The lateral diffusion of a fluorescent-labeled phospholipid, phosphatidyl-N(4-nitrobenzo-2-oxa-1,3,-diazole)ethanolamine, has been measured in binary mixtures of cholesterol and dimyristoyl phosphatidylcholine at temperatures both above and below 23.80C, the chain-melting transition temperature of this phosphatidylcholine. There is a temperature-composition region, approximately temperature less than 230C and mole fraction of cholesterol (X) less than 0.20, in which the lateral diffusion coefficient of the fluorescent probe is at least an order of magnitude smaller than it is at points outside of this temperature-composition region. At temperatures above z230C there is a significant increasing cholesterol concentration, for X > 0.2.The physical properties of bilayer membranes containing cholesterol and phosphatidylcholines have been the subject of extensive investigations employing a large number of experimental techniques (1-16). From these studies certain generalizations emerge. The addition of cholesterol to phosphatidylcholine bilayers in the fluid state (above the chain-melting temperatures of the phosphatidylcholines-i.e., above 23.80C for dimyristoyl phosphatidylcholine), leads to a decrease in the "fluidity" of the bilayer membrane. On the other hand, inclusion of cholesterol in phosphatidylcholine membranes at temperatures below the chain-melting transition temperatures leads to "fluidization" of these membranes (1-16). These generalizations have been based in part on magnetic resonance studies (spin-label paramagnetic resonance and nuclear magnetic resonance) in which the spectra are sensitive to the rotational motions of the molecules. Another interesting property of binary mixtures of dimyristoyl phosphatidylcholine or dipalmitoyl phosphatidylcholine with cholesterol is that a phosphatidylcholinelike phase transition (at t230C or -420C) can be detected in the presence of mole fraction X < 0.20 cholesterol in the binary mixture (1,3,6,7,9,10,12,16). The chain-melting transition temperature of dimyristoyl phosphatidylcholine is 23.80C, and that of dipalmitoyl phosphatidylcholine is 41.40C. Although reliable phase diagrams have been established for a number of binary mixtures of phospholipids (17-20), it has been difficult to use the published phase diagrams for binary mixtures of cholesterol and phosphatidylcholines (3, 4, 7) to account for all of the observed physical properties of these mixtures. Some of this difficulty may originate in sample heterogeneity (4).In the present paper we give the results of determinations of the lateral diffusion coefficient of the fluorescent probe phosphatidyl-N-(4-nitrobenzo-2-oxa-1,3-diazole)ethanolamine (structure I; derived from egg phosphatidylcholine) in binarymixtures of cholesterol and dimyristoyl phosphatidyicholine as a function of temperature and composition. Some diffusion coefficients for this probe in binary mixtures of dipalmitoyl phosphatidylcholine and cholesterol are also given. As will be seen, such data shed considerable light on the nature...
In 1984, the Texas Legislature funded a four-university, interdisciplinary effort to identify feasible sites for location of a very high-energy physics lab in Texas and to evaluate the comparative advantages of one site versus another. Six feasible sites were identified and a comparative site analysis was made by applying Data Envelopment Analysis (DEA), incorporating project cost, user time delay, and environmental impact data. In addition, for the efficient sites, the price weights for user time delay and environmental impact, given normalization on project cost, were analyzed and arguments were developed to bracket these pairs of price weights into an “assurance region” for the preferred site(s). The South Dallas site was found to be preferred for a wide range of conditions, while the North Houston site was sensitive to the method of indexing the impact of the environment. The method appears to be applicable to a wide range of siting problems faced by not only government, but also industry.
The lateral diffusion of a fluorescent phospholipid probe in oriented multibilayers of dimyristoylphosphatidyicholine has been measured by observing the redistribution of fluorescence after photobleaching of the membranes in a periodic pattern of parallel stripes. The diffusion constant D of the fluorescent lipid was found to vary between 1.5 X 10-11 cm2 sec at 9.60 and 2.0 X 10-10 at 22.50 in the monoclinic phase. The relationship between the composition, distribution, and motion of cell surface components and cell function is one of the major challenges of modern molecular biology. Attempts to relate lateral motion and function have been made for intact cells (1-5) as well as for model membranes having specific, well-delineated functions (6)(7)(8). Two of these studies (6, 7) included an attempt to relate the lateral mobility of haptens in model membranes to the degree of complement depletion. Because the rate of lateral diffusion of phospholipids in the "fluid" state of phosphatidylcholines has been well known from early studies using spin labels (9-12) as well as more recent photobleaching methods (13,14), the initial goal of the present work was to obtain the diffusion constants of lipids in the "solid" phase of phosphatidylcholines; if low enough, such diffusion constants could play a critical role in limiting complementmediated attack on such membranes. The elegant experiments by Wu et al. (13) clearly demonstrated a precipitous decrease in the diffusion constant of a lipid probe in dimyristoylphosphatidylcholine at the transition temperature but did not yield a diffusion constant in the lower temperature phases, a quantity of central importance in our study.In our new technique, an image consisting of alternating bright and dark stripes is projected into a sample to establish, by photobleaching of fluorescent probe molecules, a periodic variation of fluorescence intensity as a function of position. Observation of the subsequent redistribution of fluorescence intensity in the sample yields information about the motion (e.g., diffusion) of the probe molecules. THEORY AND APPARATUSAs a simple model, we consider the diffusion of fluorescent probe molecules confined to a two-dimensional surface. Photobleaching of the probe molecules by light in a periodic pattern of parallel stripes produces a corresponding periodic variation in the concentration (C) along the direction (x) perpendicular to the direction (y) of the stripes. Therefore, ?C/?by = 0, and we need only consider diffusion of fluorescent molecules in the x direction. The diffusion equation is IC(xt) = DM2C(Xt) ait oX2[1]
Text entry user interfaces have been a bottleneck of nontraditional computing devices. One of the promising methods is the virtual keyboard on touch screens. Various layouts have been manually designed to replace the dominant QWERTY layout. This paper presents two computerized quantitative design techniques to search for the optimal virtual keyboard. The first technique simulated the dynamics of a keyboard with "digraph springs" between keys, which produced a "Hooke's" keyboard with 41.6 wpm performance. The second technique used a Metropolis random walk algorithm guided by a "Fitts energy" objective function, which produced a "Metropolis" keyboard with 43.1 wpm performance.The paper also models and evaluates the performance of four existing keyboard layouts. We corrected erroneous estimates in the literature and predicted the performance of QWERTY, CHUBON, FITALY, OPTI to be in the neighborhood of 30, 33, 36 and 38 wpm respectively. Our best design was 40% faster than QWERTY and 10% faster than OPTI, illustrating the advantage of quantitative user interface design techniques based on models of human performance over traditional trial and error designs guided by heuristics.
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