bluuebunny/crossref_metadata_2025 huggingface dataset:
Acute Modulation of Adipose Tissue Lipolysis by Intravenous Estrogens
Authors: Van Pelt Rachael E. , Gozansky Wendolyn S. , Hickner Robert C. , Schwartz Robert S. , Kohrt Wendy M.
Published: April 2008

Objective: The aim of this study was to determine whether intravenous (IV) conjugated estrogens (EST) acutely enhance the suppression of whole‐body or regional subcutaneous adipose tissue (SAT) lipolysis by insulin in postmenopausal women.

Research Methods and Procedures: We assessed whole‐body lipolysis by [2H5]glycerol rate of appearance (GlycRA) and abdominal and femoral SAT lipolysis (interstitial glycerol; GlycIS) by subcutaneous microdialysis. Postmenopausal women (n = 12) were studied on two occasions, with IV EST or saline control (CON), under basal conditions and during a 3‐stage (4, 8, and 40 mU/m2/min) hyperinsulinemic, euglycemic clamp. Ethanol outflow/inflow ratio and recovery of [13C]glycerol during microdialysis were used to assess blood flow changes and interstitial glycerol concentrations, respectively.

Results: Compared with CON, EST did not affect systemic basal or insulin‐mediated suppression of lipolysis (GlycRA) or SAT nutritive blood flow. Basal GlycIS in SAT was reduced on the EST day. However, insulin‐mediated suppression of lipolysis in SAT was not significantly influenced by EST.

Discussion: These findings suggest that estrogens acutely reduce basal lipolysis in SAT through an unknown mechanism but do not alter whole‐body or SAT suppression of lipolysis by insulin.
The Growth of Children in Relation to the Timing of Obesity Development
Authors: Papadimitriou Anastasios , Gousi Theodora , Giannouli Olga , Nicolaidou Polyxeni
Published: April 2008

Objective: To examine whether there is an association between the timing of the development of obesity and children's growth.

Research Methods and Procedures: This study investigated 141 prepubertal obese children (76 girls) and 72 healthy non‐obese children (39 girls). The target height standard deviation score (SDS), the percentage weight for height, and the height SDS (H‐SDS) at presentation and at the age of 2 years were calculated. Patients were classified, according to whether obesity developed before or after the age of 3 years, as presenting with early‐onset or late‐onset obesity, respectively.

Results: Mean age (±SD) at presentation was 9.4 (2.1) years. At the age of 2 years, the H‐SDS of the children with early‐onset obesity was 1.3 (1.0) vs. 0.9 (1.3) for the late‐onset obese (p > 0.5) and 0.4 (1.0) for controls (p < 0.001), and the children with late‐onset obesity were also significantly taller than controls (p < 0.005). At presentation, children with early‐onset obesity were significantly taller than children with late‐onset obesity [1.1 (0.8) vs. 0.6 (1.0); p < 0.001] and controls [0.2 (0.8); p < 0.001]. There was no increase in H‐SDS after the age of 2 years in the late‐onset obese children (p > 0.05). H‐SDS values were below average in 21% of the children with late‐onset obesity and in only 4% of the children with early‐onset obesity.

Discussion: These findings indicate that late development of obesity is not associated with increased stature in prepubertal children; however, it may be preceded by growth acceleration in the early years of life. Growth acceleration in early life may be a predictor for future obesity.
Psychological Status and Weight‐Related Distress in Overweight or At‐Risk‐for‐Overweight Children
Authors: Young‐Hyman Deborah , Tanofsky‐Kraff Marian , Yanovski Susan Z. , Keil Margaret , Cohen Marc L. , Peyrot Mark , Yanovski Jack A.
Published: April 2008

Objective: To associate psychological status, weight‐related distress, and weight status during childhood in overweight or at‐risk‐for‐overweight children.

Research Methods and Procedures: We associated self‐report of depression, trait anxiety, and weight‐related distress (body size dissatisfaction and weight‐related peer teasing after controlling for the effects of weight) in 164 children (black 35%; age 11.9 ± 2.5 years; girls 51%) who were overweight or at‐high‐risk‐for‐overweight and were not seeking weight loss.

Results: Overall, heavier children reported more psychological and weight‐related distress. Black children reported more anxiety and body size dissatisfaction than white children, despite equivalent weights. However, psychological distress was not significantly associated with weight in white children. Girls reported more weight‐related distress than boys. Depression was associated with weight‐related teasing in all predictive models, except in the model using only black subjects. Trait anxiety was associated with report of peer teasing when using all subjects. Depression was also significantly associated with children's report of body size dissatisfaction in models using all subjects, only girls, or white subjects, but not in analyses using only boys or black subjects. For boys peer teasing was associated with body size dissatisfaction. In models including only black children, depression and trait anxiety were not significantly associated with either report of peer teasing or body size dissatisfaction.

Discussion: Regardless of race or sex, increasing weight is associated with emotional and weight‐related distress in children. However, associations of psychological status, weight, and weight‐related distress differ for girls and boys, and for black and white children.
New Insights into the Pathogenesis of Insulin Resistance in Humans Using Magnetic Resonance Spectroscopy
Authors: Petersen Kitt Falk , Shulman Gerald I.
Published: April 2008

Insulin resistance plays a major role in the pathogenesis of type 2 diabetes, yet despite much effort, the underlying factors that are responsible for it are poorly understood. In this review, we focus on some recent advances in our understanding of the pathogenesis of insulin resistance in humans that have been made using magnetic resonance spectroscopy.
Relationship of Metabolic Risk Factors and Development of Cardiovascular Disease and Diabetes
Authors: Haffner Steven M.
Published: April 2008

The prevalence of obesity and diabetes has reached pandemic proportions. Obesity, particularly in association with high waist circumference and high BMI, is an independent risk factor for coronary heart disease (CHD) and diabetes. Several large studies have shown that marginal (5 lb) to moderate (11 to 22 lb) weight gain in adulthood (age 20 to 50 years) increases the risk of chronic disease and negatively affects CHD risk status. The metabolic syndrome, a clustering of cardiovascular and metabolic risk factors that includes abdominal obesity, is increasing among adults and children and is strongly associated with the development of diabetes and CHD. Recent evidence suggests that elevated liver enzymes, an indicator of non‐alcoholic fatty liver disease, may comprise an additional component of the metabolic syndrome and may serve as a surrogate marker for type 2 diabetes, particularly if used in conjunction with C‐reactive protein.
Fatty Acid Metabolism, the Central Nervous System, and Feeding
Authors: Ronnett Gabriele V. , Kleman Amy M. , Kim Eun‐Kyoung , Landree Leslie E. , Tu Yajun
Published: April 2008

A potential role for fatty acid metabolism in the regulation of energy balance in the brain or in the periphery has been considered only recently. Fatty acid synthase (FAS) catalyzes the synthesis of long‐chain fatty acids, whereas the breakdown of fatty acids by β‐oxidation is regulated by carnitine palmitoyltransferase‐1, the rate‐limiting enzyme for the entry of fatty acids into the mitochondria for oxidation. While the question of the physiological role of fatty acid metabolism remains to be resolved, studies indicate that inhibition of FAS or stimulation of carnitine palmitoyltransferase‐1 using cerulenin or synthetic FAS inhibitors reduces food intake and incurs profound and reversible weight loss. Several hypotheses regarding the mechanisms by which these small molecules mediate their effects have been entertained. Centrally, these compounds alter the expression of hypothalamic neuropeptides, generally reducing the expression of orexigenic peptides. Whether through central, peripheral, or combined central and peripheral mechanisms, these compounds also increase energy consumption to augment weight loss. In vitro and in vivo studies indicate that at least part of C75's effects is mediated by modulation of adenosine monophosphate‐activated protein kinase, a member of an energy‐sensing kinase family. These compounds, with chronic treatment, also alter gene expression peripherally to favor a state of enhanced energy consumption. Together, these effects raise the possibility that pharmacological alterations in fatty acid synthesis/degradation may serve as a target for obesity therapeutics.
Leptin Receptor Signaling and Action in the Central Nervous System
Authors: Leshan Rebecca L. , Björnholm Marie , Münzberg Heike , Myers Martin G.
Published: April 2008

The increasing incidence of obesity in developed nations represents an ever‐growing challenge to health care by promoting diabetes and other diseases. The discovery of the hormone, leptin, a decade ago has facilitated the acquisition of new knowledge regarding the regulation of energy balance. A great deal remains to be discovered regarding the molecular and anatomic actions of leptin, however. Here, we discuss the mechanisms by which leptin activates intracellular signals, the roles that these signals play in leptin action in vivo, and sites of leptin action in vivo. Using “reporter” mice, in which LRb‐expressing (long form of the leptin receptor) neurons express the histological marker, β‐galactosidase, coupled with the detection of LRb‐mediated signal transducer and activator of transcription 3 signaling events, we identified LRb expression in neuronal populations both within and outside the hypothalamus. Understanding the regulation and physiological function of these myriad sites of central leptin action will be a crucial next step in the quest to understand mechanisms of leptin action and energy balance.
Distributed Neural Control of Energy Balance: Contributions from Hindbrain and Hypothalamus
Authors: Grill Harvey J.
Published: April 2008

Data are reviewed that support the hypothesis that the neural control of energy expenditure is distributed among several brain sites. This view contrasts with that expressed most commonly in literature, that a single site—the arcuate hypothalamic nucleus—receives and integrates signals of relevance to energy status assessment and engages the effector circuits that orchestrate responses that maintain energy balance. The data reviewed support a contribution from medullary neurons, including those of the nucleus of the solitary tract, in the integration of signals of relevance to energy balance and in the issuing of commands to local behavioral and autonomic effectors. Experimental evidence is discussed that supports the following specific conclusions: hindbrain neurons integrate oral and gastrointestinal signals and issue commands to local motor circuits that control meal size; leptin's effect on food intake may be mediated, in part, by a direct action on the hindbrain neurons that respond to gastric distention; deprivation signals, such as the fall in leptin level, affect gene expression outside of the hypothalamus with reductions in proglucagon and proopiomelanocortin message seen in nucleus of the solitary tract‐rich tissue; and that hindbrain neurons contribute to the control of energy expenditure seen with food deprivation and increases in expenditure after cold exposure or starvation. Future work is needed to define how the nucleus of the solitary tract and arcuate nodes of the central energy balance control network interact to collectively, or separately, influence specific aspects of energy balance control in the intact brain.
Rapid Glucocorticoid Actions in the Hypothalamus as a Mechanism of Homeostatic Integration
Authors: Tasker Jeffrey G.
Published: April 2008

The hypothalamic paraventricular nucleus (PVN) is a major integrative site for the control of homeostasis, including energy balance, through coordinated regulation of neuroendocrine and autonomic outputs. However, cross‐talk regulation of PVN neuroendocrine and preautonomic systems is poorly understood. The stress response invokes the coordinated control of motor, hormonal, and vegetative systems to establish homeostasis after an environmental perturbation. Elevated stress levels of circulating glucocorticoids give rise to multiple, complex physiological effects. The complexity of the glucocorticoid actions is caused by the wide range of glucocorticoid target tissues and to the broad time scale over which the actions occur. Recent studies have revealed rapid glucocorticoid actions in the hypothalamus that may provide an integrative signal linking stress with the regulation of energy and fluid homeostasis. Glucocorticoids inhibit PVN and supraoptic nucleus neurons by stimulating a rapid synthesis and retrograde release of endocannabinoids, which suppress synaptic excitation through presynaptic CB1 receptor activation. The glucocorticoid‐induced endocannabinoid synthesis is mediated apparently by a novel membrane‐associated glucocorticoid receptor found in multiple subpopulations of hypothalamic neuroendocrine cells. It may, therefore, represent a mechanism for rapid glucocorticoid control of activity among different neuroendocrine systems to coordinate a global response to stress. In support of this, leptin, a circulating adipose signal that regulates food intake and energy expenditure through central actions, blocks the glucocorticoid‐mediated endocannabinoid release in the PVN. This represents a means by which the regulation of stress and feeding may interface in the PVN, thus providing a possible mechanism for the integration of multiple homeostatic functions.
Fat and Energy Partitioning: Longitudinal Observations in Leptin‐treated Adults Homozygous for a Lep Mutation
Authors: Heymsfield Steven B. , Fong Tung M. , Gantz Ira , Erondu Ngozi
Published: April 2008

Objective: Partitioning of body energy content in the aleptinemic ob/ob mouse differs from that in wild‐type mice, but it is not known whether parallel differences exist between humans with leptin (Lep) gene mutations and healthy adults. The objective of this study was to evaluate body composition in three leptin‐treated Turkish adults homozygous for a missense mutation (C→T substitution at codon 105 resulting in Arg→Trp) of Lep.

Research Methods and Procedures: Subjects, one male and two female Turkish family members, were evaluated at baseline and treated for 18 months with r‐MetHuLeptin. Patient data (fat mass, energy content) were compared with adult sex‐specific predicted values (adjusted for weight, height, and age) derived in healthy control subjects.

Results: Weight loss with leptin treatment was substantial, ranging from 43.9% to 52.1%. At baseline and with leptin treatment, the two women had a fat mass and energy content similar (±12%) to predicted values. Baseline fat and energy content in the male patient was high compared with predicted values (e.g., fat +33%) but approached and reached normal values (e.g., fat, +2%) after 18 months of leptin therapy.

Discussion: Adult women with Lep mutations had body composition similar to normal women at baseline and with leptin treatment. In contrast, the man with a Lep mutation had high body fat in the untreated state but a normal male phenotype with leptin administration, possibly secondary to androgenic fat partitioning effects. Fat and energy partitioning can, thus, be quantitatively assessed and linked with potential hormonal mechanisms in humans with inherited disturbances in energy regulation.
Intramyocellular Lipid Content in Human Skeletal Muscle
Authors: Schrauwen‐Hinderling Vera B. , Hesselink Matthijs K.C. , Schrauwen Patrick , Kooi Marianne Eline
Published: April 2008

Fat can be stored not only in adipose tissue but also in other tissues such as skeletal muscle. Fat droplets accumulated in skeletal muscle [intramyocellular lipids (IMCLs)] can be quantified by different methods, all with advantages and drawbacks. Here, we briefly review IMCL quantification methods that use biopsy specimens (biochemical quantification, electron microscopy, and histochemistry) and non‐invasive alternatives (magnetic resonance spectroscopy, magnetic resonance imaging, and computed tomography).
Left Ventricular Mass and +276 G/G Single Nucleotide Polymorphism of the Adiponectin Gene in Uncomplicated Obesity
Authors: Iacobellis Gianluca , Petrone Antonio , Leonetti Frida , Buzzetti Raffaella
Published: April 2008

The single nucleotide polymorphism at position 276 in the adiponectin gene (APM1/ACDC +276 G>T) and left ventricular mass (LVM) have been associated with increased cardiovascular risk. We sought to evaluate whether +276 G>T variants in the adiponectin gene are correlated with LVM in uncomplicated obese subjects. APM1/ACDC +276 G>T single nucleotide polymorphism, echocardiographic indexed LVM (LVM/body surface area and LVM/height2.7), insulin sensitivity by euglycemic clamp, and plasma adiponectin levels were analyzed in 62 obese subjects without complications (51 women and 11 men; mean age, 34.2 ± 10.2 years; BMI, 38.6 ± 9.1 kg/m2). Forty lean subjects formed the control group for LVM evaluation. We found 23 (37%) uncomplicated obese subjects with the APM1/ACDC +276 G/G genotype, 25 (40%) with the G/T genotype, and 14 (23%) with the T/T genotype. G/G uncomplicated obese subjects showed significant higher LVM/body surface area and LVM/height2.7 (within the normal range in the majority of them) than uncomplicated obese subjects carrying the G/T and T/T genotypes (p < 0.01 and p < 0.05, respectively). This study showed that LVM is significantly higher in uncomplicated obese subjects carrying the G/G genotype at position 276 of the human adiponectin gene.
In Vivo Phenotyping of the ob/ob Mouse by Magnetic Resonance Imaging and 1H‐Magnetic Resonance Spectroscopy
Authors: Calderan Laura , Marzola Pasquina , Nicolato Elena , Fabene Paolo F. , Milanese Chiara , Bernardi Paolo , Giordano Antonio , Cinti Saverio , Sbarbati Andrea
Published: April 2008

Objective: We studied ob/ob and wild‐type (WT) mice to characterize the adipose tissues depots and other visceral organs and to establish an experimental paradigm for in vivo phenotyping.

Research Methods and Procedures: An in vivo evaluation was conducted using magnetic resonance imaging and 1H‐magnetic resonance spectroscopy (1H‐MRS). We used T1‐weighted images and three‐dimensional spin echo T1‐weighted images for the morphological analysis and 1H‐MRS spectra on all body mass, as well as 1H‐MRS spectra focalized on specific lipid depots [triglyceride (TG) depots] for a molecular analysis.

Results: In ob/ob mice, three‐dimensional evaluation of the trunk revealed that ∼64% of the volume consists of white adipose tissue, which is 72% subcutaneous and 28% visceral. In vivo 1H‐MRS showed that 20.00 ± 6.92% in the WT group and 58.67 ± 6.65% in the ob/ob group of the total proton content is composed of TG protons. In in vivo‐localized spectra of ob/ob mice, we found a polyunsaturation degree of 0.5247 in subcutaneous depots. In the liver, we observed that 48.7% of the proton signal is due to water, whereas in the WT group, the water signal amounted to 82.8% of the total proton signal. With the sequences used, the TG amount was not detectable in the brain or kidneys.

Discussion: The present study shows that several parameters can be obtained by in vivo examination of ob/ob mice by magnetic resonance imaging and 1H‐MRS and that the accumulated white adipose tissue displays low polyunsaturation degree and low hydrolipidic ratio. Relevant anatomical alterations observed in urinary and digestive apparatuses should be considered when ob/ob mice are used in experimental paradigms.
Assessing Weight‐Related Quality of Life in Adolescents
Authors: Kolotkin Ronette L. , Zeller Meg , Modi Avani C. , Samsa Gregory P. , Quinlan Nicole Polanichka , Yanovski Jack A. , Bell Stephen K. , Maahs David M. , de Serna Daniela Gonzales , Roehrig Helmut R.
Published: April 2008

Objective: The development of a new weight‐related measure to assess quality of life in adolescents [Impact of Weight on Quality of Life (IWQOL)‐Kids] is described.

Research Methods and Procedures: Using a literature search, clinical experience, and consultation with pediatric clinicians, 73 items were developed, pilot tested, and administered to 642 participants, 11 to 19 years old, recruited from weight loss programs/studies and community samples (mean z‐BMI, 1.5; range, −1.2 to 3.4; mean age, 14.0; 60% female; 56% white). Participants completed the 73 items and the Pediatric Quality of Life Inventory and were weighed and measured.

Results: Four factors (27 items) were identified (physical comfort, body esteem, social life, and family relations), accounting for 71% of the variance. The IWQOL‐Kids demonstrated excellent psychometric properties. Internal consistency coefficients ranged from 0.88 to 0.95 for scales and equaled 0.96 for total score. Convergent validity was demonstrated with strong correlations between IWQOL‐Kids total score and the Pediatric Quality of Life Inventory (r = 0.76, p < 0.0001). Significant differences were found across BMI groups and between clinical and community samples, supporting the sensitivity of this measure. Participants in a weight loss camp demonstrated improved IWQOL‐Kids scores, suggesting responsiveness of the IWQOL‐Kids to weight loss/social support intervention.

Discussion: The present study provides preliminary evidence regarding the psychometric properties of the IWQOL‐Kids, a weight‐related quality of life measure for adolescents. Given the rise of obesity in youth, the development of a reliable and valid weight‐related measure of quality of life is timely.
Uncoupling Protein 2 Promoter Polymorphism −866G/A, Central Adiposity, and Metabolic Syndrome in Asians
Authors: Shen Haiqing , Qi Lu , Tai E. Shyong , Chew Suok Kai , Tan Chee Eng , Ordovas Jose M.
Published: April 2008

A polymorphism in the promoter region of uncoupling protein 2 gene −866G/A has been associated with its expression levels in adipose tissue, the risk of obesity, and metabolic abnormalities. Our purpose was to examine the associations of −866G/A with body fat and the risk of metabolic syndrome in a random sample of 4018 Asians (1858 men and 2160 women) from three ethnic groups (Chinese, Malay, and Indian). The minor allele frequency of −866G/A polymorphism in South Asians was similar to that in whites. After adjustment for covariates including age, cigarette smoking, and physical activity, the −866A/A genotype was associated with higher waist‐to‐hip ratio as compared with the wild‐type genotype in Chinese and Indian men (p = 0.018 and p = 0.046, respectively). Moreover, Indian men with −866A/A genotype had a significantly increased risk of metabolic syndrome as compared with those homozygous for the wild‐type (odds ratio, 2.66; 95% confidence interval, 1.21 to 5.88; p = 0.015). Such a risk was mainly caused by the excess presence of hypertriglyceridemia and central obesity. Our findings indicate that the uncoupling protein 2 gene −866G/A polymorphism may increase the risks of central obesity and metabolic syndrome, with greater effects on Asian men.
Acute Insulin Secretion as a Predictor of Weight Gain in Healthy Humans
Authors: Silver Robert J. , Mehta Sheena , Soeldner J. Stuart , Martin Blaise C. , Warram James H. , Goldfine Allison B.
Published: April 2008

Objective: We sought to determine the role of the acute insulin secretory response to glucose (AIRg) in predicting weight gain in normoglycemic persons with no family history of diabetes, who are at low risk for development of disease.

Research Methods and Procedures: One hundred five individuals (64 men and 41 women) who underwent measures of weight and AIRg and insulin sensitivity index (SI) by intravenous glucose tolerance test between 1963 and 1983 were surveyed again for weight between 1994 and 1999, with a mean follow‐up of 26 ± 4 years.

Results: Mean change in weight was 8 ± 10 kg. Annualized weight change was calculated as change in kilograms divided by change in year and averaged 0.27 ± 0.04 kg/yr. Dividing the cohort by either median AIRg or median SI demonstrated no association of either AIRg or SI with total or annualized weight gain. Subgroup analysis by ideal body weight or gender did not alter the association. Furthermore, no association between AIRg and weight gain rate was seen within insulin‐sensitive or ‐resistant subgroups, although younger age at entry was associated with greater rates of weight gain.

Discussion: Our data suggest that neither AIRg nor SI plays a role in predicting weight gain in normoglycemic individuals with no family history of diabetes.
Sex Difference in the Effect of Puberty on the Relationship between Fat Mass and Bone Mass in 926 Healthy Subjects, 6 to 18 Years Old
Authors: Ackerman Alexandra , Thornton John C. , Wang Jack , Pierson Richard N. , Horlick Mary
Published: April 2008

Objective: Understanding factors influencing bone mineral accrual is critical to optimize peak bone mass during childhood. The epidemic of pediatric obesity and reported higher incident of fracture risk in obese children led us to study the influence of fat mass on bone mineral content (BMC) in children.

Research Methods and Procedures: Height; weight; pubertal stage; and BMC, non‐bone fat‐free mass (nbFFM), and fat mass (FM) by DXA were obtained in a multiethnic group of healthy children (444 girls/482 boys; 6 to 18 years old) recruited in the New York metropolitan area. Regression techniques were used to explore the relationship between BMC and FM, with age, height, nbFFM, pubertal stage, sex, and ethnicity as covariates.

Results: Because there were significant sex interactions, separate regression analyses were performed for girls and boys. Although ln(nbFFM) was the greatest predictor of ln(BMC), ln(FM) was also a significant predictor in prepubertal boys and all girls but not in pubertal boys. This effect was independent of ethnicity.

Discussion: FM was a determinant of BMC in all girls but in only prepubertal boys. Our study confirms nbFFM as the greatest predictor of BMC but is the first to find a sex difference in the effect of puberty on the relationship of FM to BMC. Our results suggest that, in two individuals of the same sex and weight, the one with greater fat mass will have lower BMC, especially pubertal boys. The implications of these findings for achievement of optimal peak bone mass in a pediatric population with an unprecedented incidence of overweight and “overfat” status remain to be seen.
Calculation of magnetic field noise from high-permeability magnetic shields and conducting objects with simple geometry
Authors: Lee S.-K. , Romalis M. V.
Published: April 2008

High-permeability magnetic shields generate magnetic field noise that can limit the sensitivity of modern precision measurements. We show that calculations based on the fluctuation-dissipation theorem allow quantitative evaluation of magnetic field noise, either from current or magnetization fluctuations, inside enclosures made of high-permeability materials. Explicit analytical formulas for the noise are derived for a few axially symmetric geometries, which are compared with results of numerical finite element analysis. Comparison is made between noises caused by current and magnetization fluctuations inside a high-permeability shield and also between current-fluctuation-induced noises inside magnetic and nonmagnetic conducting shells. A simple model is suggested to predict power-law decay of noise spectra beyond a quasi-static regime. Our results can be used to assess noise from existing shields and to guide design of new shields for precision measurements.
Localization and coherent dynamics of excitons in the two-dimensional optical spectrum of molecular J-aggregates
Authors: Dijkstra Arend G. , la Cour Jansen Thomas , Knoester Jasper
Published: April 2008

Two-dimensional optical spectra of J-aggregates at low temperature provide a large amount of information about the nature and dynamics of exciton states that is hidden in conventional broad band pump-probe spectra. By using numerical simulations, we study the two-dimensional absorption spectrum and find that it is dominated by a V-shaped negative peak and a blueshifted elliptic positive peak. We demonstrate a simple method to derive the energy dependence of the exciton localization size from the distance between these two features in the zero waiting time experiment. When the waiting time is turned on, the V peak is filled with an extra positive peak resulting from population relaxation. From the time evolution of this peak, energy dependent relaxation rates can be obtained. The oscillations of coherent contributions to the two-dimensional spectrum are not damped by inhomogeneous mechanisms and can be seen clearly.
Monge–Ampere grids and the multidimensional mapped Fourier method
Authors: Degani Ilan
Published: April 2008

The efficiency of a numerical method can be greatly improved by combining it with coordinate transformations tailored to a given problem. This is the basis for the mapped Fourier methods. However, obtaining “good” coordinate transformations is a major obstacle for this approach in multidimensions. Here, we calculate coordinate transformations based on solving the Monge–Ampere equation. These transformations are combined in the mapped Fourier method and applied to Schrödinger’s equation in multidimensions. Dramatic improvements in accuracy compared to the standard Fourier method were observed in eigenvalue calculations for two-dimensional systems. This work indicates that the Monge–Ampere equation may serve as a useful tool for constructing efficient representations for problems in computational quantum mechanics and other fields.
Energy band gap, intrinsic carrier concentration, and Fermi level of CdTe bulk crystal between 304 and 1067K
Authors: Su Ching-Hua
Published: April 2008

Optical transmission measurements were performed on CdTe bulk single crystals. It was found that when sliced and polished CdTe wafers were used, a white film started to develop on the sample surface and the wafer became opaque when it was heated above 530K. Therefore, a bulk crystal of CdTe was first grown in the window area by physical vapor transport. The optical transmission was then measured between 304 and 1067K and from which the energy band gap was derived. The band gaps of CdTe can fit well as a function of temperature by the Varshni expression. Using the band gap data, the high temperature electron-hole equilibrium was numerically calculated by assuming Kane’s conduction band structure and a light-hole and a heavy-hole parabolic valence bands. The calculated intrinsic carrier concentrations agree well with the experimental data previously reported. The calculated intrinsic Fermi levels between 200 and 1200K were also presented.
Electric-field-gradient tensor and charge densities in LaB6: B11 nuclear-magnetic-resonance single-crystal investigations and first-principles calculations
Authors: Herzig P. , Fojud Z. , Żogał O. J. , Pietraszko A. , Dukhnenko A. , Jurga S. , Shitsevalova N.
Published: April 2008

B 11 nuclear-magnetic-resonance measurements on a B11 enriched single crystal of LaB6 have been performed at room temperature at a frequency of 128.4MHz. The electric-field-gradient (EFG) tensor components have been determined from the angle dependence of the quadrupole splittings associated with (−3∕2↔−1∕2) and (3∕2↔1∕2) satellite transitions. These EFG components have been calculated using first-principles methods. Very satisfactory agreement with the experimental values has been obtained. The calculations show that the largest component of the EFG tensor mainly comes from the pp part of the so-called sphere component. The principal axis for the most negative EFG component is oriented in direction of the shortest and strongest bond between neighboring B atoms, which belong to two different B6 units (interoctahedral B–B bonds). The fact that these interoctahedral B–B bonds are aligned in three different directions is responsible for the observation of up to three satellite pairs, although all boron atoms are crystallographically equivalent and there exists only one EFG tensor. Apart from the EFG investigations, chemical bonding in LaB6 is analyzed from densities of states and electron-density plots.
Inhomogeneous decomposition of ultrathin oxide films on Si(100): Application of Avrami kinetics to thermal desorption spectra
Authors: Kinefuchi Ikuya , Yamaguchi Hiroki , Sakiyama Yukinori , Takagi Shu , Matsumoto Yoichiro
Published: April 2008

Thermal decomposition of ultrathin oxide layers on silicon surface was investigated with temperature programed desorption. Oxide layers were formed on Si(100) at 400°C by exposure to O2 molecular beam. Desorption spectrum for oxygen coverages between 1.7 and 2.6 ML exhibits a single dominant peak with an additional broad peak at lower temperatures. The former peak corresponds to stable binding states of O atoms at dimer bridge sites and dimer backbond sites. The high peak intensity indicates that most O atoms are at stable states. The latter peak corresponds to an unstable binding state, where O atoms are presumably trapped at dangling bonds. The SiO desorption rate from the stable binding states is well described by Avrami kinetics, suggesting that the decomposition process is spatially inhomogeneous with void formation and growth. The rate-determining step is the reaction at void perimeter even if the overlap between voids becomes quite large. The Avrami exponents determined from our experiment indicate that the increase in the initial coverage makes the oxide layer more stable and suppresses the rate of void formation at the potential nucleation sites.
Phase behavior and structure formation in linear multiblock copolymer solutions by Monte Carlo simulation
Authors: Gindy Marian E. , Prud’homme Robert K. , Panagiotopoulos Athanassios Z.
Published: April 2008

The solution phase behavior of short, strictly alternating multiblock copolymers of type (AnBn)m was studied using lattice Monte Carlo simulations. The polymer molecules were modeled as flexible chains in a monomeric solvent selective for block type A. The degree of block polymerization n and the number of diblock units per chain m were treated as variables. We show that within the regime of parameters accessible to our study, the thermodynamic phase transition type is dependent on the ratio of m∕n. The simulations show microscopic phase separation into roughly spherical aggregates for m∕n ratios less than a critical value and first-order macroscopic precipitation otherwise. In general, increasing m at fixed n, or n at fixed m, promotes the tendency toward macroscopic phase precipitation. The enthalpic driving force of phase change is found to universally scale with chain length for all multiblock systems considered and is independent of the existence of a true phase transition. For aggregate forming systems at low amphiphile concentrations, multiblock chains are shown to self-assemble into intramolecular, multichain clusters. Predictions for microstructural dimensions, including critical micelle concentration, equilibrium size, shape, aggregation parameters, and density distributions, are provided. At increasing amphiphile density, interaggregate bridging is shown to result in the formation of networked structures, leading to an eventual solution-gel transition. The gel is swollen and consists of highly interconnected aggregates of approximately spherical morphology. Qualitative agreement is found between experimentally observed physical property changes and phase transitions predicted by simulations. Thus, a potential application of the simulations is the design of multiblock copolymer systems which can be optimized with regard to solution phase behavior and ultimately physical and mechanical properties.
Glasslike behavior in aqueous electrolyte solutions
Authors: Turton David A. , Hunger Johannes , Hefter Glenn , Buchner Richard , Wynne Klaas
Published: April 2008

When salts are added to water, generally the viscosity increases, suggesting that the ions increase the strength of the water’s hydrogen-bond network. However, infrared pump-probe measurements on electrolyte solutions have found that ions have no influence on the rotational dynamics of water molecules, implying no enhancement or breakdown of the hydrogen-bond network. Here, we report optical Kerr effect and dielectric relaxation spectroscopic measurements, which have enabled us to separate the effects of rotational and transitional motions of the water molecules. These data show that electrolyte solutions behave like a supercooled liquid approaching a glass transition in which rotational and translational molecular motions are decoupled. It is now possible to understand previously conflicting viscosity data, nuclear magnetic resonance relaxation, and ultrafast infrared spectroscopy in a single unified picture.
Surface-hopping dynamics and decoherence with quantum equilibrium structure
Authors: Grunwald Robbie , Kim Hyojoon , Kapral Raymond
Published: April 2008

In open quantum systems, decoherence occurs through interaction of a quantum subsystem with its environment. The computation of expectation values requires a knowledge of the quantum dynamics of operators and sampling from initial states of the density matrix describing the subsystem and bath. We consider situations where the quantum evolution can be approximated by quantum-classical Liouville dynamics and examine the circumstances under which the evolution can be reduced to surface-hopping dynamics, where the evolution consists of trajectory segments exclusively evolving on single adiabatic surfaces, with probabilistic hops between these surfaces. The justification for the reduction depends on the validity of a Markovian approximation on a bath averaged memory kernel that accounts for quantum coherence in the system. We show that such a reduction is often possible when initial sampling is from either the quantum or classical bath initial distributions. If the average is taken only over the quantum dispersion that broadens the classical distribution, then such a reduction is not always possible.
Adiabatic model and design of a translating field reversed configuration
Authors: Intrator T. P. , Siemon R. E. , Sieck P. E.
Published: April 2008

We apply an adiabatic evolution model to predict the behavior of a field reversed configuration (FRC) during decompression and translation, as well as during boundary compression. Semi-empirical scaling laws, which were developed and benchmarked primarily for collisionless FRCs, are expected to remain valid even for the collisional regime of FRX-L experiment. We use this approach to outline the design implications for FRX-L, the high density translated FRC experiment at Los Alamos National Laboratory. A conical theta coil is used to accelerate the FRC to the largest practical velocity so it can enter a mirror bounded compression region, where it must be a suitable target for a magnetized target fusion (MTF) implosion. FRX-L provides the physics basis for the integrated MTF plasma compression experiment at the Shiva-Star pulsed power facility at Kirtland Air Force Research Laboratory, where the FRC will be compressed inside a flux conserving cylindrical shell.
Atomic complexity measures in position and momentum spaces
Authors: Angulo J. C. , Antolín J.
Published: April 2008

Fisher–Shannon (FS) and López-Ruiz, Mancini, and Calbet (LMC) complexity measures, detecting not only randomness but also structure, are computed by using near Hartree–Fock wave functions for neutral atoms with nuclear charge Z=1–103 in position, momentum, and product spaces. It is shown that FS and LMC complexities are qualitatively and numerically equivalent for these systems. New complexity candidates are defined, computed, and compared by using the following information-theoretic magnitudes: Shannon entropy, Fisher information, disequilibrium, and variance. Localization-delocalization planes are constructed for each complexity measure, where the subshell pattern of the periodic table is clearly shown. The complementary use of r and p spaces provides a compact and more complete understanding of the information content of these planes.
Optical and thermal characteristics of narrow-ridge quantum-cascade lasers
Authors: Wienold M. , Semtsiv M. P. , Bayrakli I. , Masselink W. T. , Ziegler M. , Kennedy K. , Hogg R.
Published: April 2008

Quantum-cascade lasers operating at λ≈3.9μm at room temperature with narrow w≈5μm ridge widths are described. The lateral confinement due to the narrow ridge is similar to the vertical confinement and the resulting beam is circular in cross section with a single TM00 spatial mode. The beam divergence is 46° both parallel and perpendicular to the surface. The beam quality factor along the slow axis is about M2=1.6. The narrow ridges also increase the relative lateral heat dissipation from the active region, resulting in a thermal conductance per unit area of about Gth=380WK−1cm−2 for a 3mm long laser. Maximum average power is obtained with duty cycles between 10% and 30%; in spite of the very narrow ridge, the total average power with thermoelectric cooling exceeds 60mW with a peak power of 460mW. The circularly symmetric beam with very good beam quality suggests essentially zero astigmatism and indicates that these narrow-ridge quantum-cascade lasers are well suited for applications in midinfrared spectroscopy and imaging.
The role of V2O5 on the dehydrogenation and hydrogenation in magnesium hydride: An ab initio study
Authors: Du A. J. , Smith Sean C. , Yao X. D. , Sun C. H. , Li L. , Lu G. Q.
Published: April 2008

Ab initio density functional theory calculations are performed to study the experimentally observed catalytic role of V2O5 in the recycling of hydrogen in magnesium hydride. We find that the Mg–H bond length becomes elongated when MgH2 clusters are positioned on single, two, and three coordinated oxygen sites (O1, O2, and O3) on the V2O5(001) surface. Molecular hydrogen is predicted to spontaneously form at the hole site on the V2O5(001) surface. Additionally, the activation barrier for the dissociation of hydrogen on V-doped Mg(0001) surface is 0.20eV, which is only 15 of that on pure Mg(0001) surface. Our results indicate that oxygen sites on the V2O5(001) surface and the V dopant in Mg may be important facilitators for dehydrogenation and rehydrogenation, respectively. The understanding gained here will aid in the rational design and development of Mg-based hydrogen storage materials.
The Reaction of 5-Substituted-2-Norbornenes with Phenylselenyl Chloride
Authors: Happer D. Alan R. , Francis Thomas , White Jonathan M.
Published: April 2008

In dichloromethane, 5-substituted-2-norbornenes add phenylselenyl chloride across the double bond to give adducts in which the phenylselanyl substituent is exo and the chloro endo. The relative yields of the two regioisomeric adducts are reported for several of these. For most, the formation of the 2-chloro-3-phenyselanyl adduct is favoured. The main exceptions are when the 5-substituent is exo and a good resonance donor, where the 3-chloro-2-phenylselanyl adduct is the major product. Possible factors influencing the product distribution are discussed.
Two separate UV-B radiation wavelength regions control expression of different molecular markers in Arabidopsis thaliana
Authors: Kalbina Irina , Li Shaoshan , Kalbin Georgi , Björn Lars Olof , Strid Åke
Published: April 2008

Fluence-response curves were obtained at nine wavelengths in the interval 280–360 nm for mRNA transcripts of four molecular markers induced by ultraviolet-B (UV-B) radiation in Arabidopsis thaliana (L.) Heynh.: CHS (encoding chalcone synthase), PDX1.3 (encoding an enzyme involved in formation of pyridoxine), MEB5.2 (encoding a protein with unknown function but which is strongly upregulated by UV-B), and LHCB1*3 (encoding a chlorophyll a/b binding protein). Intact Arabidopsis plants were irradiated for 3 h using a high intensity deuterium radiation source and narrow bandwith filters without supplementary PAR. The results obtained suggest the existence of two distinct UV-B signal responses: one sensitive between 300 and 310 nm and the other sensitive around 280–290 nm. Among the investigated molecular markers, CHS and PDX1.3 were regulated through the chromophore absorbing around 300 nm, whereas MEB5.2 and LHCB1*3 were regulated through the chromophore absorbing at 280–290 nm. The results obtained show that at least two signal transduction pathways exist that regulate gene expression as a result of absorption of UV-B radiation in plants.
Modeling transient collapsed states of an unfolded protein to provide insights into early folding events
Authors: Felitsky Daniel J. , Lietzow Michael A. , Dyson H. Jane , Wright Peter E.
Published: April 2008

The primary driving force for protein folding is the sequestration of hydrophobic side chains from solvent water, but the means whereby the amino acid sequence directs the folding process to form the correct final folded state is not well understood. Measurements of NMR line broadening in spin-labeled samples of unfolded apomyoglobin at pH 2.3 have been used to derive a quantitative model for transient hydrophobic interactions between various sites in the polypeptide chain, as would occur during the initiation of protein folding. Local clusters of residues with high values for the parameter “average area buried upon folding” (AABUF) form foci not only for local contacts but for long-range interactions, the relative frequencies of which can be understood in terms of differences in the extent of reduction in chain configurational entropy that occurs upon formation of nonlocal contacts. These results complement the striking correlation previously observed between the kinetic folding process of apomyoglobin and the AABUF of its amino acid sequence [Nishimura C, Lietzow MA, Dyson HJ, Wright PE (2005)
J Mol Biol
351:383–392]. For the acid-unfolded states of apomyoglobin, our approach identifies multiple distinct hydrophobic clusters of differing thermodynamic stability. The most structured of these clusters, although sparsely populated, have both native-like and nonnative character; the specificity of the transient long-range contacts observed in these states suggests that they play a key role in initiating chain collapse and folding.
HF(
v′
= 3) forward scattering in the F + H
2
reaction: Shape resonance and slow-down mechanism
Authors: Wang Xingan , Dong Wenrui , Qiu Minghui , Ren Zefeng , Che Li , Dai Dongxu , Wang Xiuyan , Yang Xueming , Sun Zhigang , Fu Bina , Lee Soo-Y. , Xu Xin , Zhang Dong H.
Published: April 2008

Crossed molecular beam experiments and accurate quantum dynamics calculations have been carried out to address the long standing and intriguing issue of the forward scattering observed in the F + H
2
→ HF(
v′
= 3) + H reaction. Our study reveals that forward scattering in the reaction channel is not caused by Feshbach or dynamical resonances as in the F + H
2
→ HF(
v′
= 2) + H reaction. It is caused predominantly by the slow-down mechanism over the centrifugal barrier in the exit channel, with some small contribution from the shape resonance mechanism in a very small collision energy regime slightly above the HF(
v′
= 3) threshold. Our analysis also shows that forward scattering caused by dynamical resonances can very likely be accompanied by forward scattering in a different product vibrational state caused by a slow-down mechanism.