Biomedical subjects

Sarah E Cross

Publications and source records attributed to Sarah E Cross.

5 recordsLinked to original sources

PubMed2006

Alkane bromination revisited: "reproportionation" in gas-phase methane bromination leads to higher selectivity for CH3Br at moderate temperatures.

The reaction of methane and bromine is a mildly exothermic and exergonic example of free radical alkane activation. We show here that the reaction of methane and bromine (CH4:Br2 > or = 1) may yield either a kinetically or a thermodynamically determined bromomethane product distribution and proceeds in two main phases between 450 and 550 degrees C under ambient pressure on the laboratory time scale. This is in contrast to the highly exothermic methane fluorination or chlorination reactions, which give kinetic product distributions, and to the endergonic iodination of methane, which yields an equilibrium distribution of iodomethanes. The first phase of reaction between methane and bromine is a relatively rapid consumption of bromine to yield a kinetic methane bromination product distribution characterized by low methane conversion, low methyl bromide selectivity, and higher polybromomethane selectivity. In the second slower phase CHxBr(4-x) reproportionation leads to significantly higher methane conversion and higher methyl bromide selectivity. For methane bromination at 525 degrees C, CH4 conversion and CH3Br selectivity reach 73.5% and 69.5%, respectively, after ample (60 s) time for reproportionation. The high selectivity and simple configuration make this pathway an attractive candidate for scale-up in halogen-mediated methane partial oxidation processes.

Journal Article↗

PubMed2006

Self-organized and highly ordered domain structures within swarms of Myxococcus xanthus.

Coordinated group movement (swarming) is a key aspect of Myxococcus xanthus' social behavior. Here we report observation of domain structures formed by multiple cells within large three-dimensional swarming groups grown on amorphous glass substrates, using the atomic force microscope (AFM). Novel analyses revealed that 90% of the wild type swarms displayed some form of preferential cell alignment. In contrast, cells with mutations in the social and adventurous motility systems displayed a distinct lack of cell alignment. Video microscopy observations of domain features of in vivo swarming M. xanthus cells were also consistent with the AFM data. The results presented here reveal that unique domain formation within swarms of wild type cells is a biologically driven process requiring the social and adventurous motility systems and is not a statistical phenomenon or thermodynamic process arising from liquid crystal behavior.

Microscopy, Atomic Force↗

PubMed2006

Functional characterization of cell-wall-associated protein WapA in Streptococcus mutans.

Streptococcus mutans is known as a primary pathogen responsible for dental caries. One of the virulence factors of S. mutans in cariogenicity is its ability to attach to the tooth surface and form a biofilm. Several surface proteins have been shown to be involved in this process. A 29 kDa surface protein named wall-associated protein A (WapA, antigen A or antigen III), was previously used as a vaccine in animal studies for immunization against dental caries. However, the function of WapA in S. mutans is still not clear. This study characterized the function of WapA in cell surface structure and biofilm formation. Compared to the wild-type, the wapA mutant had much-reduced cell chain length, diminished cell-cell aggregation, altered cell surface ultrastructure, and unstructured biofilm architecture. Furthermore, in vivo force spectroscopy revealed that the cell surface of the wapA mutant was less sticky than that of the wild-type cells. More interestingly, these phenotypic differences diminished as sucrose concentration in the medium was increased to 0.5 %. Real-time RT-PCR analysis demonstrated that sucrose strongly repressed wapA gene expression in both planktonic and biofilm cells. These results suggest that the WapA protein plays an important structural role on the cell surface, which ultimately affects sucrose-independent cell-cell aggregation and biofilm architecture.

Antigens, Bacterial↗

PubMed2004

Surface-enhanced Raman scattering from ordered Ag nanocluster arrays.

We have examined the effect of ordered silver nanocluster substrates on the surface-enhanced Raman spectrum of rhodamine 6G (R6G). Triangular shaped silver nanocluster arrays with order on the approximately 100 mum range were prepared using nanosphere lithography. Direct comparisons of R6G surface-enhanced Raman spectroscopy (SERS) signals between ordered nanocluster regions and amorphous Ag regions prepared under identical deposition conditions provide strong evidence of an electromagnetic field enhancement attributed to the unique nanocluster morphology. We have obtained order of magnitude enhancement factors for both 200 and 90 nm Ag nanocluster SERS substrates relative to Ag films.

Light↗

PubMed2004

Metformin, but not leptin, regulates AMP-activated protein kinase in pancreatic islets: impact on glucose-stimulated insulin secretion.

Metformin, a drug widely used in the treatment of type 2 diabetes, has recently been shown to act on skeletal muscle and liver in part through the activation of AMP-activated protein kinase (AMPK). Whether metformin or the satiety factor leptin, which also stimulates AMPK in muscle, regulates this enzyme in pancreatic islets is unknown. We have recently shown that forced increases in AMPK activity inhibit insulin secretion from MIN6 cells (da Silva Xavier G, Leclerc I, Varadi A, Tsuboi T, Moule SK, and Rutter GA. Biochem J 371: 761-774, 2003). Here, we explore whether 1) glucose, metformin, or leptin regulates AMPK activity in isolated islets from rodent and human and 2) whether changes in AMPK activity modulate insulin secretion from human islets. Increases in glucose concentration from 0 to 3 and from 3 to 17 mM inhibited AMPK activity in primary islets from mouse, rat, and human, confirming previous findings in insulinoma cells. Incubation with metformin (0.2-1 mM) activated AMPK in both human islets and MIN6 beta-cells in parallel with an inhibition of insulin secretion, whereas leptin (10-100 nM) was without effect in MIN6 cells. These studies demonstrate that AMPK activity is subject to regulation by both glucose and metformin in pancreatic islets and clonal beta-cells. The inhibitory effects of metformin on insulin secretion may therefore need to be considered with respect to the use of this drug for the treatment of type 2 diabetes.

AMP-Activated Protein Kinases↗