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S A Mitchell

Publications and source records attributed to S A Mitchell.

At least 19 recordsLinked to original sources
PubMed2006

Origin of second harmonic generation optical activity of a tryptophan derivative at the air/water interface.

Second harmonic generation optical activity (SHG-OA) of chiral monolayers of the tryptophan derivative N(alpha)-(tert-butoxycarbonyl)-tryptophan (BOC-Trp) at an air/water interface has been studied in detail. In combination with previously reported experimental measurements with the fundamental frequency variant Planck's 'h/' omega=2.20 eV (lambda=564 nm), new measurements with lambda=564 and 800 nm fully characterize the nonlinear susceptibility tensors of chiral and achiral (racemic) monolayers under two-photon resonant and nonresonant conditions of the fundamental frequency. A realistic computational approach including semiempirical, intermediate neglect of differential overlap (ZINDO/S) calculations has been used to calculate the nonlinear susceptibilities of model achiral and chiral monolayers composed of indole chromophores. There is satisfactory agreement between calculated and observed nonlinear susceptibilities, which constrains certain structural parameters of the monolayers including the absolute orientation of the long molecular axis of indole at the air/water interface. The origin of SHG-OA of BOC-Trp monolayers is discussed with reference of two distinct mechanisms at the microscopic level, designated type I or chiral assembly and type II or electronic coupling. Both mechanisms are studied in detail within the framework of ZINDO/S calculations. The dominant effect for the BOC-Trp monolayers is type I, involving chiral assembly of indole chromophores.

Journal Article↗
PubMed2006

Nonlinear susceptibility of a cyanobiphenyl derivative at the air/water interface: improved measurement of molecular orientation by optical second harmonic generation.

The nonlinear susceptibility tensor, chi(2), for second harmonic generation (SHG) by a monolayer of the cyanobiphenyl derivative 4-n-octyl-4'-cyanobiphenyl (8CB) at the air/water interface has been measured with the fundamental frequency variant Planck's over 2piomega=1.55 eV (lambda=800 nm). The contribution of the water subphase was estimated by measuring the magnitudes and relative phases of the nonlinear susceptibilities of the bare and monolayer-covered water surfaces. All nonzero elements of chi(2) are placed on an absolute scale by comparison with SHG in reflection from a z-cut quartz crystal. The experimental measurements for the 8CB monolayer are compared with computed susceptibilities derived by using standard time-dependent perturbation theory in conjunction with a semiempirical electronic structure model. Good agreement has been found between experimental and computational results when the average tilt angle of the cyanobiphenyl chromophore of 8CB is in the range 60-70 degrees relative to the surface normal. A critique is given of an alternative, simplified measurement procedure of the tilt angle in which only the dominant element of the molecular hyperpolarizability tensor is considered. It is shown that the simplified procedure is invalid for 8CB monolayers when the tilt angle is greater than approximately 70 degrees.

Journal Article↗
PubMed2005

Orientation and confinement of cells on chemically patterned polystyrene surfaces.

UV/ozone oxidation was combined with a photomasking technique to produce adjacent regions of different chemistry on polystyrene (PS) surfaces. The surface chemistry and topography were studied using AFM, XPS and contact angle measurements. The physicochemical patterns were visualised by the condensation of water vapour upon the surfaces and by the differential attachment of Chinese hamster ovarian (CHO) cells. The orientation of CHO cells on 55 and 125 microm wide oxidised PS strips were measured and found to be highly dependent on the width of the oxidised feature. CHO cells in relatively close proximity to a linear polar/non-polar border showed significant axial alignment along the border. CHO cells can also be confined to specific regions of the polymer surface. Cells attached to larger areas (75 microm x 75 microm) were found to have a smaller average cell size than cells attached to the smaller (56 microm x 56 microm) areas.

Animals↗
PubMed2005

The influence of electrostatic forces on protein adsorption.

In this paper we investigate the importance of electrostatic double layer forces on the adsorption of human serum albumin by UV-ozone modified polystyrene. Electrostatic forces were measured between oxidized polystyrene surfaces and gold-coated atomic force microscope (AFM) probes in phosphate buffered saline (PBS) solutions. The variation in surface potential with surface oxygen concentration was measured. The observed force characteristics were found to agree with the theory of electrical double layer interaction under the assumption of constant potential. Chemically patterned polystyrene surfaces with adjacent 5 microm x 5 microm polar and non-polar domains have been studied by AFM before and after human serum albumin adsorption. A topographically flat surface is observed before protein adsorption indicating that the patterning process does not physically modify the surface. Friction force imaging clearly reveals the oxidation pattern with the polar domains being characterised by a higher relative friction compared to the non-polar, untreated domains. Far-field force imaging was performed on the patterned surface using the interleave AFM mode to produce two-dimensional plots of the distribution of electrostatic double-layer forces formed when the patterned polystyrene surfaces is immersed in PBS. Imaging of protein layers adsorbed onto the chemically patterned surfaces indicates that the electrostatic double-layer force was a significant driving force in the interaction of protein with the surface.

Adsorption↗
PubMed2005

Second-harmonic generation optical activity of a polypeptide alpha-helix at the air/water interface.

Quantitative measurements of second-harmonic generation optical activity (SHG-OA) have been performed for alpha-helical polypeptides poly-(gamma-benzyl-L-glutamate) and poly-(gamma-ethyl-L-glutamate) adsorbed at the airwater interface, with the fundamental frequency variant Planck's over 2piomega = 2.96 eV (lambda = 417 nm). The chiral component of the nonlinear susceptibility chi(XYZ) ((2)) is small for both polymers, being comparable in magnitude with the susceptibility chi(XXZ) ((2)) of the clean airwater interface. The microscopic origin of the nonlinear response has been investigated by using semiempirical ZINDOS calculations in conjunction with standard time-dependent perturbation theory to evaluate the molecular hyperpolarizability tensor of a model alpha-helix composed of glycine residues. Calculated nonlinear susceptibilities (per monomer unit) are in good agreement with experimental measurements for both the chiral and achiral response. The computational results indicate that charge transfer transitions of the alpha-helix have a large influence on the achiral components of the hyperpolarizability tensor, and produce characteristic features in the response under suitable experimental conditions. The dominant origin of SHG-OA for the model alpha-helix is a structural effect due to the tilt of the plane of each amide group of the helix relative to the helical axis. SHG-OA is associated with the orientational distribution of isolated, achiral chromophores, and is present in the absence of electronic coupling between the amide subunits of the polypeptide alpha-helix.

Air↗
PubMed2005

Improved cellular adhesion to acetone plasma modified polystyrene surfaces.

The plasma polymerization of acetone has been used to modify polystyrene substrates for the controlled growth of human fibroblast cells. The surface modified polystyrene was studied by X-ray photoelectron spectroscopy, water contact angle and atomic force microscopy. This showed the surface oxygen levels and wettability to increase rapidly with exposure to the acetone plasma. High-resolution XPS allowed the determination of the relative amounts of surface hydroxyl, carbonyl and carboxyl groups. This showed that there was little incorporation of carboxyl groups in the deposited films. AFM measurements revealed the films to be conformal with a surface roughness equivalent to that of the underlying polystyrene substrate with film growth rates of approximately 0.5 nm min(-1). High edge-definition patterns were produced with a simple masking procedure and allowed the confinement of cells to selected areas of the substrate. These chemically patterned surfaces allowed the study of cells confined to particular regions of the substrate as a function of incubation time.

Acetone↗
PubMed2005

Internal ribosome entry segment-mediated translation during apoptosis: the role of IRES-trans-acting factors.

During apoptosis, there is a reduction in translation initiation caused by caspase cleavage of several of the factors required for the cap-dependent scanning mechanism. Under these circumstances, many proteins that are required for apoptosis are instead translated by the alternative method of internal ribosome entry. This mechanism requires the formation of a complex RNA structural element and in the presence of internal ribosome entry segment (IRES)-trans-acting factors (ITAFs), the ribosome is recruited to the RNA. The interactions of several ITAFs with IRESs have been investigated in detail, and several mechanisms of action have been noted, including acting as chaperones, stabilising and remodelling the RNA structure. Structural remodelling by PTB in particular will be discussed, and how this protein is able to facilitate recruitment of the ribosome to several IRESs by causing previously occluded sites to become more accessible.

Apoptosis↗
PubMed2005

Investigation of interactions of polypyrimidine tract-binding protein with artificial internal ribosome entry segments.

Most eukaryotic translation initiation is thought to be dependent on the 5'-cap structure of the mRNA. It is becoming apparent, however, that the mRNAs of many genes contain IRESs (internal ribosome entry segments) within the 5'-UTR (5'-untranslated region) that allow ribosomes to initiate translation independently of the 5'-cap. IRESs can enable the expression of these genes under conditions (such as viral infection, cellular stress and apoptosis) when cap-dependent translation initiation is compromised, and also provide a target for regulation of gene expression. Recent results from our laboratory and others suggest that 10% of mRNAs (approximately 4000 genes) use this mechanism to initiate translation. One of the central goals of those working in the field of translation is to identify the sequence motif(s) and proteins that are required for internal ribosome entry. We have identified recently a unique PTB (polypyrimidine tract-binding protein) motif (CCU)n that is present in a large subset of cellular IRESs, and the results suggest that PTB itself is involved either directly or indirectly in ribosome recruitment. Here, we describe further investigations of PTB with artificial sequences that harbour this motif.

5' Untranslated Regions↗
PubMed2004

UV-ozone modification of plasma-polymerised acetonitrile films for enhanced cell attachment.

Plasma polymerisation is of great interest for modifying the surface properties of biomedical devices in order to control, for example, protein adsorption and cell attachment. In this paper we present results for plasma-polymerised acetonitrile deposited onto silicon or polystyrene substrates. The chemistry of films deposited under a range of experimental conditions was studied by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). XPS provided evidence that the elemental composition of the films varied with rf power to flow rate parameter (W/F) with films produced at higher W/F being deficient in nitrogen. FTIR revealed that the plasma deposited film contained a wide range of nitrogen functional groups including amine, imine and nitrile. Oxidation of the films by exposure to radiation from a low pressure mercury vapour lamp in an air ambient increased the surface oxygen levels from 3 to 17at.% after 300 s exposure. XPS also revealed that the oxidation process proceeded via the formation of carbonyl groups at short exposure times (<60s) while longer treatment times (>60s) resulted in an increase in the concentration of carboxyl groups. To assess their potential to support cell growth, polystyrene culture dishes coated with plasma deposited films and UV-ozone oxidised films were seeded with 1BR.3.N human fibroblast cells and incubated for up to 72 h. Un-oxidised plasma-polymerised acetonitrile films were found to give comparable cell attachment densities as tissue culture polystyrene. The greatest cell attachment density was found with plasma polymer films which had been UV-ozone treated for the longest time (300 s). Enhanced attachment to this surface was attributed to the high level of carboxylic groups found on this substrate.

Acetonitriles↗
PubMed2004

Cellular attachment and spatial control of cells using micro-patterned ultra-violet/ozone treatment in serum enriched media.

Ultra-violet Ozone (UVO) modified polystyrene (PS) surfaces were analyzed by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle (CA), optical microscopy (OM) and cell culture experiments. UV/Ozone treatment up to 900 s was used to increase the surface oxygen concentration of PS surfaces from 0% to approximately 35% (unwashed) and 0% to approximately 27% (washed). The observed differences in oxygen concentration, between washed and unwashed surfaces, have been previously attributed to the removal of low molecular weight debris produced in this treatment process. Surface roughness (Rq) is known to affect cellular attachment and proliferation. AFM studies of the UV/Ozone treated PS surfaces show the surface roughness is an order of magnitude less than that expected to cause an effect. UV/Ozone treatment of PS showed a marked change in CA which decreased to approximately 60 degrees after 900 s treatment. The increased attachment and proliferation of Chinese hamster ovarian (CHO) and mouse embryo 3T3-L1 (3T3) cells on the treated surfaces compared to untreated PS were found to correlate strongly with the increase in surface oxygen concentration. Surface chemical oxidation patterns on the PS were produced using a simple masking technique and a short UV/Ozone treatment time, typically 20-45 s. The chemical patterns on PS were visualized by water condensation and the spatially selective attachment of CHO and 3T3-L1 cells cultured with 10% (v/v) serum. This paper describes an easily reproducible, one step technique to produce a well-defined, chemically heterogeneous surface with a cellular resolution using UV/Ozone modification. By using a variety of cell types, that require different media conditions, we have been able to expand the potential applications of this procedure.

3T3 Cells↗
PubMed2004

Bystander effect and adaptive response in C3H 10T(1/2) cells.

PURPOSE: To address the relationship between the bystander effect and the adaptive response that can compete to impact on the dose-response curve at low doses. MATERIALS AND METHODS: A novel radiation apparatus, where targeted and non-targeted cells were grown in close proximity, was used to investigate these phenomena in C3H 10T(1/2) cells. It was further examined whether a bystander effect or an adaptive response could be induced by a factor(s) present in the supernatants of cells exposed to a high or low dose of X-rays, respectively. RESULTS: When non-hit cells were co-cultured for 24 h with cells irradiated with 5 Gy alpha-particles, a significant increase in both cell killing and oncogenic transformation frequency was observed. If these cells were treated with 2 cGy X-rays 5 h before co-culture with irradiated cells, approximately 95% of the bystander effect was cancelled out. A 2.5-fold decrease in the oncogenic transformation frequency was also observed. When cells were cultured in medium donated from cells exposed to 5 Gy X-rays, a significant bystander effect was observed for clonogenic survival. When cells were cultured for 5 h with supernatant from donor cells exposed to 2 cGy and were then irradiated with 4 Gy X-rays, they failed to show an increase in survival compared with cells directly irradiated with 4 Gy. However, a twofold reduction in the oncogenic transformation frequency was seen. CONCLUSIONS: An adaptive dose of X-rays cancelled out the majority of the bystander effect produced by alpha-particles. For oncogenic transformation, but not cell survival, radioadaption can occur in unirradiated cells via a transmissible factor(s).

Adaptation, Physiological↗
PubMed2004

The bystander response in C3H 10T1/2 cells: the influence of cell-to-cell contact.

Although radiation-induced heritable damage in mammalian cells was thought to result from the direct interaction of radiation with DNA, it is now accepted that biological effects may occur in cells that were not themselves traversed by ionizing radiation but are close to those that were. However, little is known about the mechanism underlying such a bystander effect, although cell-to-cell communication is thought to be of importance. Previous work using the Columbia microbeam demonstrated a significant bystander effect for clonogenic survival and oncogenic transformation in C3H 10T(1/2) cells. The present study was undertaken to assess the importance of the degree of cell-to-cell contact at the time of irradiation on the magnitude of this bystander effect by varying the cell density. When 10% of cells were exposed to a range of 2-12 alpha particles, a significantly greater number of cells (P < 0.0001) were inactivated when cells were irradiated at high density (>90% in contact with neighbors) than at low density (<10% in contact). In addition, the oncogenic transformation frequency was significantly higher in high-density cultures (P < 0.0004). These results suggest that when a cell is hit by radiation, the transmission of the bystander signal through cell-to-cell contact is an important mediator of the effect, implicating the involvement of intracellular communication through gap junctions.

Alpha Particles↗
PubMed2004

Privileged structures: applications in drug discovery.

Over the past 15 years the privileged structure concept has emerged as a fruitful approach to the discovery of novel biologically active molecules. Privileged structures are molecular scaffolds with versatile binding properties, such that a single scaffold is able to provide potent and selective ligands for a range of different biological targets through modification of functional groups. In addition, privileged structures typically exhibit good drug-like properties, which in turn leads to more drug-like compound libraries and leads. The net result is the production of high quality leads that provide a solid foundation for further development. The identification of privileged structures will be discussed, emphasizing the importance of understanding the structure-target relationships that confer "privileged" status. This understanding allows privileged structure based libraries to be targeted at distinct target families (e.g. GPCRs, LGIC, enzymes/kinases). Privileged structures have been successfully exploited across and within different target families and promises to be an effective approach to the discovery and optimization of novel bioactive molecules. The application of the privileged structure approach, both in traditional medicinal chemistry and in the design of focused libraries, will be discussed with the aid of illustrative examples.

Animals↗
PubMed2002

Alverine citrate fails to relieve the symptoms of irritable bowel syndrome: results of a double-blind, randomized, placebo-controlled trial.

BACKGROUND: Alverine citrate has been used in the treatment of irritable bowel syndrome for many years. AIMS: To compare the efficacy and safety of a new formulation of alverine citrate, a 120-mg capsule, with placebo given three times daily for 12 weeks. METHODS: One hundred and seven patients with irritable bowel syndrome were entered into this three-centre, double-blind, randomized, placebo-controlled, parallel group trial. The primary end-point was relief of abdominal pain indicated by improvement in the scores for severity and frequency. Secondary efficacy variables included scores for other clinical symptoms and for overall well-being. RESULTS: The severity and frequency of abdominal pain improved in 66% and 68% of patients treated with alverine citrate vs. 58% and 69% of the placebo group, but these differences were not significant. The mean percentage reduction in the scores for abdominal pain from baseline to the final assessment, although greater in the alverine citrate group (43.7%) compared with the placebo group (33.3%), was not statistically significant. CONCLUSIONS: Alverine citrate is no better than placebo at relieving the symptoms of irritable bowel syndrome. Future trials should be designed to take into account the high and persistent placebo response seen in this condition.

Abdomen↗
PubMed2002

Cigarette smoking, appendectomy, and tonsillectomy as risk factors for the development of primary sclerosing cholangitis: a case control study.

BACKGROUND AND AIMS: The strong clinical association between primary sclerosing cholangitis (PSC) and ulcerative colitis (UC) suggests common factors in their pathogenesis. Smoking, previous appendectomy, and tonsillectomy have been associated with a decreased risk of developing UC. In this study, our aim was to examine these risk factors in patients with PSC with and without underlying inflammatory bowel disease (IBD). METHODS: The smoking habits and history of previous appendectomy and/or tonsillectomy of 170 patients with PSC, 41 without underlying IBD, 170 patients with UC but normal liver function tests, and 170 age and sex matched community controls were obtained by questionnaire. RESULTS: A total of 112 PSC patients (66%) had never smoked compared with 66 controls (39%). Only 12 PSC patients (7%) were current smokers versus 43 controls (25%). The resultant odds ratio of having PSC was 0.17 (95% confidence interval (CI) 0.08-0.35) among current smokers and 0.33 (95% CI 0.21-0.52) among ever (former+current) smokers. Among former smokers, the odds of having PSC were also significantly decreased (odds ratio 0.45, 95% CI 0.26-0.73; p<0.05). In the subgroup of PSC patients without IBD, only 5% were current smokers versus 26% of matched controls, and never smokers were overrepresented (68% v 37%). The rate of previous appendectomy was similar in all three study groups (14%, 12%, and 13%) but the frequency of tonsillectomy was reduced in the PSC group (21% v 31%; p=0.05). CONCLUSION: PSC, like UC, is a disease of non-smokers as the odds of having PSC was significantly decreased among current and former smokers. The association between non-smoking and PSC was independent of whether the PSC patient had underlying IBD. Previous tonsillectomy but not appendectomy may also be associated with a decreased risk of PSC but this warrants further study.

Adult↗
PubMed2001

Internal ribosome entry segment-mediated initiation of c-Myc protein synthesis following genotoxic stress.

Initiation of translation of the proto-oncogene c-myc can occur by either the cap-dependent scanning mechanism or by internal ribosome entry. The latter mechanism requires a complex RNA structural element that is located in the 5' untranslated region of c-myc, termed an internal ribosome entry segment (IRES). Recent work has shown that IRESs are used to maintain protein expression under conditions when cap-dependent translation initiation is compromised; for example, during mitosis, apoptosis and under conditions of cell stress, such as hypoxia or heat shock. Induction of genotoxic stress also results in a large reduction in global protein synthesis rates and therefore we investigated whether the c-myc IRES was active following DNA damage. As expected, in cells treated with either ethylmethane sulphonate or mitomycin C there was a large reduction in protein synthesis, although this was brought about by two different mechanisms. However, in each case the c-myc IRES was active and c-Myc protein expression was maintained. Finally we showed that the proteins required for this process are downstream of the p38 mitogen-activated protein kinase (MAPK)/extracellular-signal-regulated protein kinase (ERK)/MEK(MAPK/ERK kinase) signalling pathways, since pre-treatment of cells with inhibitors of these pathways before DNA damage is initiated inhibits both c-myc IRES activity and expression of c-Myc protein.

5' Untranslated Regions↗
PubMed2001

Solid-phase synthesis of O-linked glycopeptide analogues of enkephalin.

The synthesis of 18 N-alpha-FMOC-amino acid glycosides for solid-phase glycopeptide assembly is reported. The glycosides were synthesized either from the corresponding O'Donnell Schiff bases or from N-alpha-FMOC-amino protected serine or threonine and the appropriate glycosyl bromide using Hanessian's modification of the Koenigs-Knorr reaction. Reaction rates of D-glycosyl bromides (e.g., acetobromoglucose) with the L- and D-forms of serine and threonine are distinctly different and can be rationalized in terms of the steric interactions within the two types of diastereomeric transition states for the D/L and D/D reactant pairs. The N-alpha-FMOC-protected glycosides [monosaccharides Xyl, Glc, Gal, Man, GlcNAc, and GalNAc; disaccharides Gal-beta(1-4)-Glc (lactose), Glc-beta(1-4)-Glc (cellobiose), and Gal-alpha(1-6)-Glc (melibiose)] were incorporated into 22 enkephalin glycopeptide analogues. These peptide opiates bearing the pharmacophore H-Tyr-c[DCys-Gly-Phe-DCys]- were designed to probe the significance of the glycoside moiety and the carbohydrate-peptide linkage region in blood-brain barrier (BBB) transport, opiate receptor binding, and analgesia.

Amino Acid Sequence↗