Biomedical subjects

Lee D Hansen

Publications and source records attributed to Lee D Hansen.

6 recordsLinked to original sources

PubMed2005

Selective guest encapsulation by a cobalt-assembled cage molecule.

Metal-assembled resorcinarene-based cages enclose space and entrap organic molecules from water. Addition of cobalt(II) ions to a neutral, aqueous solution of a resorcinarene that has iminodiacetic acids attached to its upper rim results in the formation of cages. These cages not only entrap organic molecules, but they do so in a selective manner. Guests with optimum size, shape, and polarity are preferentially entrapped. For example, selection of p-xylene is twenty thousand times more favorable than that of m-xylene. The enthalpy of resorcinarene deprotonation and cage formation was calculated by performing calorimetry studies and ranged from -305 to -348 kJ mol(-1). The change in enthalpy of guest encapsulation varied by as much as 43 kJ mol(-1). The differences in change in free energy of guest encapsulation varied by -16 kJ mol(-1). The changes in enthalpy and free energy of guest encapsulation were used to calculate the changes in entropy, which ranged from -97 to +37 J mol(-1) K(-1). An enthalpy-entropy compensation of guest encapsulation was observed.

Journal Article↗

PubMed2005

Growth efficiency increases as relative growth rate increases in shoots and roots of Eucalyptus globulus deprived of nitrogen or treated with salt.

We used calorimetry to test whether there is a single general relationship between growth and respiration in shoots and roots of Eucalyptus globulus Labill. seedlings when stressed, irrespective of the type or severity of stress. We found that nitrogen (N) deprivation and salt treatment had no effect on the relationship between growth and respiration and little effect on absolute rates of respiration. Carbon-conversion efficiency (epsilonC) ranged from 0.7 to 0.9 for specific growth rates (R(SG)) greater than 0.3 day(-1). Above an R(SG) of 0.1 day(-1), epsilonC decreased gradually with decreasing R(SG) and between an R(SG) of 0- 0.1 day(-1), epsilonC decreased rapidly. We conclude that the relationship between epsilonC and R(SG) is not greatly affected by salt or N-deprivation stresses. Relationships between gross productivity and epsilonC may be generally applicable, in which case they could improve on the "flat-tax" approach to modeling net primary productivity from gross productivity while avoiding the complexity of more explicit models of plant respiration. However, the relationship between gross productivity and epsilonC was sensitive to temperature and the effect of temperature on epsilonC thus requires further investigation. Nitrogen deprivation caused large decreases in leaf area and shoot to root ratio, and mature leaves of N-deprived plants had lower intrinsic water-use efficiencies than leaves of plants well supplied with nutrients. Nitrogen deprivation increased apical dominance and most of the reduction in leaf area was the result of fewer secondary branches, although leaf size was also reduced. Our results suggest that N deprivation reduces productivity primarily by reducing sink size, rather than sink activity, and that apical dominance may be an important mechanism for maintaining adequate epsilonC in resource-limited conditions.

Calorimetry, Indirect↗

PubMed2004

Divalent cations increase lipid order in erythrocytes and susceptibility to secretory phospholipase A2.

Elevated concentrations of intracellular calcium in erythrocytes increase membrane order and susceptibility to secretory phospholipase A2. We hypothesize that calcium aids the formation of domains of ordered lipids within erythrocyte membranes by interacting directly with the inner leaflet of the cell membrane. The interface of these domains with regions of more fluid lipids may create an environment with weakened neighbor-neighbor interactions that would facilitate phospholipid migration into the active site of bound secretory phospholipase A2. This hypothesis was investigated by determining the effects of seven other divalent ions on erythrocyte membrane properties. Changes in membrane order were assessed with steady-state fluorescence spectroscopy and two-photon microscopy with an environment-sensitive probe, laurdan. Each ion increased apparent membrane order in model membranes and in erythrocytes when introduced with an ionophore, suggesting that direct binding to the inner face of the membrane accounts for the effects of calcium on membrane fluidity. Furthermore, the degree to which ions affected membrane properties correlated with the ionic radius and electronegativity of the ions. Lastly, erythrocytes became more susceptible to enzyme hydrolysis in the presence of elevated intracellular levels of nickel and manganese, but not magnesium. These differences appeared related to the ability of the ions to induce a transition in erythrocyte shape.

2-Naphthylamine↗

PubMed2003

Free human mitochondrial GrpE is a symmetric dimer in solution.

The co-chaperone GrpE is essential for the activities of the Hsp70 system, which assists protein folding. GrpE is present in several organisms, and characterization of homologous GrpEs is important for developing structure-function relationships. Cloning, producing, and conformational studies of the recombinant human mitochondrial GrpE are reported here. Circular dichroism measurements demonstrate that the purified protein is folded. Thermal unfolding of human GrpE measured both by circular dichroism and differential scanning calorimetry differs from that of prokaryotic GrpE. Analytical ultracentrifugation data indicate that human GrpE is a dimer, and the sedimentation coefficient agrees with an elongated shape model. Small angle x-ray scattering analysis shows that the protein possesses an elongated shape in solution and demonstrates that its envelope, determined by an ab initio method, is similar to the high resolution envelope of Escherichia coli GrpE bound to DnaK obtained from single crystal x-ray diffraction. However, in these conditions, the E. coli GrpE dimer is asymmetric because the monomer that binds DnaK adopts an open conformation. It is of considerable importance for structural GrpE research to answer the question of whether the GrpE dimer is only asymmetric while bound to DnaK or also as a free dimer in solution. The low resolution structure of human GrpE presented here suggests that GrpE is a symmetric dimer when not bound to DnaK. This information is important for understanding the conformational changes GrpE undergoes on binding to DnaK.

Amino Acid Sequence↗

PubMed2003

In-office vital tooth bleaching--what do lights add?

Aqueous hydrogen peroxide (H2O2) has been used clinically at 30% to 35% levels to lighten teeth for many years, but the process has required multiple visits. Heat and light have been used empirically in attempts to catalyze H2O2 decomposition and speed tooth lightening. The contribution of bleaching lights (LumaArch, Optilux 500, and Zoom!) to act as catalysts for lightening teeth was studied in 83 pairs of contralateral anterior maxillary and mandibular teeth on 15 human subjects. Split-arch design using centrals, laterals, and canines on one side treated with bleach plus light, were compared with contralateral teeth using bleach alone. Three researchers trained in the use of the Vitapan 3D-Master Shade Guide took shades with independently agreement within 0.5 value-chroma sum 89% of the time throughout the study Laboratory tests determined bleach gel chemistry, bleach light output, and effects on the bleaches of light alone and heat alone. Results showed that the three test lights did not lighten teeth more than their bleach gels alone. All teeth lightened to nearly the same degree (1.7 color increments), but LumaArch required 60% less time and Zoom! used 1/3 lower H2O2 concentration. Laboratory tests indicated that the proprietary chemicals mixed into each bleach gel just before use acted as catalysts and were probably responsible for more rapid lightening produced by LumaArch gel, and need for less H2O2 in Zoom! gel. Neither the heat produced by the accessory lights, nor the light output itself were responsible for catalytic activity with any of the three systems tested. Collectively, the data demonstrate positive effects from chemical catalysts added to bleaching gels. No output from any of the lights resulted in heat or light that catalyzed the gels.

Adult↗

PubMed2002

Application of an enthalpy balance model of the relation between growth and respiration to temperature acclimation of Eucalyptus globulus seedlings.

The enthalpy balance model of growth uses measurements of the rates of heat and CO(2) production to quantify rates of decarboxylation, oxidative phosphorylation and net anabolism. Enthalpy conversion efficiency (eta(H)) and the net rate of conservation of enthalpy in reduced biosynthetic products (R(SG)DeltaH(B)) can be calculated from metabolic heat rate (q) and CO(2) rate (R(CO2)). eta(H) is closely related to carbon conversion efficiency and the efficiency of conservation of available electrons in biosynthetic products. R(SG)DeltaH(B) and eta(H) can be used, together with biomass composition, to describe the rate and efficiency of growth of plant tissues. q is directly related to the rate of O(2) consumption and the ratio q:R(CO2) is inversely related to the respiratory quotient. We grew seedlings of Eucalyptus globulus at 16 and 28 degrees C for four to six weeks, then measured q and R(CO2) using isothermal calorimetry. Respiratory rate at a given temperature was increased by a lower growth temperature but eta(H) was unaffected. Enthalpy conversion efficiency - and, therefore, carbon conversion efficiency - decreased with increasing temperature from 15 to 35 degrees C. The ratio of oxidative phosphorylation to oxygen consumption (P/O ratio) was inferred in vivo from eta(H) and by assuming a constant ratio of growth to maintenance respiration with changing temperature. The P/O ratio decreased from 2.1 at 10-15 degrees C to less than 0.3 at 35 degrees C, suggesting that decreased efficiency was not only due to activity of the alternative oxidase pathway. In agreement with predictions from non-equilibrium thermodynamics, growth rate was maximal near 25 degrees C, where the calculated P/O ratio was about half maximum. We propose that less efficient pathways, such as the alternative oxidase pathway, are necessary to satisfy the condition of conductance matching whilst maintaining a near constant phosphorylation potential. These conditions minimize entropy production and maximize the efficiency of mitochondrial energy conversions as growing conditions change, while maintaining adequate finite rates of energy processing.

Acclimatization↗