortisone, 17-α-hydroxyprogesterone, and 17-α-methyltestosterone, respectively. In the tap water samples, progesterone concentrations represented the highest values being 0.22 and 1.18 ng/L in cold and hot water, respectively. They also contained testosterone (in all samples), its glucuronide metabolite (in 25% of the samples), and androstenedione (in 75% of the samples). The ultra-high liquid chromatographic method with mass spectrometric detection was used for identification of the steroids at µg/L level.

Structural elucidation of bisphenol E and bisphenol S photoinduced by-products by high-resolution electrospray ionisation mass spectrometry and tandem mass spectrometry

RATIONALE

Bisphenol E (BPE) and bisphenol S (BPS) have recently replaced bisphenol A as monomers for producing polycarbonates. However, BPE and BPS can pose hazards as they are known to be endocrine disruptors. Despite the huge increase in their use, there is a lack of data regarding the toxicity and effects of BPE and BPS.

METHODS

We investigated the photoinduced transformation of BPE and BPS when subjected to sun-simulated radiation and using TiO₂ as a photocatalyst. Analyses of BPE, BPS and their by-products were performed by high-performance liquid chromatography/high-resolution mass spectrometry (HPLC/HRMS) using an orbitrap mass analyzer in negative electrospray ionisation (ESI) mode. The chromatographic separations were achieved by employing a C18 reversed-phase column, and the transformation products (TPs) were elucidated structurally using HRMS and multistage MS experiments performed in collision-induced dissociation (CID) mode.

RESULTS

The transformation of bisphenol S involved the formation of twelve by-products, while ten TPs were detected following BPE degradation. For bisphenol S, the cleavage of the molecule is a very important transformation route, together with the hydroxylation of the substrate to provide mono- and poly-hydroxylated TPs. For bisphenol E, the two main routes were hydroxylation and ring opening. Acute toxicity for BPS, BPE and their TPs was assessed using the Vibrio fischeri assay, highlighting that their initial transformation involved the formation of TPs that were more toxic than the parent compound.

CONCLUSIONS

The HPLC/HRMS method developed was useful for characterising and identifying newly formed TPs from bisphenol E and bisphenol S. This study aimed to examine the structure of twenty by-products identified during TiO₂ -mediated photolysis and to evaluate acute toxicity over time.

Reproductive drugs and environmental contamination: quantum, impact assessment and control strategies

Industrial and municipal solid wastes, noise, pesticides, fertilizers and vehicular emission are visible pollutants responsible for environmental contamination and ill-effects on health of all living systems. But, environmental contamination due to drugs or medicines used for different purposes in humans and animals goes unseen largely and can affect the health of living system severely. During the last few decades, the usage of drugs has increased drastically, resulting in increased drug load in soil and water. Contraceptive and fertility drugs are extensively and effectively used in humans as well as animals for different purposes. Usage of these reproductive drugs in humans is increased manifold to manage reproductive problems and/or for birth control with changing lifestyles. These drugs are excreted in urine and faeces as metabolite or conjugated forms, leading to contamination of water, milk and animal produce, which are consumed directly by humans as well as animals. These drugs are not eliminated even by water treatment plant. Consumption of such contaminated water, milk, meat and poultry products results in reproductive disorders such as fertility loss in men and increase risk of different types of cancers in humans. Therefore, assessment of impact of environmental contamination by these drugs on living system is of paramount importance. The purpose of this review article is to provide a comprehensive analysis of various research and review reports on different contraceptive and fertility drugs used in human and animals, their occurrence in the environment and their ill-effects on living systems. The approaches to control this invisible menace have also been proposed.

Mathematical modeling for estrogenic activity prediction of 17β-estradiol and 17α-ethynylestradiol mixtures in wastewater treatment plants effluent

Steroid estrogens such as 17β-Estradiol (E₂) and 17α-Ethynylestradiol (EE₂) are highly potent estrogens that widely detected in environmental samples. Mathematical modelling such as concentration addition (CA) and estradiol equivalent concentration (EEQ) models are usually associated with measuring techniques to assess risk, predict the mixture response and evaluate the estrogenic activity of mixture. Wastewater has played a crucial role because wastewater treatment plant (WWTP) is the major sources of estrogenic activity in aquatic environment. The aims of this is to determine E₂ and EE₂ concentrations in six WWTPs effluent, to predict the estrogenic activity of the WWTPs effluent using CA and EEQ models where lastly the effectiveness of two models is evaluated. Results showed that all the six WWTPs effluent had relative high E₂ concentration (35.1-85.2 ng/L) compared to EE₂ (0.02-1.0 ng/L). The estrogenic activity predicted by CA model was similar among the six WWTPs (105.4 ng/L), due to the similarity of individual dose potency ratio calculated by respective WWTPs. The predicted total EEQ was ranged from 35.1 EEQ-ng/L to 85.3 EEQ-ng/L, explained by high E₂ concentration in WWTPs effluent and E₂ EEF value that standardized to 1.0 μg/L. The CA model is more effective than EEQ model in estrogenic activity prediction because EEQ model used less data and causes disassociation from the predicted behavior. Although both models predicted relative high estrogenic activity in WWTPs effluent, dilution effects in receiving river may lower the estrogenic response to aquatic inhabitants.

Multiple Reaction Monitoring for Direct Quantitation of Intact Proteins Using a Triple Quadrupole Mass Spectrometer

Methods that can efficiently and effectively quantify proteins are needed to support increasing demand in many bioanalytical fields. Triple quadrupole mass spectrometry (QQQ-MS) is sensitive and specific, and it is routinely used to quantify small molecules. However, low resolution fragmentation-dependent MS detection can pose inherent difficulties for intact proteins. In this research, we investigated variables that affect protein and fragment ion signals to enable protein quantitation using QQQ-MS. Collision induced dissociation gas pressure and collision energy were found to be the most crucial variables for optimization. Multiple reaction monitoring (MRM) transitions for seven standard proteins, including lysozyme, ubiquitin, cytochrome c from both equine and bovine, lactalbumin, myoglobin, and prostate-specific antigen (PSA) were determined. Assuming the eventual goal of applying such methodology is to analyze protein in biological fluids, a liquid chromatography method was developed. Calibration curves of six standard proteins (excluding PSA) were obtained to show the feasibility of intact protein quantification using QQQ-MS. Linearity (2-3 orders), limits of detection (0.5-50 μg/mL), accuracy (<5% error), and precision (1%-12% CV) were determined for each model protein. Sensitivities for different proteins varied considerably. Biological fluids, including human urine, equine plasma, and bovine plasma were used to demonstrate the specificity of the approach. The purpose of this model study was to identify, study, and demonstrate the advantages and challenges for QQQ-MS-based intact protein quantitation, a largely underutilized approach to date.

Measurement of estradiol, estrone, and testosterone in postmenopausal human serum by isotope dilution liquid chromatography tandem mass spectrometry without derivatization

BACKGROUND

A high-throughput, sensitive, specific, mass spectrometry-based method for quantitating estrone (E1), estradiol (E2), and testosterone (T) in postmenopausal human serum has been developed for clinical research. The method consumes 100μl human serum for each measurement (triplicates consume 300μl) and does not require derivatization. We adapted a commercially available 96-well plate for sample preparation, extraction, and introduction into the mass spectrometer on a single platform.

METHODS

Steroid extraction from serum samples and mass spectrometer operational parameters were optimized for analysis of estradiol and subsequently applied to other analytes. In addition to determining the limit of detection (LOD) and limit of quantitation (LOQ) from standard curves, a serum LOQ (sLOQ) was determined by addition of known steroid quantities to serum samples. Mass spectrometric method quantitative data were compared to results using a state-of-the-art ELISA (enzyme-linked immunosorbent assay) using stored serum samples from menopausal women.

RESULTS

The LOD, LOQ, sLOQ was (0.1pg, 0.3pg, 1pg/ml) for estrone, (0.3pg, 1pg, 3pg/ml) for estradiol, and (0.3pg, 1pg, 30pg/ml) for testosterone, respectively. Mass spectrometry accurately determined concentrations of E2 that could not be quantified by immunochemical methods. E1 concentrations measured by mass spectrometry were in all cases significantly lower than the ELISA measurements, suggesting immunoreactive contaminants in serum may interfere with ELISA. The testosterone measurements broadly agreed with each other in that both techniques could differentiate between low, medium and high serum levels.

CONCLUSIONS

We have developed and validated a scalable, sensitive assay for trace quantitation of E1, E2 and T in human serum samples in a single assay using sample preparation method and stable isotope dilution mass spectrometry.

'Extreme mass spectrometry': the role of mass spectrometry in the study of the Antarctic environment

A focus on the studies of the Antarctic environment that have been performed by mass spectrometry is presented herein; our aim is to give evidence of the essential role of this instrumental technique in the framework of the scientific research in Antarctica, with a comprehensive review on the main literature of the last two decades. Due to the wideness of the topic, the present review is limited to the determination of organic pollutants, natural molecules and biomarkers in Antarctica, thus excluding elemental analysis and studies on inorganic species. The work has been divided into five sections, on the basis of the considered environmental compartment: air; ice and snow; seawater, pack ice and lakes; soil and sediments; and organisms and biomarkers.

Mechanism of formation of the major estradiol product ions following collisional activation of the molecular anion in a tandem quadrupole mass spectrometer

The importance of the mass spectral product ion structure is highlighted in quantitative assays, which typically use multiple reaction monitoring (MRM), and in the discovery of novel metabolites. Estradiol is an important sex steroid whose quantitation and metabolite identification using tandem mass spectrometry has been widely employed in numerous clinical studies. Negative electrospray ionization tandem mass spectrometry of estradiol (E2) results in several product ions, including the abundant m/z 183 and 169. Although m/z 183 is one of the most abundant product ions used in many quantitative assays, the structure of m/z 183 has not been rigorously examined. We suggest a structure for m/z 183 and a mechanism of formation consistent with collision induced dissociation (CID) of E2 and several stable isotopes ([D4]-E2, [(13)C6]-E2, and [D1]-E2). An additional product ion from E2, namely m/z 169, has also been examined. MS(3) experiments indicated that both m/z 183 and m/z 169 originate from only E2 [M - H](-) m/z 271. These ions, m/z 183 and m/z 169, were also present in the collision induced decomposition mass spectra of other prominent estrogens, estrone (E1) and estriol (E3), indicating that these two product ions could be used to elucidate the estrogenic origin of novel metabolites. We propose two fragmentation schemes to explain the CID data and suggest a structure of m/z 183 and m/z 169 consistent with several isotopic variants and high resolution mass spectrometric measurements.

Spatial and temporal evaluations of estrogenic activity in tap water served by a water plant in Wuhan, China

This study aimed to evaluate the spatial and temporal characteristics of estrogenic activities in tap water served by a water plant in Wuhan, China. Tap water samples were monthly collected from the three sampling sites with different distances of distribution network from the plant during April 2010-March 2011: Min (less than 0.1km), Mid (approximately 4km) and Max (approximately 8km). Estrogenic activities of solid phase-extracted tap waters were measured by using recombinant yeast assay incorporated with and without exogenous metabolic activation system (rat liver S9 fractions) and expressed as 17β-estradiol equivalents (EEQ). Pro-estrogenic and estrogenic activity in tap water ranged from 151.4 to 1395.6pg EEQ/L and 35.2 to 1511pg EEQ/L, respectively. Average pro-estrogenic activity (680.3pg EEQ/L) was significantly higher than estrogenic activity (412.8pg EEQ/L) throughout the whole year. The pro-estrogenic activity significantly increased with the extending of distribution network, and was also statistically correlated with water temperature and pH. However, pro-estrogenic and estrogenic activity was not altered across four seasons. Our results suggest that the pro-estrogenic and estrogenic chemicals are present in tap water served by the water plant.

Seasonal distribution, source investigation and vertical profile of phenolic endocrine disrupting compounds in Dianchi Lake, China

Phenolic endocrine disrupting compounds, including nonylphenol-di-ethoxylate (NP2EO), nonylphenol-mono-ethoxylate (NP1EO), 4-nonylphenol (4-NP), bisphenol A (BPA), 4-cumylphenol (4-CP) and 4-tert-octylphenol (4-t-OP), were investigated in water, surface sediment and sediment cores in Dianchi Lake to track their seasonal distributions, pollution sources and historical trends. The concentrations of NP2EO, NP1EO, 4-NP, BPA, 4-CP and 4-t-OP were up to 295.14, 448.48, 45.28, 530.33, 8.96 and 21.37 ng L(-1) in water, and up to 297.11, 809.63, 4.58, 166.87, 3.62 and 40.69 ng g(-1) dry weight in surface sediment, respectively. Except BPA in water, concentrations of all the other phenolic compounds in both of the matrices were higher in January than in July, 2011. The concentrations decreased significantly with an increase in distance from the sampling locations which were adjacent to the urban areas (Kunming City, Chenggong City and Jinning City). The pollution of phenolic EDCs came mainly from industry, agriculture and daily life. The relationships between the concentrations of target compounds and the six water quality parameters were evaluated. There were significant positive correlations between concentrations of phenolic compounds in water and in surface sediment. For sediment cores, three clearly separated maxima occurred in segments 0-5 cm (the late 2000s), 5-10 cm (the early and mid of 2000s) and 20-25 cm (the mid of 1980s), respectively. NP2EO, NP1EO and BPA were the three dominant compounds in the lake.

Assessment of source water contamination by estrogenic disrupting compounds in China

Detection of estrogenic disrupting compounds (EDCs) in drinking waters around China has led to rising concerns about health risks associated with these compounds. There is, however, a paucity of studies on the occurrence and identification of the main compounds responsible for this pollution in the source waters. To fill this void, we screened estrogenic activities of 23 source water samples from six main river systems in China, using a recombinant two-hybrid yeast assay. All sample extracts induced significant estrogenic activity, with E2 equivalents (EEQ) of raw water ranging from 0.08 to 2.40 ng/L. Additionally, 16 samples were selected for chemical analysis by gas chromatography-mass spectrometry. The EDCs of most concern, including estrone (E1), 17beta-estradiol (E2), 17alpha-ethinylestradiol (EE2), estriol (E3), diethylstilbestrol (DES), estradiol valerate (EV), 4-t-octylphenol (4-t-OP), 4-nonylphenols (4-NP) and bisphenol A (BPA), were determined at concentrations of up to 2.98, 1.07, 2.67, 4.37, 2.52, 1.96, 89.52, 280.19 and 710.65 ng/L, respectively. Causality analysis, involving comparison of EEQ values from yeast assay and chemical analysis identified E2, EE2 and 4-NP as the main responsible compounds, accounting for the whole estrogenic activities (39.74% to 96.68%). The proposed approach using both chemical analysis and yeast assay could be used for the identification and evaluation of EDCs in source waters of China.

Hydrogen rearrangement and ring cleavage reactions study of progesterone by triple quadrupole mass spectrometry and density functional theory

The fragmentation mechanisms of progesterone have been studied by triple quadrupole tandem mass spectrometry (MSMS) and density functional theory (DFT). Mechanisms leading to major product ions are proposed. The data suggest that progesterone fragments preferentially via hydrogen and other rearrangements lead to neutral losses. These fragmentations are quite complex and are preceded by σ-bond cleavages in most cases. Four major pathways for progesterone fragmentation are proposed involving: (1) cleavage of ring B at C9-C10, (2) cleavage of C6-C7 bond in ring B through m/z 191, (3) two types of cleavages of ring D, and (4) ketene elimination in ring A. Pathways (1)-(3) proceed via charge-remote fragmentations while pathway (4) proceeds via charge-site initiated mechanism. The geometry of product ions in these pathways were optimized using DFT at the B3LYP/6-311G(d,p) level of theory from which the free energies of the pathways were calculated. The effect that the choice of basis sets and density functionals has on the results was tested by performing additional calculations using B3LYP/6-31G(d) and B3PW91/6-311G(d,p).