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At least 1,369 records · Page 76Linked to original sources
PubMed2004

One-bond spin-spin coupling constants of X-1H proton donors in complexes with X-H-Y hydrogen bonds, for X = 13C, 15N, 17O, and 19F: predictions, comparisons, and relationships among 1J X-H, 1K X-H, and X-H distances.

Ab initio calculations at the equation-of-motion coupled cluster (EOM-CCSD) level of theory have been carried out to investigate one-bond (13)C-(1)H, (15)N-(1)H, (17)O-(1)H, and (19)F-(1)H coupling constants in a systematic study of monomers and hydrogen-bonded complexes. Computed coupling constants ((1)J(X-H)) for monomers are in good agreement with available experimental data. All reduced Fermi-contact terms and reduced coupling constants ((1)K(X-H)) for monomers and complexes are positive. Plots of (1)K(X-H) versus the X-H distance for the 16 monomers and the 64 complexes in which these monomers are proton donors exhibit significant scatter. However, a linear relationship has been demonstrated for the first time between coupling constants and X-H distances for different X atoms by plotting the ratios of the coupling constants for complexes and corresponding monomers versus the ratios of distances for complexes and corresponding monomers times the square of the Pauling electronegativity. Since the ratio removes the dependence of coupling constants on the magnetogyric ratios of X, this relationship holds for both (1)K(X-H) and (1)J(X-H). The decrease in reduced coupling constants ((1)K(X-H)) as the X-H distance increases is due primarily to the increased proton-shared character of the hydrogen bond.

Journal Article↗
PubMed2005

Strikingly long C...C distances in 1,2-disubstituted ortho-carboranes and their dianions.

Neutral and especially dianionic 6- and 12-vertex closo ortho-carboranes (o-carboranes) 1,2-R2-1,2-C2BnHn (R = H, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, as well as e-, CH2-, NH-, O-, SiH2-, PH-, and S- exhibit extremely large variations (over 1 A!) of the cage CC distances, from 1.626 to 2.638 A, at the B3LYP/6-31G//B3LYP/6-31G DFT level. These CC "bond lengths," among the longest ever reported, generally are greater in the icosahedral than those in the corresponding octahedral systems and depend strongly on the substituents. While 1,2-(NH2)2-1,2-C2B10H10 has the longest Cc...Cc separation in neutral species (1.860 A), Cc...Cc distances can be much larger in the corresponding dianions. These range from 1.823 A (R- = e-) to 2.638 A (R- = CH2-) for 1,2-(R-)2-1,2-C2B10H10 and from 1.626 A (R- = SiH2-) to 3.099 A (R- = NH-) for 1,2-(R-)2-1,2-C2B4H4. Remarkably, there is no abrupt discontinuity over the entire range of CC lengths. Consequently, the relationship between the gradual changes in the distances and the nature of the bonding was analyzed by means of the form of the Kohn-Sham orbitals, the Wiberg Cc...Cc bond indices, and Bader AIM method. Cluster carboranes, and possibly other heteroboranes, thus appear to offer unique opportunities for modulating Cc...Cc distances.

Journal Article↗
PubMed2005

Fluorophore-quencher distance correlation functions from single-molecule photon arrival trajectories.

We generalize and simplify the method of Yang and Xie (J. Chem. Phys. 2002, 117, 10965) to obtain distance correlation functions from photon arrival trajectories of single fluorophores whose lifetime, [k(r)](-1), depends on the distance to a quencher. It is assumed that this distance does not change during the fluorescence lifetime. The experimental trajectory is first transformed by replacing the delay time (i.e., the interval between the photon arrival and the nearest laser pulse) by a certain function of this delay time. This function is the inverse Laplace transform of r(k), which is the solution of k(r) = 1. The correlation function of the transformed data then directly gives the distance correlation function. Illustrative examples include Förster energy transfer and quenching due to electron transfer.

Diffusion↗
PubMed2006

Scanning tunneling microscopy and scanning tunneling spectroscopy studies of planar and nonplanar naphthalocyanine on graphite (0001). Part 2: tip-sample distance-dependent I-V spectroscopy.

Tip-sample distance-dependent current-voltage tunneling spectroscopy on monolayers of base-free naphthalocyanine (Nc), a planar molecule, and tin-naphthalocyanine (SnNc), a nonplanar molecule, has been studied on a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface using a variable-temperature STM at 50 K under ultra-high vacuum conditions. The current-voltage curves show an unsymmetrical diode-like nature especially at large tip-sample distances in both cases. Normalized differential conductivity of all spectra has been considered for further analysis. The ionization and electron affinity levels are compared with the single-molecule local density of states (LDOS) near the Fermi energy using a theoretical calculation for Nc and SnNc. A tip-sample distance-dependent highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap shrinking is observed in the case of Nc, in which the filled levels of the molecules are pinned while the unfilled levels near the Fermi energy are shifting toward lower energy. In contrast, there is no such HOMO-LUMO gap shrinking in the case of the SnNc decreasing tip-sample distance. However, a subsequent increase in the tunneling current was observed by almost 1 order of magnitude compared with Nc. A model is proposed to explain this phenomenon where the Nc-graphite interface is considered as a pure capacitive interface.

Journal Article↗
PubMedDate not supplied

Monte Carlo calculations on polypeptide chains. VIII. Distribution functions for the end-to-end distance and radius of gyration for hard-sphere models of randomly coiling poly(glycine) and poly(L-alanine).

A Monte Carlo study of the distribution functions for the end-to-end distance and radius of gyration for hard-sphere models of poly(glycine) and poly(L-alanine) random coils has been conducted in the chain-length range n = 3 to 100 monomer units for both unperturbed chains and chains perturbed by long-range interactions (excluded volume effects). The distribution functions for the radius of gyration in all cases have been very precisely calculated, those for the perturbed end-to-end distance less precisely, and those for the unperturbed end-to-end distance least precisely. Empirical distribution functions of the form W(p) = ap-b exp(-cp-d) for the reduced end-to-end distance p = r/"r-2"-one-half and a similar form for the reduced radius of gyration could be least-squares fit to the Monte Carlo data. The expansion factors alpha-r and alpha-s were calculated vs. chain length and were used to test various versions of the two-parameter theory of the excluded volume effect. To be consistent with the chain-length dependence of alpha-r and alpha-s as determined by the Monte Carlo calculations, each of these theories required two different binary cluster integrals, a beta-r based on alpha-r and a beta-s based on alpha-s, both of which were strongly chain-length dependent. Both of these results suggest that the two-parameter theory is not applicable to the models used in this study. It was also found that, except for very short chain lengths, plots of ln alphs-r vs. ln n were linear, and thus that alpha-r could be estimated for long chain lengths. Comparison of these estimates with the experimental data on four polypeptide chains in one-earth solvents that the hard-sphere models used in this study yield expansion factors that do not seriously overestimate the magnitude of the excluded volume effect.

Alanine↗
PubMed1998

Evolutionary distances and identification of Candida species in clinical isolates by randomly amplified polymorphic DNA (RAPD).

Fast and reliable identification of different species of the genus Candida is important to define adequate therapeutic decisions, because the different species have highly variable susceptibilities to antifungal drugs; azoles and amphothericin B. Accurate statistical records on case history and epidemiological studies also depend on effective identification. To address this problem we established a RAPD method that enabled direct identification of five very common species of Candida. Initially, reference band patterns were established for C. albicans, C. tropicalis, C. parapsilosis, C. glabrata and C. krusei. One of the primers, M2, showed remarkably conserved intra-specific patterns of approximately 10 bands each, ranging in size from 2.0 to 0.1 kb. These patterns were significantly different and species-specific. Few bands were conserved between different species of Candida, which was assumed to be consistent with their phylogenetic relatedness. In addition, band patterns were constant and reproducible and DNA isolated from single colonies yielded sufficient DNA for identification. The reference band patterns were then used, in blind experiments, to identify species of Candida in 50 randomly chosen samples, including clinical isolates and ATCC strains. RAPD results were 100% consistent with results obtained by conventional diagnostic methods and were achieved in one day instead of several days taken by conventional methods. Because ideal identification methods should be consistent with phylogeny and taxonomy we tested whether RAPD could be used to calculate genetic distances. Comparison of RAPD phylogenetic trees with 18S rRNA trees showed significant differences in tree topologies which indicated that RAPD data could not accurately measure the relative distances between different species. Also, computer simulations of RAPD random patterns were used to test whether the observed degree of RAPD band pattern similarities could occur at random. These simulations suggested that the level of inter-specific band pattern similarities observed in our data could be obtained at random, while intraspecific pattern similarities could not. RAPD would be helpful to discriminate between isolates but not to quantitate the differences. We suggest that the inaccurate estimate of genetic distances from RAPD is a general limitation of the technique and not a specific problem of our identification method. Because of the repetitive character of the target sequences, genetic distances calculated from RAPD could be affected by paralogy, namely, recombination and duplication events not parallel with speciation events.

Candida↗
PubMed2002

Associations between distance lags, groundwater velocities, and detection efficiencies in groundwater monitoring networks.

Effects of distance lags between landfills and monitoring wells on contaminant detection capability were quantified in several groundwater velocity settings. Detection efficiency calculations were made with and without imposing a time limit on contaminant travel. In general, longer distance lags yielded higher detection efficiencies. However, detection efficiencies decreased as monitoring wells approached a buffer zone boundary imposing a maximum permissible contaminant transport distance. Imposing a time limit on contaminant travel substantially reduced detection efficiency in low velocity settings, especially at longer distance lags. Time limits were less significant in high velocity settings where contaminants more quickly reached monitoring wells. Detection efficiencies also decreased as velocity increased, but decreases were minor once the velocity reached a threshold value.

Environmental Monitoring↗
PubMed1999

Six minute walking distance in healthy elderly subjects.

The six minute walking distance (6MWD) test is a commonly used test to estimate functional exercise capacity in patients with chronic diseases including chronic obstructive lung disease. Surprisingly, no attempt has been made to establish normal values for the 6MWD. The aim of this study, therefore, was to evaluate the 6MWD in healthy elderly volunteers and to evaluate its determining factors. Fifty-one healthy subjects aged 50-85 yrs volunteered to participate in the trial. All subjects were free of diseases that could interfere with performance in a walking test. Tests were performed in a quiet 50-m long hospital corridor. Patients were encouraged every 30 s to continue walking as quickly as possible. Walking distance averaged 631+/-93 m and was 84 m greater in the male compared to female subjects (p<0.001). The 6MWD showed significant correlations with age (r=-0.51, p<0.01) and height (r=0.54, p<0.01). Stepwise multiple regression analysis showed that age, height, sex and weight were independent contributors to the 6MWD in healthy subjects, thus explaining 66% of the variability. It is concluded that the six minute walking distance can be predicted adequately using a clinically useful model in healthy elderly subjects. Its variability is explained largely by age, sex, height and weight. Results of the six minute walking distance may be interpreted more adequately if expressed as a percentage of the predicted value.

Aged↗
PubMed2003

Polymer structure of a light-cured resin composite in relation to distance from the surface.

The aim of the investigation was to study the polymer structure of a light-cured resin composite in relation to the distance from the irradiated surface. Ten cylinders (4 x 8 mm) of composite were light-cured in a mold at 580 mW cm-2 for 40 s. The cylinders were expressed from the mold and, after 1 wk of dry storage at 37 degrees C, embedded in dental stone. Grinding parallel to the long axis of the cylinders on carborundum paper exposed the resin composite. The Wallace hardness in relation to distance from the irradiated surface was measured before and after 1 d of ethanol storage. Before ethanol storage and at distances from the surface of 0.5-3.5 mm, no difference in hardness was recorded. The ethanol storage gave rise to a softening of the resin composite. The softening recorded at 3.0-3.5 mm was significantly more pronounced than the softening taking place at 0.5-2.5 mm. This result was explained by a slower polymerization occurring at the greater distances from the irradiated surface, resulting in a polymer with reduced crosslink density.

Composite Resins↗
PubMed1996

Distance and distraction effects in the apprehension of spatial relations.

Theories of the apprehension of spatial relations differ in the predictions they make about the effects of distance between the arguments of spatial relations and the effects of distracting stimuli presented along with the arguments. One theory predicts no effect of distance, another predicts a monotonic increase in reaction time (RT) with distance, and a third predicts a monotonic decrease. Most theories predict slower RTs but reasonable accuracy when distractors are present, but 1 theory predicts chance-level accuracy. These predictions were tested in 3 sentence-picture comparison experiments, in which subjects searched for targets exemplifying the relations above and below. Distance had no effect when no distractors were present. Distractors slowed RT but did not reduce accuracy to chance levels. These results suggest modifications to many of the theories of apprehension.

Humans↗
PubMed1996

Substantial outgassing of CO from comet Hale-Bopp at large heliocentric distance.

When comet C/1995 O1 (Hale-Boop) was discovered, at a distance of seven astronomical units from the sun, it was more than one hundred times brighter than comet Halley at the same distance. A comet's brightness is derived from the reflection of sunlight from dust grains driven away from the nucleus by the sublimation of volatile ices. Near the sun, sublimation of water ice (a main constituent of comet nuclei) is the source of cometary activity; but at its current heliocentric distance, Hale-Boop is too cold for this process to operate. Other comets have shown activity at large distances, and in the case of comet Schwassmann-Wachmann 1, carbon monoxide has been detected in quantities sufficient to generate its observed coma. Here we report the detection of CO emission from Hale-Boop, at levels indicating a very large rate of outgassing. Several other volatile species were searched for, but not detected. Sublimation of CO therefore appears to be responsible for the present activity of this comet, and we anticipate that future observations will reveal the onset of sublimation of other volatile species as the comet continues its present journey towards the sun.

Carbon Monoxide↗
PubMed1989

Genetic analyses of pupation distance in Drosophila melanogaster.

The inheritance of Drosophila melanogaster larval pupation behaviour is investigated in sixteen reciprocal crosses between field collected lines. These lines were made isogenic for the two major autosomes enabling the data to be analyzed using contrast analysis of variance and biometrical genetic analysis. Results of both analyses showed that the trait "pupation distance", the distance larvae pupate from food, fits a simple additive model of inheritance with no dominance. A chromosomal analysis showed that both the second and third chromosomes act additively on pupation distance and that the third pair of chromosomes had a much larger effect than the second. Significant variability exists in the distance D. melanogaster larvae pupate from fruit in nature. This phenotypic variation results from both heritable variation and variation from environmental sources. When the moisture content of the environment surrounding food is modified, gene by environment interactions also contribute to variation in the phenotype. Selective pressures which may act on larval differences in pupation site choice are discussed.

Analysis of Variance↗
PubMed2005

Relationships of age and weekly running distance to BMI and circumferences in 41,582 physically active women.

OBJECTIVE: To assess in women whether age-related increases in adiposity are dependent on exercise, and, contrariwise, whether exercise-related declines in adiposity are dependent on age. RESEARCH METHODS AND PROCEDURES: Cross-sectional analyses were conducted of 41,582 female runners. RESULTS: Age affected the relationships between vigorous exercise and adiposity. The decline in BMI per kilometer per week run was linear in 18 to 23 year olds and became increasingly non-linear (convex) with age. Waist, hip, and chest circumferences declined significantly with running distance across all age groups, but the declines were significantly greater in older than younger women, particularly among shorter distance runners. The relationships between body circumferences and running distance became increasingly convex in older women. Conversely, vigorous exercise diminished the apparent increase in adiposity with age. The increase in average BMI with age was greatest in women who ran <8 km/wk, intermediate in women who ran 8 to 15 km/wk or 16 to 31 km/wk, and least in those who averaged over 32 km/wk. Before age 45, waist circumference rose for those who ran 0 to 7 km/wk, showed no significant relationship to age for those who ran 8 to 39 km/wk, and declined in those who ran 40 to 55 and 56 km/wk and more. Age related-increases in hip and chest circumferences before 45 years old were significantly less in women who ran longer weekly distances. DISCUSSION: These cross-sectional associations are consistent with the hypothesis that exercise may mitigate age-related increases in adiposity and that age affects exercise-induced reductions in adiposity (although causality remains to be determined experimentally).

Adiposity↗
PubMed2003

Habitat differentiation vs. isolation-by-distance: the genetic population structure of Elymus athericus in European salt marshes.

We investigated genetic differentiation among populations of the clonal grass Elymus athericus, a common salt-marsh species occurring along the Wadden Sea coast of Europe. While E. athericus traditionally occurs in the high salt marsh, it recently also invaded lower parts of the marsh. In one of the first analyses of the genetic population structure in salt-marsh species, we were interested in population differentiation through isolation-by-distance, and among strongly divergent habitats (low and high marsh) in this wind- and water-dispersed species. High and low marsh habitats were sampled at six sites throughout the Wadden Sea. Based on reciprocal transplantation experiments conducted earlier revealing lower survival of foreign genotypes we predicted reduced gene flow among habitats. Accordingly, an analysis with polymorphic cross-species microsatellite primers revealed significant genetic differentiation between high and low marsh habitats already on a very small scale (< 100 m), while isolation-by-distance was present only on larger scales (60-443 km). In an analysis of molecular variance we found that 14% of the genetic variance could be explained by the differentiation between habitats, as compared to only 8.9% to geographical (isolation-by-distance) effects among six sites 2.5-443 km distant from each other. This suggests that markedly different selection regimes between these habitats, in particular intraspecific competition and herbivory, result in habitat adaptation and restricted gene flow over distances as small as 80 m. Hence, the genetic population structure of plant species can only be understood when considering geographical and selection-mediated restrictions to gene flow simultaneously.

Ecosystem↗
PubMed2002

Survival from upper gastrointestinal cancer in New Zealand: The effect of distance from a major hospital, socio-economic status, ethnicity, age and gender.

BACKGROUND: Upper gastrointestinal cancer is the third most common cause of cancer mortality in New Zealand. Prognosis of these and other cancers is possibly affected by the accessibility of hospitals. Several studies have revealed a reduction in survival that correlates with increasing distance from a major cancer centre. The aim of the present study was to analyse any correlation between survival from upper gastrointestinal cancer and distance from a major centre, socio-economic status, gender, age and ethnicity. METHODS: Details of all 3351 patients diagnosed with cancers of the oesophagus, stomach, pancreas, liver and biliary tract between 1 January 1995 and 31 December 1997 were retrieved from the New Zealand cancer registry. The effect of age, gender, ethnicity, socio-economic status and distance from a major centre was analysed using univariate and multivariate regression analysis to identify any associations with survival. CONCLUSIONS: Increasing age and Maori descent were the only consistent indicators of poorer survival in this study. The relationship between distance and survival was shown to be complex and in this study deprivation had no effect on the prognosis of upper gastrointestinal cancer.

Aged↗
PubMed1992

Sleeping distance in relation to sexual state in the Arctic blue fox.

The paper provides comparative data concerning sleeping distance in relation to sexual state and hierarchical dominance in the Arctic blue fox. Increasing levels of vulval swelling and electrical resistance of vaginal tract were inversely related to sleeping distance between the highest-ranking male and each female on heat. A mating pair typically slept no further than 50 cm from each other. One month after copulation, the distance was already more than 200-250 cm. It can be concluded that oestrus-associated changes in sexual hormone levels have an influence on observed changes in sleeping distance.

Animals↗
PubMedDate not supplied

The effect of distance from injection site to the brainstem using spinal sufentanil.

BACKGROUND AND OBJECTIVES: Intrathecal (IT) sufentanil is commonly used in parturients to provide rapid onset of labor analgesia without motor block. This practice, although widely used, has been associated with severe respiratory depression in some patients. The mechanism of this respiratory depression is unclear, however, rapid cephalad movement and interaction with parenteral opioids are 2 frequently cited explanations for this complication. Because this complication has occurred only in women with heights between 150 and 157 cm, we elected to study the effect of the distance from injection site to the cisterna magna (CM) on peak brainstem cerebrospinal fluid (CSF) concentrations. METHODS: Ten adult ewes were injected with IT sufentanil (0.3 microg/kg) at a mean distance of either 71 cm (65 to 78 cm) from the brainstem (pelvic group) or 37 cm (34 to 42 cm) from the brainstem (thoracic group). CSF was then sampled at 5-minute intervals from the CM. RESULTS: Measurable CM concentrations of sufentanil were noted in the brainstem at 20 and 25 minutes, respectively, for the thoracic and the pelvic groups. Peak sufentanil concentrations from the thoracic group were nearly 10-fold higher (0.553 +/- 0.43 ng/mL) compared with the pelvic group (0.064 +/- 0.002 ng/mL) when measured in the CM (P =.023). CONCLUSIONS: Our results suggest that sufentanil migrates relatively large distances in the IT space. Injection site (distance from the brainstem) appears to be a prominent factor in determining brainstem concentrations and subsequent respiratory depression after spinal administration. Reg Anesth Pain Med 2001;26:306-309.

Analgesics, Opioid↗
PubMed2001

The body weight-walking distance product as related to lung function, anaerobic threshold and peak VO2 in COPD patients.

The product of walking distance and body weight (D x W) mimics the work of walking. We hypothesized the superiority of D x W to walking distance (D) alone in any correlation with lung function, anaerobic threshold (AT) and maximal oxygen uptake (VO2max). We further hypothesized that the D x W product for a 6-min walk test (6 MWT) would correlate with the AT and VO2max because all three are markers of exercise ability. Thirty-three male chronic obstructive pulmonary disease (COPD) patients with mean forced expiratory volume in 1 sec (FEV1) of 1.2+/-0.4 l (range 0.58-1.86 l) were enrolled. Six patients were excluded due to inability to achieve a maximal test. Lung function and self-assessed every-day activities using a oxygen-cost diagram were evaluated before entry of the study. A maximal effort ramp-pattern cardiopulmonary exercise test (CPET) and a 6 MWT were conducted in random order. Borg score, heart rate, and O2 saturation with pulse oximetry (SpO2) were measured during both exercise tests. VO2 AT and minute ventilation were also measured during the CPET. Correlations were sought between the distance covered in the 6 MWT, and the D x W product with AT, VO2max and other variables. The average D and D x W were 456 m and 27.5 kg km(-1), respectively. D x W was superior to D alone when correlated with the VO2max and AT determined from the CPET, while modestly correlated with the change (delta) in Borg score and delta SpO2 in the 6 MWT and self-assessed every-day activities. Distance x weight product was correlated with the AT and VO2max. In addition, D x W was better correlated with diffusing capacity for carbon monoxide and vital capacity than D alone. We conclude that D x W mimics the work of walking better than D and is suggested as a parameter for evaluation of patients' fitness if gas exchange measurements are not available.

Activities of Daily Living↗