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At least 109 records · Page 6Linked to original sources
PubMed1977

Application of radiationless energy transfer for distance measurements across membranes. Transition Probabilities for radiationless energy transfer within several assemblies of donors and acceptors at stationary distances.

For several 2-or 3-dimensional configurations of stationary donors and acceptors on or near a spherical membrane shell the transition probabilities for radiationless energy transfer are calculated, using Förster's approximation obtained for Coulombic dipole-dipole interaction of the transition moments. It turns out that the difference in the refractive indices for membrane (lipid) and bulk phase (water) has only a small influence on the transition probabilities. Furthermore, the curvature of biological cell surfaces can be neglected, but affects the energy transfer across small vesicles. The ratio thickness/radius of small vesicles can be determined by measuring fluorescence quenching of excited donors by acceptors on the other side of the membrane.

Energy Transfer↗
PubMed1996

Neural processing of stereopsis as a function of viewing distance in primate visual cortical area V1.

1. The influence of viewing distance on disparity selectivity was investigated in area V1 of behaving monkeys. While the animals performed a fixation task, cortical cells were recorded extracellularly in the foveal representation of the visual field. Disparity selectivity was assessed by using static random dot stereograms (RDSs) through red/green filters flashed over the central fixation target. To determine the influence of the viewing distance, a color video monitor was positioned at fixed distances of 20, 40, or 80 cm. The same RDSs with the same angular size of dots were used at the three distances. 2. Disparity sensitivity was tested on 139 cells, of which 78 were analyzed at two or more distances and the rest (61) at a single distance. When disparity selectivity was analyzed at a given distance, about half the cells were found to be selective at 40 or 80 cm, but only a third at 20 cm. Near cells were > or = 1.5 times more common than far cells at all three distances. The latency distribution of the responses of disparity-selective (DS) cells was similar at all three distances, with a mean distribution centered around 60 ms. 3. Changing the viewing distance drastically affected the neural activity of the V1 neurons. The visual responsiveness of 60 of 78 cells (77%) was significantly changed. Disparity selectivity could be present at a given distance and absent at other(s), with often a loss of visual response. This emergence of disparity coding was the strongest effect (28 of 78 or 36%) and occurred more frequently from short to long distances. Among the cells that remained disparity insensitive at all recorded distances (31 of 78 or 40%), about half also showed modulations of the amplitude of the visual response. For cells that remained DS at all recorded distances (13 of 78 or 17%), changing the viewing distance also affected the sharpness (or magnitude) of disparity coding in terms of level of visual responsiveness and those changes were often combined with variations in tuning width. In only two cells did the peak of selectivity type change. Finally, the activity of four DS cells was not affected at all by the viewing distance. 4. Another effect concerned the level of ongoing activity (OA), defined as being the neural activity in darkness preceding the flash of the visual stimulus while the monkey was fixating the small bright target. Changing the viewing distance resulted in significant changes in OA level for more than half of the cells (41 of 78 or 53%). The most common effect was an increase in OA level at the shorter distance. The modulations of both visual responsiveness and OA could occur simultaneously, although they often had opposite signs. Indeed, the two effects were statistically independent of each other, i.e., modulations of visual responses were not related to the level of excitability of the neurons. 5. Control experiments were performed that showed that the effects of changing the viewing distance were not due to the retinal patterns in that the modulations of visual responsiveness were independent of the dot density. Seventeen cells were also tested for a possible effect of vergence by the use of prisms. When there was an effect of distance, it could be closely or partially reproduced by using prisms. These controls, together with the effects observed on OA, strongly suggest that the modulations of neural activity of the V1 neurons by the viewing distance are extraretinal in origin, probably proprioceptive. 6. The modulation of visual responsiveness by the viewing distance in the primary visual cortex indicates that integration of information from both retinal and extraretinal sources can occur early in the visual processing pathway for cortical representation of three-dimensional space. A functional scheme of three-dimensional cortical circuitry is discussed that shows cortical areas where disparity selectivity and modulations of visual activity by the angle of gaze have been described so far.

Animals↗
PubMed1980

Nuclear Overhauser effect and cross-relaxation rate determinations of dihedral and transannular interproton distances in the decapeptide tyrocidine A.

The following interproton distances are reported for the decapeptide tyrocidine A in solution: (a) r(phi) distances between NH(i) and H alpha (i), (b) r(psi) distances between NH (i + 1) and H alpha (i), (c) r(phi psi) distances between NH(i + 1) and NH(i), (d) NH in equilibrium NH transannular distances, (e) H alpha in equilibrium H alpha transannular distances, (f) r x 1 distances between H alpha and H beta protons, (g) NH(i) in equilibrium H beta (i) distances, (h) NH (i + 1) in equilibrium H beta (i) distances, (i) carboxamide-backbone protons and carboxamide-side chain proton distances, (j) side chain proton-side chain proton distances. The procedures for distance calculations were: NOE ratios and calibration distances, sigma ratios and calibration distances, and correlation times and sigma parameters. The cross-relaxation parameters were obtained from the product, say, of NOE 1 leads to 2 and the monoselective relaxation rate of proton 2; the NOEs were measured by NOE difference spectroscopy. The data are consistent with a type I beta-turn/ type II' beta-turn/ approximately antiparallel beta-pleated sheet conformation of tyrocidine A in solution and the NOEs, cross-relaxation parameters, and interproton distances serve as distinguishing criteria for beta-turn and beta-pleated sheet conformations. It should be borne in mind that measurement of only r phi and r psi distances for a decapeptide only defines the ( phi, psi)-space in terms of 4(10) possible conformations; the distances b-j served to reduce the degeneracy in possible (phi, psi)-space to one tyrocidine A conformation. The latter conformation is consistent with that derived from scalar coupling constants, hydrogen bonding studies, and proton-chromophore distance measurement, and closely resembles the conformation of gramicidin S.

Amino Acids↗
PubMed1989

Distances as degrees of freedom.

Tertiary contact distance information of varying resolution for large biological molecules abounds in the literature. The results provided herein develop a framework by which information of this type can be used to reduce the allowable configuration space of a macromolecule. The approach combines graph theory and distance geometry. Large molecules are represented as simple, undirected graphs, with atoms, or groups, as vertices, and distances between them as edges. It is shown that determination of the exact structure of a molecule in three dimensions only requires the specification of all the distances in a single tetrahedron, and four distances to every other atom. This is 4N-10 distances which is a subset of the total N(N-1)/2 unique distances in a molecule consisting of N atoms. This requirement for only 4N-10 distances has serious implications for distance geometry implementations in which all N(N-1)/2 distances are specified by bounded random numbers. Such distance matrices represent overspecified systems which when solved lead to non-obvious distribution of any error caused by inherent contradictions in the input data. It is also shown that numerous valid subsets of 4N-10 distances can be constructed. It is thus possible to tailor a subset of distances using all known distances as degrees of freedom, and thereby reduce the configuration space of the molecule. Simple algebraic relationships are derived that relate sets of distances, and complicated rotations are avoided. These relationships are used to construct minimum, complete sets of distances necessary to specify the exact structure of the entire molecule in three dimensions from incomplete distance information, and to identify sets of inconsistent distances. The method is illustrated for the flexible structural types present in large ribosomal RNAs: 1.) A five-membered ring; 2.) a chemically bonded chain with its ends in contact (i.e., a hairpin loop); 3.) the spatial orientation of two separate molecules, and; 4.) an RNA helix that can have variation in individual base pairs, giving rise to global deviation from standardized helical forms.

Base Composition↗
PubMed1995

MR of the cerebral operculum: topographic identification and measurement of interopercular distances in healthy infants and children.

PURPOSE: To evaluate the role of axial, coronal, and sagittal MR in identification of surface landmarks of the cerebral operculum and to determine the reference values of interopercular distances of each hemisphere in healthy infants and children on MR images. METHODS: Two hundred fourteen cerebral opercula of 35 healthy infants and 72 healthy children were retrospectively evaluated from 107 routine MR brain examinations. The surface landmarks of the operculum and interopercular distances of each hemisphere, which were subjectively divided into anterior interopercular distance (anterior sylvian width) and posterior interopercular distance (posterior sylvian width), were recorded from axial, coronal, and sagittal MR images, respectively. The mean value of anterior interopercular distance of each hemisphere was obtained by averaging two linear measurements of the anterior sylvian width from lateral, sagittal, and axial planes of the same side. Likewise, the posterior interopercular distance of each side of the brain was obtained from averaging of two measurements on lateral, sagittal, and coronal planes. RESULTS: The landmarks of the operculum were best identified by sagittal MR, followed by axial and coronal images. The average values of left anterior interopercular distance, right anterior interopercular distance, left posterior interopercular distance, and right posterior interopercular distance in infants were 1.9 +/- 1.3, 1.6 +/- 1.1, 0.4 +/- 0.7, and 0.2 +/- 0.4 mm, and in children, 0.9 +/- 1.3, 1.0 +/- 1.4, 0.03 +/- 0.23, and 0.01 +/- 0.07 mm, respectively. Infants showed significantly wider interopercular distances than children. Left anterior interopercular distance was significantly wider than right in infants, but not in children. Male children displayed a more significant increase in anterior interopercular distance than did female children. There was no statistic difference in measurements of anterior interopercular distance and posterior interopercular distance between female and male infants. CONCLUSIONS: The operculum should be evaluated with MR in three planes. Infants may show conspicuous sylvian fissures that should not exceed 4.5 mm (mean + 2 SD) anteriorly on axial and sagittal planes and 1.8 mm posteriorly on sagittal and coronal planes. Healthy children who have fully developed opercula should have an anterior interopercular distance of no more than 3.5 mm and a posterior interopercular distance of 0.5 mm.

Adolescent↗
PubMed2005

The effect of relative distance enlargement on visual acuity in the visually impaired.

BACKGROUND: Prescribing magnification is typically based on distance or near visual acuity. This presumes a constant minimum angle of visual resolution with working distance and therefore enlargement of an object moved to a shorter working distance (relative distance enlargement). This study examines this premise in a visually impaired population. METHODS: Distance letter visual acuity was measured prospectively for 380 low vision patients (distance visual acuity between 0.3 and 2.1 logMAR) over the age of 57 years, along with near word visual acuity at an appropriate distance for near lens additions from +4 D to +20 D. Demographic information, the disease causing low vision, contrast sensitivity, visual field and psychological status were also recorded. RESULTS: Distance letter acuity was significantly related to (r = 0.84) but on average 0.1 +/- 0.2 logMAR better (1 +/- 2 lines on a logMAR chart) than near word acuity at 25 cm with a +4 D lens addition. In 39.8 per cent of patients, near word acuity was more than 0.1 logMAR worse than distance letter acuity. In 11.0 per cent of subjects, near visual acuity was more than 0.1 logMAR better than distance letter acuity. The group with near word acuity worse than distance letter acuity also had lower contrast sensitivity. The group with near word acuity better than distance letter acuity was less likely to have age-related macular degeneration. Smaller print size could be read by reducing working distance (achieved by using higher near lens additions) in 86.1 per cent, although not by as much as predicted by geometric progression in 14.5 per cent. DISCUSSION: Although distance letter and near word acuity are highly related, they are on average 1 logMAR line different and this varies significantly between individuals. Near word acuity did not increase linearly with relative distance enlargement in approximately one in seven visually impaired, suggesting that the measurement of visual resolution over a range of working distances will assist appropriate prescribing of magnification aids.

Aged↗
PubMed2005

Comparison of phenotypic and molecular distances to predict heterosis and F1 performance in Ethiopian mustard (Brassica carinata A. Braun).

Predicting heterosis and F1 performance from the parental generation could largely enhance the efficiency of breeding hybrid or synthetic cultivars. This study was undertaken to determine the relationship between parental distances estimated from phenotypic traits or molecular markers with heterosis, F1 performance and general combining ability (GCA) in Ethiopian mustard (Brassica carinata). Nine inbred lines representing seven different geographic regions of Ethiopia were crossed in half-diallel. The nine parents along with their 36 F1s were evaluated in a replicated field trail at three locations in Ethiopia. Distances among the parents were calculated from 14 phenotypic traits (Euclidean distance, ED) and 182 random amplified polymorphic DNA (RAPD) markers (Jaccard's distances, JD), and correlated with heterosis, F1 performance and GCA sum of parents (GCAsum). The correlation between phenotypic and molecular distances was low (r=0.34, P< or =0.05). Parents with low molecular distance also had low phenotypic distance, but parents with high molecular distance had either high, intermediate or low phenotypic distance. Phenotypic distance was highly significantly correlated with mid-parent heterosis (r=0.53), F1 performance (r=0.61) and GCA (r=0.79) for seed yield. Phenotypic distance was also positively correlated with (1) heterosis, F1 performance and GCA for plant height and seeds plant(-1), (2) heterosis for number of pods plant(-1), and (3) F1 performance for 1,000 seed weight. Molecular distance was correlated with GCAsum (r=0.36, P< or =0.05) but not significantly with heterosis and F1 performance for seed yield. For each parent a mean distance was calculated by averaging the distances to the eight other parents. Likewise, mean heterosis was estimated by averaging the heterosis obtained when each parent is crossed with the other eight. For seed yield, both mean ED and JD were significantly correlated with GCA (r=0.90, P< or =0.01 for ED and r=0.68, P< or =0.05 for JD) and mean heterosis (r=0.79, P< or =0.05 for ED and r=0.77, P< or =0.05 for JD). In conclusion, parental distances estimated from phenotypic traits better predicted heterosis, F1 performance and GCA than distances estimated from RAPD markers.

Brassica↗
PubMed2005

Structure and function of distance runners' heart.

OBJECTIVE: To compare ultra-long distance runners' heart morphologic and functional parameters at rest with those of long distance runners' and middle distance runners'. MATERIALS AND METHODS: Standard Doppler, M-mode and 2-D-mode echocardiography was performed at rest to 22 middle, 31 long and 11 ultra-long adult male distance runners. RESULTS: Long and ultra-long distance runners' left ventricular mass and left ventricular mass index were larger (p<0.05) than that of middle distance runners' (groups' means--approximately 288, 305 and 250 g as well as 153, 160 and 130 g/m(2), respectively) due to both larger (p<0.05) end-diastolic interventricular wall thickness (10.6, 11.1 and 9.8 mm, respectively) and left ventricular posterior wall thickness (10.7, 11.5 and 10.0 mm, respectively). Ultra-long distance runners' left ventricular mass and mass index did not differ significantly from long distance runners' (p>0.05), but end-diastolic posterior wall thickness was higher (p<0.05). Relative left ventricular wall thickness was larger in ultra-long distance runners as compared with middle distance runners (0.402 and 0.362, respectively; p<0.05). Ultra-long distance runners' right ventricular end-diastolic diameter was significantly larger (p<0.05) than that of middle and long distance runners (groups' means--25.8, 20.7 and 21.4 mm, respectively). Right ventricular end-diastolic free wall was thicker in ultra-long distance runners as compared with middle distance runners (groups' means--6.7 and 5.9 mm, respectively; p<0.05). Diastolic left ventricular function (evaluated as E/A) as well as end-diastolic left ventricular diameter (groups' mean--55.5-56.4 mm) did not differ between groups (p>0.05). CONCLUSIONS: The hypertrophy of ultra-long (as well as long) distance runners' myocardium of both ventricles is more pronounced than that of middle distance runners'.

Adolescent↗