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Biomedical subjects

R K Thomas

Publications and source records attributed to R K Thomas.

At least 37 records · Page 2Linked to original sources
PubMed2004

Neutron reflection from liquid interfaces.

Recent applications of neutron reflectometry to the study of wet interfaces are described. An outline is given of the basic principles that allow the techniques to determine composition and structure in a variety of situations. These are the adsorption of surfactant molecules at air/liquid and solid/liquid interfaces, the shape of the segment-density profiles of different types of polymer, including block copolymers and polyelectrolytes, adsorption in mixed surfactant and polymer/surfactant systems, and interfacial systems of biophysical interest.

Adsorption↗
PubMed2003

The distribution of toluene in intercalation complexes of a vermiculite and alkyl trimethylammonium bromides.

Neutron diffraction and isotopic labeling have been used to determine the distribution of toluene sorbed into the interlamellar space of complexes of vermiculite with alkyl trimethylammonium bromide complex of alkyl chain lengths C(12), C(14), and C(16) and in a mixed C(12)/C(16) complex. The toluene remains in a liquid-like form and fills and swells the interlamellar space space up to a point where there remains a small amount of overlap of the alkyl chains of the surfactant. If the complex is between vermiculite and one surfactant only this results in the toluene concentration being reduced in the center of the interlamellar space, with maxima on either side. On the other hand, in a complex of mixed-length surfactants, the toluene distribution reaches its maximum at the center of the interlamellar space.

Journal Article↗
PubMed2003

The structure of mixed nonionic surfactant monolayers at the air-water interface: the effects of different alkyl chain lengths.

The structure of mixed nonionic surfactant monolayers of monodecyl hexaethylene glycol (C10E6) and monotetradecyl hexaethylene glycol (C14E6) adsorbed at the air-water interface has been determined by specular neutron reflectivity. Using partial isotopic labeling (deuterium for hydrogen) of the alkyl and ethylene oxide chains of each surfactant, the distribution and relative positions of the chains at the interface have been obtained. The packing of the two different alkyl chain lengths results in structural changes compared to the pure surfactant monolayers. This results in changes in the relative positions of the alkyl chains and of the ethylene oxide chains at the interface. The role of the alkyl chain length is contrasted with that of the ethylene oxide chain length, determined from results reported previously on the nonionic surfactant mixture of monododecyl triethylene glycol (C12E3) and monododecyl octaethylene glycol (C12E8).

Journal Article↗
PubMed2002

The intercalation of a vermiculite by cationic surfactants and its subsequent swelling with organic solvents.

We have measured the dimensions of the interlamellar space following intercalation of a vermiculite by a range of cationic surfactants and followed the subsequent swelling of the organoclay compounds with several organic solvents. A single vermiculite (Eucatex) was used with three series of surfactants, N-alkyltrimethylammonium bromides, N,N'-dialkyldimethylammonium bromides, and the gemini cationic surfactants, alpha,omega-bis (N-alkyldimethylammonium) alkanes. In all cases well-defined stoichiometric compounds are obtained and the amount of surfactant intercalating the layer indicates that there are two factors controlling this amount, charge neutralization of the clay and hydrophobic packing. Packing arguments are used to deduce the fraction of non-charge-neutralizing material in the interlamellar space. It is clear that by altering the surfactant charge and structure it is possible to control the degree to which adsorption beyond charge neutralization occurs in these complexes, which is important when the capacity of such complexes to sorb other materials is considered. The general pattern of swelling of the surfactant/vermiculite complex by toluene suggests that the maximum expansion of the intralamellar space is limited by the longest chain in the surfactant. In contrast to earlier results we found that these vermiculites could be swollen by alkanes as well as aromatic solvents. This is attributed to the greater hydrophobicity of the interior of an organoclay formed from a clay of higher charge density.

Journal Article↗
PubMed2002

Comparison of the coadsorption of benzyl alcohol and phenyl ethanol with the cationic surfactant, hexadecyl trimethyl ammonium bromide, at the air-water interface.

A comparison of the coadsorption of benzyl alcohol and phenyl ethanol with the cationic surfactant, hexadecyl trimethyl ammonium bromide, C16TAB, at the air-water interface is made using the specular reflection of neutrons. The phenyl ethanol is more surface active than the benzyl alcohol, and competes more effectively with the C16TAB for the interface. The structure of the C16TAB component in the mixed monolayer is compared with the structure of the pure C16TAB monolayer at an equivalent area per molecule. The addition of the aromatic alcohol subtly alters the conformation of the C16TAB and draws it closer to the aqueous subphase. The center of the alcohol distribution is located in the interface adjacent to the C6 group of the C16TAB alkyl chain closest to the headgroup. Compared to the benzyl alcohol, the more hydrophobic phenyl ethanol is slightly farther away from the headgroup, and has a greater impact on the conformation of the alkyl chain of the C16TAB.

Journal Article↗
PubMed2002

Adsorption of nonionic mixtures at the air-water interface: effects of temperature and electrolyte.

Specular neutron reflection has been used to investigate the effects of temperature and added electrolyte on the adsorption of nonionic surfactants and nonionic surfactant mixtures at the air-water interface. For the alkyl poly-oxyethylene oxide nonionic surfactants, C(n)EO(m), the adsorption at the air-water interface is independent of temperature for surfactants with shorter ethylene oxide groups, whereas there is an increasing tendency for increased adsorption with temperature for surfactants with longer ethylene oxide groups. The addition of "salting in" (sodium thiocyanate, NaSCN) and "salting out" (sodium chloride, NaCl, sodium sulphate, Na2SO4) electrolyte results in reduced and enhanced adsorption, respectively, for C12EO8, whereas both types of electrolyte result in enhanced adsorption for C12EO12. The addition of electrolyte does not substantially alter the temperature dependence of the adsorption of the pure monolayers. For the nonionic mixtures of C12EO3/C12EO8 increasing temperature results in a surface richer in the least surface-active component, C12EO8. For the same nonionic mixture, the addition of "salting in" and "salting out" electrolyte results in an reduced and increased adsorption, respectively. The addition of "salting in" electrolyte results in a surface more rich in C12EO3, whereas for the addition of both "salting in" and "salting out" electrolyte the surface composition is essentially unaltered.

Journal Article↗
PubMed2001

Loss of heterozygosity in the Hodgkin-Reed Sternberg cell line L1236.

Hodgkin-Reed Sternberg cells are derived from germinal centre B-cells in most cases. Somatic mutations affecting their rearranged immunoglobulin genes were detected, rendering potential functional rearrangements non-functional. Under physiological conditions such cells would be designated to undergo apoptosis within the germinal centre. In search for the specific transforming event that prevents Hodgkin-Reed Sternberg cells from programmed cell death, cytogenetic analyses were broadly performed but did not reveal specific chromosomal aberrations. Analysis of these cells on the molecular level is difficult to perform due to the scarcity of the cells in the lymphoma tissue and the given limitations of in situ studies. To overcome these limitations, the cell line L1236, known to be derived from Hodgkin-Reed Sternberg cells in situ, was chosen for allelotype analysis. Using a panel of microsatellite loci assigned to nearly all chromosomal arms, regions of loss of heterozygosity were detected on chromosomal arms 6p, 9q and 17p. The size of lost segments was estimated by amplification of additional microsatellite loci mapped to the respective regions. Further analyses of single Hodgkin-Reed Sternberg cells will reveal whether LOH affecting these regions is a recurrent event in HD and to which extent the smallest commonly affected region can be estimated.

Alleles↗
PubMed2001

Beta-casein adsorption at the hydrophobized silicon oxide-aqueous solution interface and the effect of added electrolyte.

The effect of the presence of NaCl, CaCl(2), or MgCl(2) at the same ionic strength on the structure of beta-casein layers adsorbed on hydrophobic surfaces has been investigated by neutron reflectivity measurements. The data were fitted to a four-layer model. The volume fraction versus distance profiles have a similar shape whether beta-casein is adsorbed from NaCl, CaCl(2), and MgCl(2) of the same ionic strength or whether the protein concentration is lowered 10 times. In particular at larger distances from the surface, the volume fraction values are low and similar. However, close to the hydrophobic surface the volume fraction of protein decreases in the order CaCl(2) > MgCl(2) > NaCl. We have also used a specific proteolytic enzyme, endoproteinase Asp-N, which cleaves off the hydrophilic part of beta-casein, as a tool to reveal the interfacial structure of the protein. For all the different types of added electrolytes, endoproteinase Asp N only affects the outermost beta-casein layer. Subsequent addition of beta-casein in all cases led to large increases in amounts adsorbed and in the thickness of the outer layers.

Adsorption↗
PubMed2001

beta-Casein adsorption at the silicon oxide--aqueous solution interface.

Neutron reflectometry was used to investigate the time-dependent beta-casein adsorption at the silica-aqueous solution interface. The transient and steady-state structural characteristics of the adsorbed layer were determined from reflectivity curves, fitted to three-layer and two-layer models. The results show that the beta-casein adsorption to silica is very slow. The adsorption process involves the formation of an inner dense protein layer with a mean thickness of about 30 A onto which a more hydrated outer layer is self-associated. The surface excess and the total layer thickness of the asymmetric bilayer were, after 5 h adsorption time, estimated to be about 6.5 mg/m2 and 105 A, respectively. The adsorption behavior observed on silica contrasts with that previously reported for hydrophobic substrates, where beta-casein adsorption is much more rapid and the final surface excess is less than half of that observed for silica. Rinsing the silica surface with protein-free buffer resulted in a substantial desorption; much more pronounced than observed for hydrophobic substrates. This behavior suggests a weak adsorption affinity for a fraction of the adsorbed casein molecules; most likely the outer self-associated casein molecules in the adsorbed bilayer. The comparative desorption from hydrophobic surfaces was shown to be marginal. The difference between the layer structures adopted on hydrophobic and hydrophilic surfaces is also mirrored in the effects that the addition of a specific proteolytic enzyme (endoproteinase Asp-N) has on the adsorbed layer properties. The rinsing and endoproteinase cleavage processes result together in more than 80% reduction of the originally adsorbed mass at the silica surface. Only a thin but dense adsorbed layer remains after these treatments. The corresponding reduction reported for the hydrophobic adsorbent system was only about 20%. It is concluded that beta-casein adsorption on silica results in the formation of an asymmetric surface bound bilayer that stands in strong contrast to the monolayer structure formed at hydrophobic surfaces. This finding support the previous results obtained by using ellipsometry. The study also shows that neutron reflection, despite its limitations in time resolution, can be used for studying dynamic interfacial phenomena in protein systems.

Adsorption↗
PubMed2001

The effects of nucleus basalis magnocellularis lesions in Long-Evans hooded rats on two learning set formation tasks, delayed matching-to-sample learning, and open-field activity.

Rats with quisqualic acid lesions of the nucleus basalis magnocellularis (nBM) and control rats were compared in discrimination reversal learning set (DRLS) and olfactory discrimination learning set (ODLS) tasks, a delayed matching-to-sample task (DMTS), and open-field activity. Evidence of learning set formation was seen in control rats but not in nBM-lesioned rats in both the DRLS and ODLS tasks. Better-than-chance performances were seen for both groups in DMTS, indicating no impairment after nBM lesions. There were no group differences in open-field activity. These findings suggest that the nBM is important for higher cognitive processing such as "learning to learn" and thus is important for a complex form of reference memory. In addition, perseverational, working memory, and attentional deficits could not explain learning set impairment after nBM lesions.

Animals↗
PubMed2001

The undiscovered Argentine pharmaceutical market.

The Argentine health care system is more highly developed than most systems in Latin America and gives most of its citizens access to a reasonable level of care. Now after 50 years of government control the system is becoming privatized, which provides new possibilities for health care marketers. One company developed an innovative approach to pharmaceutical marketing in Argentina that, while perhaps not appropriate for the U.S. market, may reap rewards in certain cultures and provide lessons for others.

Argentina↗
PubMed2001

HIV services for a new generation.

Expanded treatment capabilities, supportive services, and education are available to control the AIDS epidemic as it spreads among new, harder-to-reach populations. Unfortunately, disparities in health status among these groups often prevent them from getting preventive care and treatment. To build awareness among today's at-risk population, health care marketers must address the needs of their target audience. With a well-planned and sensitive approach, they can work with practitioners to bring these services to the people who need them most.

Adolescent↗
PubMed2000

Surfactant layers at the air/water interface: structure and composition.

The use of neutron reflectometry to study the structure and composition of surfactant layers adsorbed at the air/water interface is reviewed. A critical assessment of the results from this new technique is made by comparing them with the information available from all other techniques capable of investigating this interface.

Adsorption↗
PubMed2000

Orbital abscess: management and outcome.

PURPOSE: To discuss the diagnosis, management and outcome of various types of orbital abscess. METHODS: The medical records of 13 patients diagnosed and treated for orbital abscess were reviewed. The sources of infection included: paranasal sinusitis (n = 5), odontogenic origin of infection (n = 4), one each, temporal fossa abscess, palatal abscess, furuncle on the nose, and secondary to retrobulbar injection of steroid. Computed tomographic scans revealed the presence of an abscess in all 13 cases. Associated findings on CT scan included: sinus disease (n = 8), cavernous sinus thrombosis (n = 2) and subdural empyema (n = 2). All patients were treated with intensive, multiple, intravenous antibiotics and early surgical drainage. RESULTS: Purulent material collected surgically from the orbit cultured Staphylococcus aureus (n = 3), two each Pseudomonas aeruginosa, Proteus mirabilis, Acinetobacter species and one each beta-haemolytic Streptococci, Citrobacter frundi and Enterobacter. Final visual acuity was good in 6 patients (6/12-6/6) and no light perception in 6 others. Visual acuity could not be recorded in the infant. The other complications were intracranial abscess (n = 4), cavernous sinus thrombosis (n = 2) and restricted ocular motility (n = 1). CONCLUSIONS: A high index of suspicion is necessary, along with early institution of appropriate diagnostic imaging, and aggressive medical and surgical treatment for a favourable outcome in cases of orbital abscess.

Abscess↗
PubMed2000

The feminization of American medicine. The transformation of health care from a masculine orientation to a more feminine orientation requires a new marketing approach.

Although the transformation of American medicine in the last part of the twentieth century has been well documented, a major development that has significant implications for health care marketing has been essentially overlooked. This development has been the shift in health care from a primarily masculine orientation to a primarily feminine orientation. This shift has been driven by, among other factors, the transition from an acute care orientation to a chronic care orientation and calls for a qualitatively different approach to the marketing of health services.

Acute Disease↗
PubMed2000

Topical diclofenac sodium for treatment of postoperative inflammation in cataract surgery.

PURPOSE: To study the effect of a topical non-steroidal anti-inflammatory drug as an alternative to topical steroids for postoperative control of inflammation in cataract surgery. METHODS: The effect of diclofenac sodium 0.1% following cataract surgery was studied and compared to routine corticosteroid, dexamethasone phosphate 1% in a prospective, double-blind randomized study. Both groups were similar in baseline parameters. Postoperative inflammatory response, intraocular pressure and best-corrected visual acuity following standard extracapsular cataract extraction were assessed in both groups in the initial 21 days and the severity of these parameters was graded. The severity of postoperative inflammatory response to the two drugs was graded at 1, 3, 7, 14 and 21 days. Intraocular pressure and visual acuity at baseline and endpoint were compared and statistically analyzed. RESULTS: The two groups did not differ statistically in treatment effect for any of the variables including aqueous cells, flare, ciliary congestion, Descemet's folds, visual acuity and intraocular pressure (p < 0.001). However there seemed to be a trend towards quicker improvement with corticosteroid when cells in the anterior chamber were considered. There were no side effects from topical diclofenac, and it was well tolerated. CONCLUSION: Diclofenac sodium is as effective as topical corticosteroid and can be used as an alternative in routine postoperative treatment following uncomplicated cataract surgery.

Anterior Chamber↗