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Journal Article↗
Biomedical subjects
Cynthia K Larive
Publications and source records attributed to Cynthia K Larive.
High-performance liquid chromatographic-nuclear magnetic resonance investigation of the isomerization of alachlor-ethanesulfonic acid.
The metabolism of the acetanilide herbicide alachlor in soils leads to the formation of alachlor-ethanesulfonic acid (alachlor-ESA) as one of the major transformation products of this compound. The unique structure of alachlor and its metabolites allows the formation of two diastereomers (s-trans and s-cis) due to the hindered rotation of the amide bond connected to a rigid aromatic ring. Although these stereoisomers do interconvert by rotation about the amide bond, the rate of interconversion is slow allowing separation of the isomers on the chromatographic time scale. Once separated, the unique nuclear magnetic resonance signals of each isomer can be used to monitor the rate of isomerization. This paper reports the on-line separation and detection of the rotational diastereomers using high-performance liquid chromatography-nuclear magnetic resonance (HPLC-NMR) to efficiently measure the isomerization rate of alachlor-ESA.
Acetamides↗
Analytical applications of NMR diffusion measurements.
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Journal Article↗
Characterization of humic substances: implications for trihalomethane formation.
Humic substances are precursors of carcinogenic trihalomethanes (THMs) formed during disinfection by chlorination in water treatment processes. In an effort to understand the relationship between trihalomethane formation potential (THMFP) and physicochemical properties of humic substances, UV-visible absorbance, fluorescence in emission and synchronous scan modes, and NMR spectra were measured for several aquatic fulvic and humic acids. For comparison, a soil fulvic acid was also examined using these methods. The feasibility of the gradient modified spin-echo (GOSE) NMR experiment to selectively measure singlet resonances arising from isolated protons was examined. In addition, diffusion coefficients were measured for DMSO solutions of the fulvic acids using BPPLED and GOSE-edited pulse sequences. Although none of the methods tested produced results that correlated with THMFP, the GOSE intensities determined for different regions of the NMR spectra did reflect the relative abundance of different types of functional groups produced by lignin oxidation. In addition, the GOSE-edited diffusion results suggest that the isolated protons, those most reactive to chlorination, are more likely contained in the larger molecular weight fractions of fulvic acids.
Journal Article↗
Educational approaches for analytical science.
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Journal Article↗
NMR diffusion analysis of surfactant-humic substance interactions.
Surfactants can be introduced into the environment through wastewater or by direct contamination. Understanding the fate and transport of surfactants in the environment is important in assessing their role as pollutants. Humic substances are complex heterogeneous mixtures of decomposition products of natural organic materials. They are environmentally important because they are known to solubilize and transport organic pollutants. Therefore humic substances are likely to affect the environmental fate of surfactants. Diffusion coefficients measured with pulsed-field gradient nuclear magnetic resonance spectroscopy are used in this study to examine the intermolecular interactions of the surfactants sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) in the presence of various humic substances. These results indicate that humic substances enhance the aggregation of SDS prior to micellization with a more pronounced effect observed for the more hydrophobic humic materials. The positively charged surfactant CTAB forms stable ion pairs with the humic substances.
Journal Article↗
Effects of three pharmaceutical and personal care products on natural freshwater algal assemblages.
Treated wastewaters in the United States contain detectable quantities of surfactants, antibiotics, and other types of antimicrobial chemicals contained in pharmaceutical and personal-care products (PPCPs) that are released into stream ecosystems. The degradation characteristics of many of these chemicals are not yet known, nor are the chemical properties of their byproducts. They also are not currently mandated for removal under the U.S. Clean Water Act. Three representative PPCPs were individually tested in this study using a series of laboratory dilution bioassays: Ciprofloxacin (an antibiotic), Triclosan (an antimicrobial agent), and Tergitol NP 10 (a surfactant), to determine their effects on natural algal communities sampled both upstream and downstream of the Olathe, KS wastewater treatment plant (WWTP). There were no significant treatment effects on algal community growth rates during the exponential phase of growth, but significant differences were observed in the final biomass yields (p < 0.001). All three compounds caused marked shifts in the community structure of suspended and attached algae at both the upstream and downstream sites (p < 0.05). Increasing the concentrations of all three compounds over a 3 orders of magnitude range also caused a consistent decline in final algal genus richness in the bioassays. Our results suggest that these three PPCPs may potentially influence both the structure and the function of algal communities in stream ecosystems receiving WWTP effluents. These changes could result in shifts in both the nutrient processing capacity and the natural food web structure of these streams.
Anti-Infective Agents↗
Transferred nuclear overhauser effect in nuclear magnetic resonance diffusion measurements of ligand-protein binding.
The drug discovery process relies on characterizing structure-activity relationships, since specific ligand-target interactions often result in important biological functions. Measuring diffusion coefficients by nuclear magnetic resonance spectroscopy is a useful way to study binding, because changes can be detected when a small ligand interacts with a macromolecular target. Diffusion coefficients can be miscalculated, however, due to magnetization transfer between the receptor and ligand. This transferred nuclear Overhauser effect (trNOE) disrupts the observed signal decay due to diffusion as a function of the experimental diffusion time. Since longer diffusion times also selectively edit free ligand signal, the measured diffusion coefficients become biased toward the fraction of bound ligand. Despite this discrepancy, under these experimental conditions, the trNOE selectively influences the measured signals of binding ligands and can be used to gain insight into ligand-protein interactions. These phenomena have been studied for caffeine and L-tryptophan, which bind to human serum albumin, and the antimalarial agent trimethoprim, which interacts with dihydrofolate reductase. The results provide insight into the nature of ligand-protein binding and are thus useful for elucidating the molecular features of the ligand that interact with the protein.
Caffeine↗
A mechanistic study of danazol dissolution in ionic surfactant solutions.
This study examined the dissolution mechanism of the neutral drug danazol into solutions of the ionic surfactant sodium dodecyl sulfate (SDS). The effect of counterion concentration on drug dissolution was also studied by controlling the solution ionic strength (IS). The laminar flow apparatus of Shah and Nelson was chosen to measure in vitro dissolution rates for its simulation of physiological hydrodynamics. A mathematical model was developed to test the proposed mechanism for dissolution. Transport of the dissolved drug away from the tablet surface is the slow step in the process. Two major physicochemical properties, drug solubility in surfactant solutions and the effective diffusion coefficients used in the model, were measured in separate experiments for use in the transport model. Pulsed field proton nuclear magnetic resonance spectroscopy ((1)H NMR) was used to measure the drug diffusion coefficient. Actual drug dissolution rates were determined by multiplying the measured effluent drug concentration in the aqueous medium by its flow rate. The assumption of a transport-controlled dissolution rate was tested by plotting the measured dissolution rates as a function of medium flow rate in a log-log plot. A slope of 1/3 is predicted by the model and slopes of 0.26 to 0.32 were found experimentally, suggesting that the transport controlled mechanism is accurate. The model-predicted dissolution rates were compared with the experimental data. For SDS solutions without IS control, the model calculated data are 20-35% lower than the experimental results, whereas with IS control, the error is only 0.4-4%. We believe that there is significant electrostatic interaction between micelles in processes with low IS or poor IS control. In that situation, the nuclear magnetic resonance (NMR)-measured drug diffusivity would not be its actual value in the dissolution process.
Algorithms↗
Nutrient level, microbial activity, and alachlor transformation in aerobic aquatic systems.
Alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide) is a moderately toxic herbicide that is frequently found in agriculturally impacted surface waters. To assess primary mechanism(s) that affect its fate in aquatic systems, two field experiments were performed using large mesocosms (n=39) and smaller microcosms with and without microbial inhibitors (n=16). The mesocosm experiment tested the effect of fertility conditions on alachlor fate, assessing alachlor disappearance over time under oligotrophic (total phosphorus (TP) <12 microg/L) through hypereutrophic (TP>80 microg/L) water conditions. Whereas, the microcosm experiment assessed alachlor fate in the presence of microbial inhibitors that selectively blocked eubacterial (chloroamphenicol, streptomycin, and penicillin combined), eukaryotic (cycloheximide), and universal (all inhibitors) microbial activity. First-order alachlor transformation rate coefficients ranged from 0.006 to 0.042 day(-1) when microbial inhibitors were not present (half-lives from 16 to 122 days) with the highest rates occurring in hypereutrophic waters. Statistics indicated that mean TP, and universal and eubacterial small sub-unit rRNA level most closely correlated with transformation rate. Further, the inhibitor study indicated that alachlor transformation was biotic (>90%), but that high transformation rates only occurred when eubacterial and eukaryotic domains were both metabolically active. Our results confirm that alachlor transformation is primarily biotic; however, efficient biotransformation only occurs when both major microbial domains in aerobic systems are active.
Acetamides↗
Analysis of molecular square size and purity via pulsed-field gradient NMR spectroscopy.
The size (volume) of a large tetrametallic molecular square, that has resisted characterization by mass spectrometry, has been determined by pulsed-field gradient NMR spectroscopy, a technique that reports on self-diffusion coefficients. These scale inversely with hydrodynamic radii, which in turn scale approximately as the cube of the assembly's mass. The technique has also been used to determine whether NMR spectral complexities observed for the new compound are due to contamination with chemically related assemblies, or instead reflect the intrinsic structural complexities of the compound itself.
Journal Article↗
Insights into the cITP process using on-line NMR spectroscopy.
Recently, capillary isotachophoresis (cITP) has been coupled on-line with nuclear magnetic resonance (NMR) to enhance analysis of dilute charged analytes through sample concentration and separation. This study focuses on the unique detection capabilities of NMR to noninvasively examine the cITP process and obtain diagnostic information. With their enhanced mass sensitivity, microcoil NMR probes provide optimal detection for cITP/NMR. Whereas previous studies used deuterated buffers, a 1H NMR observable leading electrolyte, tetramethylammonium acetate, is employed here to better track cITP progression. Fortuitously, the 1H chemical shift of the acetate methyl resonance depends on pD. Hence, by using a calibration curve, the solution pD can be determined on-line during cITP. Similarly, intracapillary temperature can be measured in cITP/NMR by observing the HOD chemical shift. To obtain accurate chemical shift measurements, charge-neutral tert-butyl alcohol is added to all cITP electrolyte solutions as an internal reference. As an ancillary benefit, line width measurements of the ubiquitous tert-butyl alcohol enable NMR spectral resolution to be examined throughout the experiment. Capable of providing quantitative results, NMR simultaneously determines the concentrations of the leading ion, sample, and counterion over the course of the cITP experiment.
Calibration↗
Capillary isotachophoresis/NMR: extension to trace impurity analysis and improved instrumental coupling.
Building upon its promising initial performance, the online coupling of capillary isotachophoresis (cITP) to nuclear magnetic resonance (NMR) is extended to trace impurity analysis. By simultaneously concentrating and separating dilute charged species on the basis of their electrophoretic mobility, cITP greatly facilitates NMR structural elucidation. cITP/NMR appears particularly attractive for identifying trace charged synthetic and natural organic compounds obscured by large excesses of other components. A 9.4 microL injection of 200 microM (1.9 nmol) atenolol in a 1000-fold excess of sucrose (200 mM) is analyzed by cITP/NMR. A microcoil, the most mass sensitive NMR probe, serves as the detector as it provides optimal NMR observation of the capillary-scale separation. cITP successfully isolates the atenolol from the sucrose while concentrating it 200-fold to 40 mM before presentation to the 30 nL observe volume microcoil, thereby enabling rapid 1H NMR spectral acquisition of atenolol (experimental time of 10 s) without obstruction from sucrose. For this particular probe and sample, the stacking efficiency is near the theoretical limit as 67% of the sample occupies the 1 mm long microcoil during peak maximum. A multiple-coil probe with two serial 1 mm long microcoils arranged 1 cm apart has been developed to facilitate peak trapping and sample band positioning during cITP/NMR.
Atenolol↗
Analysis of protein/ligand interactions with NMR diffusion measurements: the importance of eliminating the protein background.
Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) is a well-established method for the determination of translational diffusion coefficients. Recently, this method has found applicability in the combinatorial arena with the introduction of affinity NMR for characterizing protein/ligand interactions. Although affinity NMR has been reported to be an effective method for the identification of active compounds in a complex mixture, there are limitations of this method. We have developed a simple mathematical model to predict optimum concentration ratios of the ligand and protein in order to observe maximum changes in the ligand diffusion coefficient upon protein binding. The ligand/protein systems of L-tryptophan and ibuprofen binding to human serum albumin were chosen to demonstrate the usefulness of this model. However, even when the conditions of the mathematical model are satisfied, the spectral background arising from the protein in proton-detected experiments can be problematic. To this end, we have employed spectral subtraction of the protein spectrum to yield ligand diffusion coefficients that are in agreement with those predicted by simulation.
Ligands↗
The 2D-J-DOSY experiment: resolving diffusion coefficients in mixtures.
Many diffusion-ordered spectroscopy (DOSY) NMR techniques have recently been developed to aid in the deconvolution of complex mixtures. Spectroscopic separation based on chemical and physical properties facilitates the identification of mixture components while eliminating time-consuming separation steps and preserving the chemical environment. One way to improve resolution in such experiments is to spread the spectroscopic information into two dimensions. The 2D-J-DOSY experiment has been designed to resolve mixture components in terms of a chemical shift and proton coupling constant as well as distinguishing them on the basis of translational diffusion. Acquiring a series of spectra as a function of gradient amplitude permits the determination of diffusion coefficients for components that cannot be resolved in the one-dimensional (1D) (1)H NMR spectrum. Comparison of the resulting values with those obtained through the traditional 1D diffusion experiment for a mixture of sugars validates The 2D-J-DOSY technique.
Carbohydrates↗