Biomedical subjects

D W Armstrong

Publications and source records attributed to D W Armstrong.

At least 55 records · Page 3Linked to original sources

PubMed1999

Novel living skin replacement biotherapy approach for wounded skin tissues.

A novel living skin replacement (LSR) biotherapy concept, addressing the challenging problems related to tissue regeneration and wound healing, is presented for the treatment of skin burns, traumatic injuries and ulcerations. LSR combines elements of cell therapy along with those of tissue engineering to allow for the regeneration of wounded skin. It takes advantage of biodegradable microspheres onto which donor skin epidermal and dermal cells can be attached and expanded in vitro for subsequent direct application down to the deepest recesses of the wound bed. The key element of the biotherapy is the ability of the skin cells to migrate freely from the microspheres into the wound for regeneration of the tissues. The large surface to volume ratio of the microspheres allows for the delivery of appropriate cell numbers while minimizing the amount of biomaterial to be resorbed. This novel approach presents a number of advantages over existing therapies including facilitated cell manipulations, ease of storage and transportation, rapid clinical intervention due to the elimination of any surgical suturing or stapling, and a more natural three-dimensional tissue remodeling and anatomical compliance. Preliminary in vitro and in vivo evidence of the LSR functionality and its potential benefits is presented.

Animals↗

PubMed1998

Effect of temperature on retention of enantiomers of beta-methyl amino acids on a teicoplanin chiral stationary phase.

The isocratic retention of enantiomers of beta-methyl amino acids (beta-methyltyrosine, beta-methylphenylalanine, beta-methyl-tryptophan and beta-methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) was studied on a teicoplanin-containing chiral stationary phase at different temperatures and at different mobile phase compositions, using the reversed-phase mode. With variation of both mobile phase composition and temperature, almost baseline separations could be achieved for all four enantiomers of sterically hindered amino acids. The retention factors and selectivity factors for the enantiomers of all investigated compounds decreased with increasing temperature. The natural logarithms of the retention factors (ln k) of the investigated compounds depended linearly on the inverse of temperature (1/T). van 't Hoff plots afforded thermodynamic parameters, such as the apparent change in enthalpy (delta H degree), the apparent change in entropy (delta S degree) and the apparent change in Gibbs free energy (delta G degree) for the transfer of analyte from the mobile to the stationary phase. The thermodynamic constants (delta H degree, delta S degree and delta G degree) were calculated in order to promote an understanding of the thermodynamic driving forces for retention in this chromatographic system.

Amino Acids↗

PubMed1998

Nicotine enantiomers and oxidative stress.

Nicotine affects a variety of cellular processes ranging from induction of gene expression to secretion of hormones and modulation of enzymatic activities. The objective of this study was to characterize the toxicity of nicotine enantiomers as well as their ability to induce oxidative stress in an in vitro model using Chinese hamster ovary (CHO) cells. Colony formation assay has demonstrated that (-)-nicotine is the more toxic of the enantiomers. At 6 mM concentrations, (-)-nicotine was found to be approximately 28- and 19-fold more potent than (+)-, and (+/-)-nicotine (racemic), respectively. Results also indicated that the toxicity of (+/-)-nicotine is higher than that of (+)-nicotine. (-)-Nicotine at a 10 mM concentration substantially decreased glutathione (GSH) levels (46% decrease). In addition, a 3-fold increase in malondialdehyde (MDA) level was evident in cells after exposure to 10 mM (-)-nicotine. Increased lactate dehydrogenase (LDH) activities in the media demonstrated that cellular membrane integrity was disturbed in nicotine treated cells. In the presence of superoxide dismutase (SOD) and catalase (CAT), the LDH activities returned to control value in 24 h with all concentrations of (-)-, (+)-, and (+/-)-nicotine. The decreases in LDH activities in the presence of the radical scavenging enzymes SOD and CAT suggest that membrane damage may be due to free radical generation.

Animals↗

PubMed1998

Bubble fractionation of enantiomers from solution using molecularly imprinted polymers as collectors.

Adsorptive bubble separation methods have been used to enrich components from both heterogeneous and homogeneous solutions. These methods are particularly effective for processing large solution volumes at low cost. Previous work demonstrated that chiral, surface-active collectors could be used to enrich enantiomers from homogeneous solution in a foam fractionation process. In a significant extension of this work, the use of highly selective molecularly imprinted polymers (MIPs) and heterogeneous solutions for the bubble flotation of enantiomers was evaluated. The high selectivity and ease of recycling of the MIP make this a potentially powerful approach for process-scale separations from large-volume bulk solutions. New MIPs were produced with low swelling properties which allowed them to retain enantioselectivity after numerous recyclings.

Adsorption↗

PubMed1998

Characterization and evaluation of d-(+)-tubocurarine chloride as a chiral selector for capillary electrophoretic enantioseparations.

A new macrocyclic of the bis(benzylisoquinoline) alkaloid family, d-(+)-tubocurarine chloride (DTC), has been evaluated as a chiral selector for the separation of optical isomers of organic carboxylates using capillary electrophoresis (CE). The pertinent physicochemical properties, such as absorption spectrum, isoionic point, and solution conformation, of DTC were determined. The effects of varying such experimental parameters as DTC concentration, pH, and methanol content in the running buffer were assessed. CE separation of the enantiomers of 18 different compounds was achieved using DTC as the chiral selector under optimized background electrolytic conditions.

Carboxylic Acids↗

PubMed1998

High-performance liquid chromatographic separation of enantiomers of unusual amino acids on a teicoplanin chiral stationary phase.

A glycopeptide antibiotic, teicoplanin, was used as chiral stationary phase for the high-performance liquid chromatographic (HPLC) separation of enantiomers of more than 30 unnatural amino acids, such as phenylalanine and tyrosine analogues and analogues containing 1,2,3,4-tetrahydroisoquinoline, tetraline, 1,2,3,4-tetrahydro-2-carboline, cyclopentane, cyclohexane, cyclohexene, bicyclo[2.2.1]heptane or heptene skeletons. Excellent resolutions were achieved for most of the investigated compounds by using a hydro-organic mobile-phase system. The effects of organic modifier content, temperature and flow-rate on the resolution were investigated and the conditions of separation were optimized.

Amino Acids↗

PubMed1998

Mechanism of capillary electrophoresis enantioseparations using a combination of an achiral crown ether plus cyclodextrins.

The addition of an achiral crown ether (18-crown-6) to a cyclodextrin-based separation can significantly affect the capillary electrophoresis (CE) enantioresolution of organic racemates that contain a primary amine functional group. In most cases an enhancement of the enantioseparation was observed. However, there are also cases where the addition of 18-crown-6 was detrimental to a cyclodextrin-based CE enantioseparation. The effect of concentration of the two complexing additives as well as the effect of pH and added potassium ion were examined. A specific three-body complex involving simultaneous, dual inclusion complex formation can be used to explain both the enhanced and diminished enantioselectivities observed when 18-crown-6 is added to the run buffer.

Amines↗

PubMed1998

Highly enantioselective HPLC separations using the covalently bonded macrocyclic antibiotic, ristocetin A, chiral stationary phase.

The macrocyclic glycopeptide, ristocetin A, was covalently bonded to a silica gel support and evaluated as a liquid chromatographic (LC) chiral stationary phase (CSP). Over 230 racemates were resolved in either the reversed-phase mode, the normal-phase mode, or the polar-organic mode. The retention behavior and selectivity of this CSP were examined in each mode. Optimization of separations on this column is discussed. The ristocetin A CSP appeared to be complimentary to other glycopeptide CSPs (i.e., vancomycin and teicoplanin). Column stability was excellent. The CSP was not irreversibly altered when going from one mobile phase to another.

Anti-Bacterial Agents↗

PubMed1998

Evaluation of the macrocyclic antibiotic avoparcin as a new chiral selector for HPLC.

Avoparcin is a macrocyclic glycopeptide antibiotic structurally related to vancomycin, teicoplanin, and ristocetin A. When attached to 5-microns spherical silica gel, the avoparcin proved to be an effective chiral stationary phase (CSP) that could be used in the reversed-phase, normal-phase, and polar-organic modes. The avoparcin CSP was complimentary to the other macrocyclic glycopeptide CSPs in that it could resolve some racemates that the others could not, and vice versa. Some important compounds resolved on the avoparcin CSP include verapamil, thyroxine, mephenytoin, and 2-imidazolidone-4-carboxylic acid. The use of this CSP and the optimization of separations on it are discussed. Avoparcin appears to be a useful addition to this family of CSPs.

Anti-Bacterial Agents↗

PubMed1998

Metabolic and endocrine responses to cold air in women differing in aerobic capacity.

PURPOSE: The purpose of this study was to measure resting metabolic rate, plasma norepinephrine, and plasma immunoreactive beta endorphin during exposures to cold air during two consecutive 5-d periods, separated by 2 weekend days, in two groups of women differing in aerobic fitness. METHODS: Plasma norepinephrine (NE), plasma immunoreactive beta-endorphin (IBE), and resting metabolic response (RMR) were measured during repeated exposures to 3.5 degrees C air in two groups of women differing in aerobic fitness. Ten women, separated into highly fit (HFW) and less fit (LFW) groups, sat in 22 degrees C air for 45 min followed by 45 min in 3.5 degrees C air each day during two consecutive 5-d periods separated by two weekend days. RESULTS: Norepinephrine was not different between groups during warm air exposure; however, following 45 min of cold air, NE was two times higher in HFW compared with that in LFW (P < 0.001). Plasma IBE was elevated (P < 0.02) in HFW compared with that in LFW but was not affected by exposure to cold on any test day. Warm RMR was not different between groups and remained unchanged during the study period. Cold RMR was significantly higher in LFW compared with that in HFW (P < 0.01). Resting metabolic rate peaked at 30% of VO2peak in LFW by the 5th minute of cold exposure on day 1 before declining to 21% and remaining steady. In contrast, RMR in HFW peaked at about 13% and then fell to 9.4% before slowly increasing to 14% by the end of 45 min. On other test days HFW RMR increased to 14% of VO2peak and rose slowly through 45 min of cold exposure while LFW RMR peaked at 24% of VO2peak before declining to 20% and remaining steady. CONCLUSIONS: Our findings suggest that, in women, aerobic fitness alters the endocrine and metabolic responses to acute cold air exposure. The norepinephrine response is exaggerated in highly fit women exposed to cold but not the metabolic response. Immunoreactive beta endorphin was not affected by exposure to cold but was elevated in highly fit women. We further conclude that the temperature threshold for acclimation to cold air by women may be higher than the air temperature used in this study.

Acclimatization↗

PubMed1997

Methods for the estimation of binding constants by capillary electrophoresis.

Capillary electrophoresis (CE) has developed into a particularly effective means to determine apparent equilibrium constants for molecular association in solution (e.g., to micelles, cyclodextrins, antibiotics, proteins, RNA, DNA, etc.). The various experimental, graphical and mathematical approaches for determining association constants are reviewed. In CE, association constants can be calculated because there is a relationship between substrate concentration and the measured electrophoretic mobility of the solute. Most of the approaches for obtaining association constants by CE are conceptually and mathematically related to one another. Likewise, they are analogous to many spectroscopic techniques that are used for obtaining association constants. The advantages, limitations and proper use of the various CE approaches are examined.

Electrophoresis, Capillary↗

PubMed1997

Capillary electrophoretic enantioseparations using macrocyclic antibiotics as chiral selectors.

Macrocyclic antibiotics are the newest class of chiral selector used in capillary electrophoresis (CE) enantioseparations. Two basic types of compounds have proven to produce useful enantioseparations. They are the ansa compounds and the glycopeptides. The ansa compounds best resolve basic or cationic racemates while the glycopeptides best resolve acidic or anionic analytes. The structure and use of these chiral selectors are discussed. The factors affecting separation and the separation mechanism are considered as well.

Anti-Bacterial Agents↗

PubMed1996

Investigation on enantiomeric separations of fluorenylmethoxycarbonyl amino acids and peptides by high-performance liquid chromatography using native cyclodextrins as chiral stationary phases.

A systematic study was carried out to investigate enantiomeric separations of fluorenylmethoxycarbonyl (FMOC) amino acids and their peptides. Twenty amino acids were derivatized by 9-fluorenylmethyl chloroformate (FMOC-Cl) and its analogues, FMOC-glycyl-Cl and FMOC-beta-alanyl-Cl. All derivatives were chromatographed on native beta- and gamma-cyclodextrin columns using acetonitrile as the main mobile phase component. The results indicated that glycyl and beta-alanyl groups between FMOC and amino acid moieties enhanced chiral selectivities of amino acid derivatives. The addition of modifiers, triethylamine, acetic acid and methanol, into the mobile phase caused alterations in retention, enantiorecognition and elution order. The structures of amino acids and the type of chiral stationary phase employed exhibited significant impacts on chiral resolutions. It is also found that the number and position of glycyl moieties affect the retentions and enantioselectivities of FMOC derivatized glycyl containing peptides.

Alanine↗

PubMed1996

Comparison and modeling study of vancomycin, ristocetin A, and teicoplanin for CE enantioseparations.

The structurally related glycopeptide antibiotics vancomycin, ristocetin A, and teicoplanin can all be used as chiral selectors in capillary electrophoresis (CE). Both experimental and modeling studies were done to elucidate their similarities and differences. There are identifiable morphological differences in the aglycon macrocyclic portions of these three compounds. In addition, there are other structural distinctions that can affect their CE enantioselectivity, migration times, and efficiency. Teicoplanin is the most distinct of the three and is the only one that is surface active. Its aggregational properties appear to affect its enantioselectivity among other things. The similar but not identical structures of the three glycopeptides produce similar but not identical enantioselectivities. This leads to the empirically useful "principle of complementary separations", in which a partial resolution with one chiral selector can be brought to baseline with one of the others. Overall, ristocetin A appears to have the greatest applicability for CE enantioseparations.

Anti-Bacterial Agents↗

PubMed1996

Facile liquid chromatographic enantioresolution of native amino acids and peptides using a teicoplanin chiral stationary phase.

The glycopeptide antibiotic teicoplanin is shown to be a highly effective stationary phase chiral selector for the resolution of underivatized amino-acid and imino-acid enantiomers. Fifty four of these compounds (including all chiral protein amino acids) as well as a number of dipeptides were resolved. Hydro-organic mobile phases are used and no buffers or added salts are needed in most cases. Hence the purified analytes are easily isolated in pure form, if needed, by evaporating of the solvent. The effect of pH, organic modifier type and amount are discussed. The enantioselective separation mechanism is examined using both molecular modeling and retention data. The strongest stereoselective interaction is for carboxy-terminated D-amino-acids. In case of peptides, it is not necessary for these to be a D-, D-, terminal sequence for strong interactions. In some cases, including Ala-Ala, the L-, D-, terminal sequence showed greater interaction with the teicoplanin chiral stationary phase.

Amino Acid Sequence↗

PubMed1996

In vitro study of the metabolic effects of D-amino acids.

While the L-configuration of amino acids predominates in all known living systems, D-enantiomers of amino acids have been detected with highly sensitive chromatographic techniques in human physiological fluids. In the present study, the survival of Chinese hamster ovary cells (CHO) and HeLa cells was inhibited by exposure to high concentrations of some D- or L-amino acids. Inhibition of colony formation, though, was not necessarily observed to be chiral-dependent. Some L-amino acids (LAAs) were found to be toxic while other D-amino acids (DAAs) were innocuous in both cultures. This is contradictory to the previous observations that DAAs were generally considered to be harmful. Frequently it was implied, although not experimentally proven, that the LAAs were not toxic. One of the metabolites produced by oxidative deamination of D- or LAAs is hydrogen peroxide (H2O2), a reactive oxygen species (ROS) that is decomposed by catalase. Increased intracellular H2O2 can result in peroxidation of lipids. We measured catalase activity and the lipid peroxide levels (LPO) after incubating cells in either D- or LAAs. The amino acids (AAs) that were found to inhibit colony formation were found to be associated with higher levels of catalase activity and LPO. Therefore, we hypothesize that enhanced ROS generation may be, in part, responsible for the observed toxicity of some amino acids.

Amino Acids↗

PubMed1996

Evaluation of the concentration and enantiomeric purity of selected free amino acids in fermented malt beverages (beers).

Even though amino acids are important trace components in the brewing of beers, they have not been extensively evaluated in these beverages. Studies involving the enantiomeric composition of these amino acids are even less prevalent. A brief summary of the brewing process for malt beverages is given. The total concentration and enantiomeric composition of three amino acids (leucine, phenylalanine, and proline) were determined in 25 different beers. Proline tended to have the highest average absolute concentration and the lowest percentage of the D-enantiomer in most samples. In some cases the relative amounts of D-phenylalanine and D-leucine exceeded 10% of the individual amino acids. The enantiomeric composition of the amino acids in different beer samples did not vary as extensively as the absolute concentrations. The reason for the concentration differences between proline and the other amino acids is discussed. A knowledge of amino acid concentrations and enantiomeric compositions appears to be useful in characterizing specific beers and brewing processes.

Amino Acids↗

PubMed1995

Effect of micelles and mixed micelles on efficiency and selectivity of antibiotic-based capillary electrophoretic enantioseparations.

Vancomycin (an oligophenolic, glycopeptide, macrocyclic antibiotic) has been shown to be a superb chiral selector for anionic and neutral compounds. It was found that adding sodium dodecyl sulfate to the run buffer increased efficiency by over 1 order of magnitude, decreased analysis times, and reversed the elution order of the enantiomers. This allows for control of the retention order as well as the resolution of enantiomers in complex mixtures in a single run. A mechanism is proposed which explains all of the observed effects and is verified experimentally. Since vancomycin is present in both the micelle and in free solution, previously proposed micelle-selector models are, at best, limiting cases. A general equation is derived which can be used to describe all possible interactions, including those with the capillary wall, if needed. Also, it is shown that electrophoretic mobilities and not migration times must be used to calculate binding constants of a solute to the micelle, the chiral selector, or both. Furthermore, it is shown that a neutral marker molecule cannot be used to accurately correct mobilities that have been altered due to changes in solution viscosity. While this work utilizes the practical vancomycin-micelle system, the general conclusions and theory apply to most other analogous CE systems as well.

Anti-Bacterial Agents↗