Mechanical calorimetry of large dimyristoylphosphatidylcholine vesicles in the phase transition region.
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Calorimetry↗
Biomedical subjects
R Kwok
Publications and source records attributed to R Kwok.
Thermoelasticity of large lecithin bilayer vesicles.
Micromechanical experiments on large lecithin bilayer vesicles as a function of temperature have demonstrated an essential feature of bilayer vesicles as closed systems: the bilayer can exist in a tension-free state (within the limits of experimental resolution, i.e., less than 10(-2) dyn/cm). Furthermore, because of the fixed internal volume, there is a critical temperature at which the vesicle becomes a tension-free sphere. Below this temperature, thermoelastic tension builds up in the membrane and the vesicle's internal pressure increases while the surface area remains constant. Above this temperature, the vesicle's surface area increases while the tension and internal pressure are negligible. Without mechanical support, the vesicles fragment into small vesicles because they have insufficient surface rigidity. In the upper temperature range we have measured the increase of surface area with temperature. These data established the thermal area expansivity to be 2.4 X 10(-3)/degrees C. At constant temperature, we used either pipet aspiration with suction pressures up to 10(4) dyn/cm2 or compression against a flat surface with forces up to 10(-2) dyn to produce area dilation of the vesicle surface on the order of 1%. The rate of increase of membrane tension with area dilation was calculated, which established the elastic area compressibility modulus to be 140 dyn/cm. The tension limit that produced lysis was observed to be 3-4 dyn/cm (equivalent to 2-3% area increase). The product of the elastic area compressibility modulus, the thermal area expansivity, and the temperature gives the reversible heat of expansion at constant temperature for the bilayer. This value is 100 ergs/cm2 at 25 degrees C, or approximately 5 kcal/mol of lecithin. Similarly, the product of the thermal area expansivity multiplied by the area compressibility modulus determines the rate of increase of thermoelastic tension with decrease in temperature when the area is held constant, i.e., -0.34 dyn/cm/degrees C.
Elasticity↗
Calibration of beam deflection produced by cellular forces in the 10(-9)--10(-6) gram range.
An experimental procedure and method of analysis are presented for calibration of a thin-beam force transducer. The beam transducer can be produced and calibrated with a minimum coefficient of 10 ng (10(-5) dyne) force per micron (10(-4) cm) deflection, i.e., kB approximately 0.1 dyne/cm. Since beam deflections on the order of 0.1 micron can be detected, forces of a few nanograms can be resolved. Such forces are common in mechanical experiments on microscopic bodies, e.g., biological cells, artificial membrane capsules, droplets, etc.
Biomechanical Phenomena↗
Studies of effector cell, antibody, and target cell interactions in natural cell-mediated cytotoxicity.
IgG antibodies bound to effector cells through Fc receptors were observed to determine the specificity of natural cell-mediated cytotoxicity (NCMC) against cultured target cells. When effector lymphocytes were isolated from the peripheral blood of most individuals, they already possessed natural antibodies specific for antigens on cultured cells. Since they lacked IgG antibodies specific for antigens on sheep red blood cell (SRBC) targets, natural cytotoxicity against SRBC was almost non-existent. Effector cells incubated in IgG anti-SRBC became specifically cytotoxic to SRBC. In the process, NCMC and antibody-dependent cell-mediated cytotoxicity was diminished, indicating that arming with anti-SRBC replaced natural antibodies and occupied Fc receptors on effector cells. Thus, treating effector cells with serum may result in increased or decreased cytotoxicity depending upon the specificity of antibodies within the serum. This type of modulation of NCMC occurs at the interaction between antibody Fc and Fc receptors and can explain blocking and unblocking.
Animals↗
The rearrangement of methoxylated phenylanthranils.
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Acridines↗
The 1998 Baltimore Particulate Matter Epidemiology-Exposure Study: part 1. Comparison of ambient, residential outdoor, indoor and apartment particulate matter monitoring.
A combined epidemiological exposure panel study was conducted during the summer of 1998 in Baltimore, Maryland. The objectives of the exposure analysis component of the 28-day study were to investigate the statistical relationships between particulate matter (PM) and related co-pollutants from numerous spatial boundaries associated with an elderly population, provide daily mass concentrations needed for the epidemiological assessment, and perform an extensive personal exposure assessment. Repeated 24-h integrated PM2.5 (n=394) and PM10 (n=170) data collections corresponding to stationary residential central indoor, individual apartment, residential outdoor and ambient monitoring were obtained using the same sampling methodology. An additional 325 PM2.5 personal air samples were collected from a pool of 21 elderly (65+ years of age) subjects. These subjects were residents of the 18-story retirement facility where residential monitoring was conducted. Mean daily central indoor and residential apartment concentrations were approximately 10 microg/m3. Outdoor and ambient PM2.5 concentrations averaged 22 microg/m3 with a daily range of 6.7-59.3 microg/m3. The slope of the central indoor/outdoor PM2.5 mass relationship was 0.38. The average daily ratio of PM2.5/PM10 mass concentrations across the measurement sites ranged from 0.73 to 0.92. Both the central indoor and mean apartment PM2.5 mass concentrations were highly correlated with the outdoor variables (r>0.94). The lack of traditionally recognized indoor sources of PM present within the facility might have accounted for the high degree of correlation observed between the variables. Results associated with the personal monitoring effort are discussed in depth in Part 2 of this article.
Aged↗
The 1998 Baltimore Particulate Matter Epidemiology-Exposure Study: part 2. Personal exposure assessment associated with an elderly study population.
An integrated epidemiological-exposure panel study was conducted during the summer of 1998 which focused upon establishing relationships between potential human exposures to particulate matter (PM) and related co-pollutants with detectable health effects. The study design incorporated repeated individual 24-h integrated PM2.5 personal exposure monitoring. A total of 325 PM2.5 personal exposure samples were obtained during a 28-day study period using a subject pool of 21 elderly (65+ years of age) residents of an 18-story retirement facility near Baltimore, Maryland. Each sample represented a unique 24-h breathing zone measurement of PM2.5 mass concentration. PM2.5 and PM10 mass concentrations collected from the apartments of the subjects as well as residential and ambient sites were compared to individual and mean PM2.5 personal exposures. Daily PM2.5 personal exposure concentrations ranged from 2.4 to 47.8 microg/m3 with an overall individual study mean of 12.9 microg/m3. Mean PM2.5 personal exposures were determined to be highly correlated to those representing the central indoor (r=0.90) and ambient sites (r=0.89). Subjects reported spending an average of 92% of each day within the confines of the retirement center. Based upon measured and modeled exposures, a mean PM2.5 personal cloud of 3.1 microg/m3 was estimated. Data collected from these participants may be unique with respect to the general elderly population due to the communal lifestyle within the facility and reported low frequency of exposure to sources of PM.
Activities of Daily Living↗
Comparison of PM2.5 and PM10 monitors.
An extensive PM monitoring study was conducted during the 1998 Baltimore PM Epidemiology-Exposure Study of the Elderly. One goal was to investigate the mass concentration comparability between various monitoring instrumentation located across residential indoor, residential outdoor, and ambient sites. Filter-based (24-h integrated) samplers included Federal Reference Method Monitors (PM2.5-FRMs), Personal Environmental Monitors (PEMs), Versatile Air Pollution Samplers (VAPS), and cyclone-based instruments. Tapered element oscillating microbalances (TEOMs) collected real-time data. Measurements were collected on a near-daily basis over a 28-day period during July-August, 1998. The selected monitors had individual sampling completeness percentages ranging from 64% to 100%. Quantitation limits varied from 0.2 to 5.0 microg/m3. Results from matched days indicated that mean individual PM10 and PM2.5 mass concentrations differed by less than 3 microg/m3 across the instrumentation and within each respective size fraction. PM10 and PM2.5 mass concentration regression coefficients of determination between the monitors often exceeded 0.90 with coarse (PM10-2.5) comparisons revealing coefficients typically well below 0.40. Only one of the outdoor collocated PM2.5 monitors (PEM) provided mass concentration data that were statistically different from that produced by a protoype PM2.5 FRM sampler. The PEM had a positive mass concentration bias ranging up to 18% relative to the FRM prototype.
Air Pollutants↗