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At least 775 records · Page 43Linked to original sources
PubMed2004

Singularity of proton transport in salts of orthoperiodic and orthotellurium acids: theoretical modeling using density functional calculations.

Density functional B3LYP calculations have been performed to investigate proton transport in orthoperiodic and orthotellurium acids, their salts MIO(6)H(4)(M = Li, Rb, Cs) and CsH(5)TeO(6), dimers of the salt*acid type MIO(6)H(4)*H(5)IO(6)(M = Rb, Cs), CsIO(6)H(4)*H(6)TeO(6), CsHSO(4)*H(6)TeO(6), Cs(2)SO(4)*H(6)TeO(6), and also in double-substituted and binary salts Rb(2)H(3)IO(6) and Rb(4)H(2)I(2)O(10). It has been shown that the energy of salt dimerization is 33-35 kcal mol(-1) and the activation barrier for proton migration between the neighboring octahedrons of the salt*acid --> acid*salt type is calculated to be 3-13 kcal mol(-1). The activation energy of the proton migration along the octahedron, 20-30 kcal mol(-1), is comparable with the barrier for water molecule separation. Quantum-chemical calculations correlate with the results of X-ray and electrochemical studies.

Journal Article↗
PubMed2004

Theoretical model of the three-dimensional structure of a sugar-binding protein from Pyrococcus horikoshii: structural analysis and sugar-binding simulations.

The three-dimensional structure of a sugar-binding protein from the thermophilic archaea Pyrococcus horikoshii has been predicted by a homology modelling procedure and investigated for its stability and its ability to bind different sugars. The model was created by using as templates the three-dimensional structures of a maltodextrin-binding protein from Pyrococcus furiosus, a trehalose-maltose-binding protein from Thermococcus litoralis and a maltodextrin-binding protein from Escherichia coli. According to the suggestions from the CASP (Critical Assessment of Structure Prediction) meetings, the homology modelling strategy was applied by assessing an accurate multiple sequence alignment, based on the high structural conservation in the family of ATP-binding cassette transporters to which all these proteins belong. The model has been deposited in the Protein Data Bank with the code 1R25. According to the origin of the protein, several characteristics in the organization of the secondary-structure elements and in the distribution of polar and non-polar amino acids are very similar to those of thermophilic proteins, compared with proteins from mesophilic organisms, and are analysed in detail. Finally, a simulation of the binding of several sugars in the binding site of this protein is presented, and interactions with amino acids are highlighted in detail.

Amino Acid Sequence↗
PubMedDate not supplied

Blood usage in transfusion-dependent patients. A theoretical model.

A model for blood requirements of transfusion-dependent patients is presented to relate the transfusion interval in days (I), the percentage fall in hemoglobin (F), and the time in days after a transfusion during which no red cell loss occurs (S). The model applies to both normal blood (S = 0) and to young red cell (YRBC) transfusions, giving I = (S + 120)/(200/F - 1) The model predicts prolongation of transfusion interval by use of YRBCs, and is also of use in establishing whether excess red cell destruction or significant red cell production is present in a transfusion-dependent patient receiving either normal blood or YRBCs.

Anemia, Aplastic↗
PubMed2003

Cardiac MR in robotic heart surgery for preoperative identification of the target vessel and precise port placement--a theoretical model.

BACKGROUND: The identification of the ideal anastomosis site and the proper port placement are critical for the success of closed-chest robotic surgery. We investigated a new systematic procedure for precise port placement for TECABs. METHODS: We used trigonometry and a human thoracic model to determine the optimal working angles between anastomotic plane, instruments, and endoscope. We then applied the results to seven human subjects as follows: 1. A navigation grid was located extrathoracically before cardiac MR examination. 2. The ideal anastomosis site was defined with the MR. Intrathoracic distances and angles were computed with cardiac MR software and projected onto the thorax. 3. The ideal port placement points were marked on the thorax. RESULTS: The optimal working angle between endoscope and instruments was 35 degrees. 0 degrees and 90 degrees angles were associated with a significant reduction in visualization, technical ease, quality and anastomosis time. The course of the LAD was identified in all seven volunteers with MR. Mean deviation of the endoscope port from the medioclavicular line was 4.3+/-2.1 cm and of the instrument ports from the anterior axillary line 8.4+/-2.4 cm. CONCLUSIONS: Cardiac MR in combination with the navigation grid proved suitable for the visualization of coronary vessels for individually calculating port placement points on the thorax.

Adult↗
PubMed2005

Theoretical modeling of vibroelectronic quantum states in complex molecular systems: solvated carbon monoxide, a test case.

In this paper we extend the perturbed matrix method by explicitly including the nuclear degrees of freedom, in order to treat quantum vibrational states in a perturbed molecule. In a previous paper we showed how to include, in a simple way, nuclear degrees of freedom for the calculation of molecular polarizability. In the present work we extend and generalize this approach to model vibroelectronic transitions, requiring a more sophisticated treatment.

Journal Article↗
PubMed2001

Blood flow and oxygen delivery to human brain during functional activity: theoretical modeling and experimental data.

Coupling of cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO(2)) in physiologically activated brain states remains the subject of debates. Recently it was suggested that CBF is tightly coupled to oxidative metabolism in a nonlinear fashion. As part of this hypothesis, mathematical models of oxygen delivery to the brain have been described in which disproportionately large increases in CBF are necessary to sustain even small increases in CMRO(2) during activation. We have explored the coupling of CBF and oxygen delivery by using two complementary methods. First, a more complex mathematical model was tested that differs from those recently described in that no assumptions were made regarding tissue oxygen level. Second, [(15)O] water CBF positron emission tomography (PET) studies in nine healthy subjects were conducted during states of visual activation and hypoxia to examine the relationship of CBF and oxygen delivery. In contrast to previous reports, our model showed adequate tissue levels of oxygen could be maintained without the need for increased CBF or oxygen delivery. Similarly, the PET studies demonstrated that the regional increase in CBF during visual activation was not affected by hypoxia. These findings strongly indicate that the increase in CBF associated with physiological activation is regulated by factors other than local requirements in oxygen.

Adult↗
PubMed1976

Presynaptic calcium currents and their relation to synaptic transmission: voltage clamp study in squid giant synapse and theoretical model for the calcium gate.

A voltage clamp study of the presynaptic terminal in squid stellate ganglion has given quantitative results relating inward Ca2+ current to presynaptic membrane potential and postsynaptic response to inward Ca2+ current. The results indicate an S-shaped curve for the relationship between presynaptic potential and Ca2+ current and a linear relationship between Ca2+ current and postsynaptic potential. A similar S-shaped curve was found for the time-dependent properties of the Ca2+ conductance. Based on these results a mathematical model was developed which accounts for the experimental results in this and previously published papers by other authors. The model suggests that five subunits are involved in the Ca2+ gate and that the subunits change noncooperatively from an inactive to an active form upon membrane depolarization.

Animals↗
PubMed1980

Theoretical model for the cooperative equilibrium binding of myosin subfragment 1 to the actin-troponin-tropomyosin complex.

Recent experimental data on the equilibrium binding of myosin subfragment 1 (S-1) to regulated actin filaments in the presence and in the absence of Ca(2+) are analyzed by using a linear Ising model. In the model, each tropomyosin-troponin unit (including seven sites on the actin filament) can be in one of two possible states, which have different intrinsic free energies and different binding constants for S-1. Bound S-1 molecules do not interact with each other. There are nearest-neighbor (pair) interactions between these units that depend on the state of each member of the pair and on the number of Ca(2+) bound to one member of the pair. There are two sources of positive cooperativity in this system: the fact that seven actin sites change state together as part of a single unit; and the existence of attractive nearest-neighbor interactions between units. Parameters in the model are evaluated by fitting the data, both in the presence and in the absence of Ca(2+). Several extensions of this model are discussed.

Actins↗
PubMed1993

Interpretation of two compact planetary nebulae, IC 4997 and NGC 6572, with aid of theoretical models.

Observations of two dense compact planetary nebulae secured with the Hamilton Echelle spectrograph at Lick Observatory combined with previously published UV spectra secured with the International Ultraviolet Explorer enable us to probe the electron densities and temperatures (plasma diagnostics) and ionic concentrations in these objects. The diagnostic diagrams show that no homogenous model will work for these nebulae. NGC 6572 may consist of an inner torordal ring of density 25,000 atoms/cm3 and an outer conical shell of density 10,000 atoms/cm3. The simplest model of IC 4997 suggests a thick inner shell with a density of about 107 atoms/cm3 and an outer envelope of density 10,000 atoms/cm3. The abundances of all elements heavier than He appear to be less than the solar values in NGC 6572, whereas He, C, N, and O may be more abundant in IC 4997 than in the sun. IC 4997 presents puzzling problems.

Journal Article↗
PubMed2003

Theoretical modelling, experimental studies and clinical simulations of urethral cooling catheters for use during prostate thermal therapy.

Urethral cooling catheters are used to prevent thermal damage to the urethra during thermal therapy of the prostate. Quantification of a catheter's heat transfer characteristics is necessary for prediction of the catheter's influence on the temperature and thermal dose distribution in periurethral tissue. Two cooling catheters with different designs were examined: the Dornier Urowave catheter and a prototype device from BSD Medical Corp. A convection coefficient, h. was used to characterize the cooling ability of each catheter. The value of the convection coefficient (h = 330 W m(-2) C(-1) for the Dornier catheter. h = 160 W m(-2) C(-1) for the BSD device) was obtained by comparing temperatures measured in a tissue-equivalent phantom material to temperatures predicted by a finite element method simulation of the phantom experiments. The coefficient was found to be insensitive to the rate of coolant flow inside the catheter between 40 and 120 ml min(-1). The convection coefficient method for modelling urethral catheters was incorporated into simulations of microwave heating of the prostate. Results from these simulations indicate that the Dornier device is significantly more effective than the BSD catheter at cooling the tissue surrounding the urethra.

Burns↗
PubMed1992

The family physician's role in caring for the dying patient and family: a comprehensive theoretical model.

Care for the dying patient is a difficult task which arouses many emotions in all participants. Many physicians feel inadequately prepared to cope with the many diverse challenges. A three-dimensional model is described which intends to help physicians better define their role in caring for dying patients. The model comprises direct involvement with the dying patients and their families, the physician's own needs and personal development and the co-operation with other care-givers. This model can be implemented at three levels, according to the physician's experience, skills, feelings, and the situation. The model can serve as a basis for extensive research and as a foundation for curricular innovation for students, residents and continuing medical education.

Bereavement↗
PubMed1986

Intergenerational solidarity: an examination of a theoretical model.

This study provided an empirical test of a widely cited proposed model of intergenerational solidarity. The model emphasizes that association, affect, and consensus are interdependent dimensions of solidarity and offers an explanatory rationale for parent-child solidarity. Path analysis techniques were used with a sample of 279 older rural-transitional parents. The results showed little support for the model, but the data indicated that residential propinquity and mutual helping behavior were strong predictors of intergenerational association. The importance of the indirect effect of sex linkage via mutual help to associational solidarity also received strong support. The results showed that the proposed model was useful in explaining objective solidarity (association) but not subjective solidarity (consensus and affection) and that these variables were not dimensions of one construct.

Adult↗
PubMedDate not supplied

The development of cytogenetically normal, abnormal and mosaic embryos: a theoretical model.

Assisted reproduction and preimplantation genetic diagnosis (PGD) involve various complicated techniques, each of them with its own problems. However, the greatest problem with PGD for chromosome abnormalities is not of a technical nature but is a biological phenomenon: chromosomal mosaicism in the cleavage stage embryo. Here, we present a hypothetical, quantitative model for the development of chromosomally normal, abnormal and mosaic embryos. The arising of mosaicism in 2-8-cell embryos was described by a binomial probability model on the occurrence of mitotic events inducing chromosomal changes in the blastomeres. This model converted the 'mean' rate of mosaicism found in cross-sectional studies (60%) into an equal rate of mosaic embryos at arrival at the 8-cell stage (59.8%). The disappearance of > 90% of the mosaic embryos or the mosaicism itself from surviving embryos during the morula stage was explained by mitotic arrest of most of the mitotically changed cells under increasing cell cycle control. In our model, 25.9 and 14.3% of the embryos at the 8-cell stage are normal and abnormal respectively. The remaining 59.8% of the embryo shows mosaicism: 34.6% of abnormal/normal cells and 25.2% of abnormal/abnormal cells. The high proportion of abnormal and mosaic embryos together explains the high rate of abnormal laboratory findings in PGD for chromosomal abnormalities and aneuploidy screening. The poor representation of a 1- or 2-cell biopsy for the 7- or 6-cell post-biopsy embryo in the case of mosaicism explains the high rate of false-negative and false-positive results.

Chromosome Aberrations↗