Stable isotope approaches, applications, and issues related to polyunsaturated fatty acid metabolism studies

The use of stable isotope tracers for investigating fatty acid metabolism in human subjects has increased substantially over the last decade. Advances in analytical instrumentation, commercial availability of labeled substrates, and safety considerations are major reasons for this increased use of stable isotope tracers. Several experimental design options are available for using either deuterium or carbon-13 as tracers for fatty acid and lipid studies. Options include feeding a pulse dose of labeled fat or a mixture containing two or more labeled fats. Multiple doses of the labeled fat can be fed at timed intervals to increase enrichments. Administration by injection or continuous intravenous infusion is an alternative. Another option is to use diets containing foods from plants that have slightly higher natural carbon-13 enrichment. Each basic experimental design has its specific strengths, and the best choice of experimental design depends on the study objectives. Stable isotope studies have been used to address a variety of questions related to unsaturated fatty acid metabolism in humans. Examples are provided that illustrate the use of stable isotopes to investigate oxidation of docosahexaenoic acid, desaturation of linoleic and linolenic acids in infants and adults, incorporation of long-chain n−6 and n−3 fatty acids, bioequivalency of linolenic acid in primates, ¹³C nuclear magnetic resonance spectra of arachidonic acid in living rat brain, and effect of triacylglycerol structure on absorption. Radioisotope and stable isotope tracer studies in animals and humans are responsible for much of our understanding of fatty acid and lipid metabolism. However, tracer studies have limitations, and there are some unresolved issues associated with isotope studies. Examples of unresolved issues are quantification of isotope data, validity of in vivo fatty acid metabolite results, kinetic modeling, subject variability, and use of blood lipid data as a reflection of tissue lipid metabolism. Resolving these issues, developing novel methodology, and applying stable isotope tracer methods to questions related to PUFA metabolism are broad areas of interesting and challenging research opportunities.     

Computational studies of the gramicidin channel

Ion channels are highly specific membrane-spanning protein structures which serve to facilitate the passage of selected ions across the lipid barrier. In the past decade, molecular dynamics simulations based on atomic models and realistic microscopic interactions with explicit solvent and membrane lipids have been used to gain insight into the function of these complex systems. These calculations have considerably expanded our view of ion permeation at the microscopic level. This Account will mainly focus on computational studies of the gramicidin A channel, one of the simplest and best characterized molecular pore.     

Simulation studies of nearest-neighbor distribution functions and related structural properties for hard-sphere systems

Molecular dynamics simulations have been carried out to investigate nearest-neighbor distribution functions and closely related quantities for the system of hard-spheres. The nearest-neighbor distribution function and the exclusion probability function were computed to examine the density dependence on the structural ‘void’ and ‘particle’ properties. Simulation results were used to access the applicabilities of various theoretical predictions based on the scaled-particle theory, the Percus-Yevick equation, and the Carnahan-Starling approximation. For lower density systems the three different approximations give the nearest-neighbor distribution functions which are very close to one another and also to the resulting simulation data. Among those theoretical predictions, the Carnahan-Starling approximation gives remarkably good agreement with the simulation data even for higher density systems. Also calculated is the nth moment of the nearest-neighbor distribution functions, in which the corresponding length scale is directly related to the measurement of the characteristic pore-size distribution.     

Further studies on the isomerization of polyunsaturated fatty acids by potassium tertiary butoxide

Summary A temperature and time study revealed that the best conditions for the isomerization and estimation of polyunsaturated acids are heating in a sealed tube for 2 hrs. at 140° with potassium-t-butoxide in t-butanol. Under these conditions conjugation of linolenic and arachidonic acids appears to have attained completion. With linoliec acid a higher degree of conjugation than by other methods and a shift of peak to lower wavelength (231–232 mμ) are observed. Extinction coefficients obtained by this method for isomerized pure linoleic (k₂₃₂, 98.0) and linolenic (k₂₆₈, 138.4) acids are the highest so far reported. Ultraviolet and infrared examination of the conjugated trienes isolated from isomerized linolenic acid concentrate showed that, under these conditions, trienes similar to both alpha- and beta-eleostearic acids are produced. Analyses of samples of oils for component polyunsaturated acids, by this method, compare well with those by other methods. The presence of small amounts of a tetraenoic acid in linseed oil is noted. Postdoctoral Fellow in physiological chemistry. This work was supported in part from funds granted by the Ohio State University Research Foundation to the university for aid in fundamental research. Presented at the 31st Fall Meeting of the American Oil Chemists' Society, Cincinnati, O., September 30 to October 2, 1957.     

Computational studies on the diffusion behaviour of alkylaromatics in large pore zeolites

Alkylation of aromatics over solid acid catalysts such as zeolites, has emerged in the recent past as a viable alternative to conventional Friedel–Crafts alkylation over environmentally hostile catalysts. We studied the diffusion behaviour of ethylbenzene (EB), isobutylbenzene (IBB), o-, m- and p-isobutylethylbenzene (IBEB) in various zeolites such as offretite (OFF), cancrinite (CAN), ZSM-12 (MTW) and ZSM-18 (MEI) by computational procedures. The periodic variations of interaction energy between the molecules and zeolite framework in the calculated diffusion energy profiles are used to predict the energy barrier for diffusion. We analyzed the results to understand the product selectivity in the formation of IBEB in the transalkylation/disproportionation reaction between IBB and EB. The results indicated that the zeolites with channel-like pores are more suitable than those with cage-like pores to achieve better selectivity. The zeolites with channels whose diameters are close to the dimensions of the molecules and those which do not have intersecting channels are better selective catalysts. The efficiency of shape selective production of p-IBEB in these zeolites will be in the order MEI < OFF ∼ MTW < CAN as predicted from their diffusion energy barriers. The detailed analysis of the configurations of the molecules in the most favourable and unfavourable adsorption location, indicate that the p-IBEB has favourable interaction energy in all the four zeolites with different pore architecture, compared to o- and m-IBEB except for MEI. It could be concluded that the pore architecture plays a dominant role in controlling the adsorption and diffusion characteristics of these molecules. The actual values of interaction energy themselves are indication of their adsorption behaviour inside the pores of the zeolite. This revised version was published online in June 2006 with corrections to the Cover Date.     

Computational studies of the primary phototransduction event in visual rhodopsin

This Account addresses recent advances in the elucidation of the detailed molecular rearrangements due to the primary photochemical event in rhodopsin, a prototypical G-protein-coupled receptor (GPCR) responsible for the signal transmission cascade in the vertebrate vision process. The reviewed studies provide fundamental insight on long-standing problems regarding the assembly and function of the individual residues and bound water molecules that form the rhodopsin active site, a center that catalyzes the 11-cis/all-trans isomerization of the retinyl chromophore in the primary step of the phototransduction mechanism. Emphasis is placed on the authors' recent computational studies, based on state-of-the-art quantum mechanics/molecular mechanics (QM/MM) hybrid methods, addressing the structural refinement of the retinyl chromophore binding site in high-resolution X-ray structures of bovine visual rhodopsin, the energy storage mechanism, and the molecular origin of spectroscopic changes due to the primary photochemical event.     

Computational studies of the activation of lipases and the effect of a hydrophobic environment

We have investigated the activation pathway of three wild type lipases and three mutants using molecular dynamics techniques combined with a constrained mechanical protocol. The activation of these lipases involves a rigid body hinge-type motion of a single helix, which is displaced during activation to expose the active site and give access to the substrate. Our results suggest that the activation of lipases is enhanced in a hydrophobic environment as is generally observed in experiments. The energy gain upon activation varies between the different lipases and depends strongly on the distribution of the charged residues in the activating loop region. In a low dielectric constant medium (such as a lipid environment), the electrostatic interactions between the residues located in the vicinity of the activating loop (lipid contact zone) are dominant and determine the activation of the lipases. Calculations of the pKas qualitatively indicate that some titratable residues experience significant pK shifts upon activation. These calculations may provide sufficient details for an understanding of the origin and magnitude of a given electrostatic effect and may provide an avenue for exploring the activation pathway of lipases.      

Biological effects and safety issues related to long-chain polyunsaturated fatty acids in infants

The purpose of this workshop at the American Oil Chemists’ Society Symposium, “PUFA in Infant Nutrition: Consensus and Controversies”, was to enumerate the safety issues raised by the prospect of supplementing infant formulas with long-chain polyunsaturated fatty acids (LC-PUFA), to evaluate the evidence that these concerns are problematical, or theoretically problematical, and to identify the safety issues most in need of resolution. This was approached by reviewing briefly the known biological effects of LC-PUFA and how these effects might give rise to concerns about safety of LC-PUFA as components of infant formulas. Some of these issues were then discussed in more detail by invited participants, all of whom had submitted abstracts concerning the issue discussed. The pertinent aspects of all issues discussed during the workshop are summarized. In addition, since the symposium was held over 2 yr ago, an addendum summarizing additional data reported since the symposium that either support or refute issues discussed during the workshop also is included. Summary of workshop “Biological Effects and Safety Aspects of PUFA Related to Infants”, held at the AOCS symposium: PUFA in Infant Nutrition: Consensus and Controversies, November 7–9, 1996, Barcelona, Spain.     

Computational and experimental studies of mercury adsorption on unburned carbon present in fly ash

Unburned carbon (UBC) present in fly ash has been shown to adsorb mercury. In this work mercury adsorption onto the surface of UBC particles was investigated by using both computational and experimental methods. The UBC surfaces were assumed to be similar to that of graphene (single-layer graphite). The theoretical predictions using the Hartree–Fock method found that the zigzag edge of the carbonaceous cluster (C₂₅H₉) used provides stronger forces to attract mercury compared to the armchair edge (C₂₄H₈), probably resulting in greater mercury removal from flue gases. The adsorption of mercury on the simulated UBC surface (C₂₅H₉) was found to be a chemical process, with the predicated adsorption energy of 288.632 kJ/mol at room temperature. Furthermore, as temperature increases the adsorption energy slightly raises. The experimental studies showed that decreasing the particle size of UBC particles resulted in higher mercury uptake. Increasing the bed length resulted in higher mercury uptakes. Particle size can affect the sorbent capacity, and in this study UBC particles with size ranging between 125 and 250 μm seem to be more effective for mercury adsorption.     

Computational and experimental studies of electrohydrodynamic atomization for pharmaceutical particle fabrication

Electrohydrodynamic atomization (EHDA) is a promising method for the fabrication of micro‐ and nanosized particles with narrow‐size distribution and better morphologies in comparison to conventional methods of particle fabrication. A computational model was developed in this study to simulate the fluid and particle dynamics in an EHDA chamber, and thereby providing a means of predicting particle collection efficiencies at various operating conditions. Experiments were also conducted using a new design of the EHDA chamber. It was found that nitrogen flow rate, solution flow rate and voltage difference between the nozzle and ring can significantly affect the particle collection efficiency of the EHDA process. Electric field and electric potential profiles in the chamber were significantly affected by the combined voltages of the nozzle and ring. In general, a good qualitative agreement in particle collection efficiencies was obtained from experiments and simulations. The computational model developed in this study provided a means of understanding the various processes involved in micro‐ and nano‐sized particle fabrication using the EHDA methodology. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

三（正）丁基锡氢化物存在下的自由基环化反应

三(正)丁基锡氢化物在 AIBN 存在下可使某些分子中含有重键的卤化物或羰基化合物发生分子内环化反应,形成各种环状化合物,而且反应条件温和,收率高。     

Computational studies of turbulent premixed flame based dump combustor

The present work reports the computational studies of turbulent premixed flame based dump combustor. The effects of various flow parameters such as inlet Reynolds number, inlet turbulence intensity and expansion ratio on important flow quantities like axial velocity, turbulent kinetic energy and turbulent dissipation rate have been studied extensively. It was found out that the axial velocities and the radial velocities within the flowfield are many times higher as compared to the cold flow case due to the heat release and volumetric expansion. The reattachment length for the reacting flow case is found to be lower than that of the cold flow cases due to higher recirculation velocities. A maximum reduction of 64.6% compared to the cold flow reattachment length was observed in the case of turbulence intensity, I = 10%, equivalence ratio,  = 1.00 with an expansion ratio of 2. In contrast, minimum of 41.51% reduction in recirculation length was observed in the case of I = 5%,  = 0.5 for the same expansion ratio. This reduction was quite significant at higher inlet turbulence intensities and at higher equivalence ratios.     

Some thermal studies on polystyrylpyridine resins and related model compounds

Five model compounds representing part of the structure of polystyrylpy-ridines have been studied using differential scanning calorimetry (DSC) and pyrolysis-gas chromatography-mass spectrometry (P-GC-MS). Cured resin samples have also been examined by pyrolysis-gas chromatography-mass spectrometry and thermogravimetry. DSC showed that, with one exception, all the model compounds were crystalline as supplied and recrystallisation occurred readily on melting and cooling. The breakdown products in P-GC-MS, with the exception of 1-methyleneindene (produced from three of the model compounds), could all be explained on the basis of general bond scission followed by H or C₆H₅ transfer. Thermogravimetry indicated the minimum cure cycle necessary to give optimum thermal stability on a weight loss basis.     

Computational and experimental studies of electrospray deposition process in pharmaceutical micro-pattern formation

Electrospray deposition on a substrate through a mask is a simple (single-step) yet versatile and robust approach to generate biodegradable polymeric particle patterns. Different methods including photolithography, soft lithography and ink jetting have been employed for automated micro-pattern fabrication; however, most of them are limited to the investigation of material properties of substrates with high-cost and complex procedures. In the present work, two slightly different experimental setups were used to investigate the effect of different operational parameters in electrospray particle deposition on both mask and substrate. The sample consists of an aqueous solution of polymer, solvent and drug. In addition to the experimental section, a mathematical model was developed to track the particle trajectories and focusing effect in electrospray deposition process on the substrate.The final results confirm that the clearest particle pattern and the best focusing effect on the substrate can be achieved with long distance between the nozzle and the substrate, high voltage difference between the nozzle and the mask, short process time and low solution flow rate. On the contrary, a smooth and integrated layer on the mask can be formed with a short distance between the nozzle and substrate in which no clear pattern can be recognized. Furthermore, micro-fibers can be observed on the mask when the voltage difference between the nozzle and substrate is not high.