PubMed HealthSearch

SEARCH · PubMed Health

Results for “quantum computing”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3Linked to original sources

Osteoporosis: the state of the art in 1987: a review.

Osteoporosis affects approximately 15 to 20 million people in the United States and is the underlying cause of 1.3 million new fractures per year in people over age 45. The more common risk factors recognized in this disorder are older age, female sex, white race, physical inactivity, and early menopause. We now have available equipment which can measure bone density at various sites. These include single- and dual-photon densitometry, and single and dual quantitative computed tomography. These procedures are a quantum improvement over plain x-ray in the assessment of the severity of osteoporosis, but measurement at one site may not reflect the density at other sites. The value of these techniques in screening the general population for osteoporosis remains to be demonstrated. They are valuable when used to monitor patients longitudinally to assess the progression of disease and the effects of specific therapeutic regimens. There is no established effective therapy for osteoporosis so prevention is the goal. The effectiveness of different programs of physical activity in preventing bone loss and fractures is unknown but isotonic exercises three times a week for thirty minutes is recommended. There is general agreement that adequate calcium intake is important for maintenance of skeletal integrity, but there is no proof that a high dietary calcium alone will prevent osteoporosis. Estrogen therapy clearly prevents the accelerated bone loss which occurs in all white women at the time of menopause, but the question still remains who should be started on estrogens, and within what period of time after menopause are estrogens still useful in preventing postmenopausal bone loss, and for how long do we continue hormone therapy. Many questions are left to be answered but at least now osteoporosis is recognized as a major medical problem and much research is being done to answer the above questions.

Aged

On the suitability of semiempirical calculations as sources of force field parameters.

The suitability of Dewar's Hamiltonians as a source of bonded force field parameters is explored from the comparison analysis between up to 270 semiempirically derived force field parameters and experimentally derived values reported in some of the most popular force fields. From the statistical analysis of the results, some general conclusions about the semiempirical parametrization are formulated.

Mathematical Computing

Dynamic control of inositol 1,4,5-trisphosphate-induced Ca2+ release: a theoretical explanation for the quantal release of Ca2+.

A theoretical model has been elaborated in order to describe the kinetics of Ca2+ release induced by inositol 1,4,5-trisphosphate (IP3). The model is based on the existence of a key molecule that controls the interconversion of open and closed forms of the Ca2+ channel. The model can quantitatively explain the previously obtained experimental observations that showed that a continuous IP3 stimulus leads to a biphasic Ca2+ release and that successive IP3 additions provoke repetitive bursts of Ca2+ release. Other published interpretations of these observations are discussed.

Calcium

Future in biomolecular computation.

Large-scale computations for biomolecules are dominated by three levels of theory: rigorous quantum mechanical calculations for molecules with up to about 30 atoms, semi-empirical quantum mechanical calculations for systems with up to several hundred atoms, and force-field molecular dynamics studies of biomacromolecules with 10,000 atoms and more including surrounding solvent molecules. It can be anticipated that increased computational power will allow the treatment of larger systems of ever growing complexity. Due to the scaling of the computational requirements with increasing number of atoms, the force-field approaches will benefit the most from increased computational power. On the other hand, progress in methodologies such as density functional theory will enable us to treat larger systems on a fully quantum mechanical level and a combination of molecular dynamics and quantum mechanics can be envisioned. One of the greatest challenges in biomolecular computation is the protein folding problem. It is unclear at this point, if an approach with current methodologies will lead to a satisfactory answer or if unconventional, new approaches will be necessary. In any event, due to the complexity of biomolecular systems, a hierarchy of approaches will have to be established and used in order to capture the wide ranges of length-scales and time-scales involved in biological processes. In terms of hardware development, speed and power of computers will increase while the price/performance ratio will become more and more favorable. Parallelism can be anticipated to become an integral architectural feature in a range of computers.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical Phenomena

Nonlinear polychromatic and noise artifacts in x-ray computed tomography images.

The variance of the image noise in computed X-ray transmission tomography (CT) due to quantum noise is in a first approximation a nonlinear function of X-ray attenuation. Beam hardening in CT is also a nonlinear function of attenuation. We present a theoretical study of both phenomena. Computer simulations and numerical results show that both nonlinear dependencies have quite similar effects on image quality. We also show how the two-dimensional distribution of the noise variance in a CT image is a weighted superposition of images obtained by backprojecting integer powers of the noiseless projection data corresponding to the scanned object. The streak-like pattern in the image noise due to the anisotropic nature of the noise cross-correlation function is discussed. We also discuss how these nonlinear phenomena affect noise filtering and tissue characterization using statistical parameters.

Electronics, Medical

Subunits in quantal transmission at the mouse neuromuscular junction: tests of peak intervals in amplitude distributions.

The regular spacing of peaks throughout the amplitude distribution of miniature end-plate potentials, quantal evoked end-plate potentials and quantal currents was demonstrated using autocorrelations and power density spectra calculated from the number of events in the successive bins of the histograms built by Matteson et al. (1979), Kriebel & Florey (1983) and Erxleben & Kriebel (1984). At the same mouse neuromuscular junction, the calculated interpeak was constant for evoked and spontaneous quantal releases, throughout sequential sampling and after change of bin size. The presence of regular peak intervals supports the hypothesis that quantal potentials are composed of potential subunits the size of the smallest subminiature potential. Challenging the hypothesis of an acetylcholine quantum composed of acetylcholine subunits, a postsynaptic origin of the subunit is proposed on the basis of the spatial arrangement in rows of the ACh receptors. The ACh-saturating patch evoked by a quantum release (Land et al., 1980, 1981) activates 10-20 rows of receptors, which is roughly the number of subunits composing a quantal event. Therefore the position of the ACh patch or the continuous variations in its size might cause stepwise variations in the total number of ACh receptors activated by an ACh quantum.

Acetylcholine

Computer-assisted structure-activity studies of chemical carcinogens. A heterogeneous data set.

A structure-activity relations study has been performed on a heterogeneous set of organic compounds to develop predictive ability for carcinogenic potential. The compounds employed came from more than 12 structural classes and numbered 130 carcinogens and 79 noncarcinogens. A set of 28 calculated molecular structure descriptors was identified that supported a linear discriminant function able to completely separate 192 compounds into the carcinogenic and noncarcinogenic classes. A predictive ability of 90% for carcinogens and 78% for noncarcinogens was obtained in randomized testing. The results demonstrate that pattern-recognition methods can be used to analyze a diverse set of compounds each represented by calculated molecular structure descriptors for a common biological activity.

Carcinogens

Quantum mechanics and cellular information processing: the self-assembly paradigm.

Biological cells have greater information processing efficiency than the programmable computers used to model them. In part this is due to the larger number of interactions that can contribute to function. General arguments suggest that systems in which quantum features play a prominent role are more powerful than classical physical-dynamical analogs. A hypothetical model, involving macromolecular self-assembly, is used to illustrate how the parallelism inherent in the quantum mechanical wave function could play a role in cellular pattern processing. Signals impinging on the external membrane of the cell trigger the release of specifically shaped macromolecules. These aggregate into a mosaic shape features that reflect different groupings of the signal input patterns. The shape features are in turn read out and connected to effector actions by adaptor molecules. The self-assembly model fits into a more general hierarchical scheme of biological information processing in which macroscopic signals are transduced to mesoscopic and then microphysical representations, processed largely at the microphysical level, and then amplified for macroscopic action. The physical dynamics are controlled by proteins and other macromolecules that are molded through the evolutionary process of variation and selection. The organizational requirements for evolutionary moldability and for efficient information processing function are completely consistent. They include high dimensionality, multiplicity of weak interactions, and hierarchical-compartmental structure.

Biological Evolution

Conformational properties of molecules by ab initio quantum mechanical energy minimization.

The recent literature on the determination of minimum energy conformations by ab initio quantum mechanical techniques is reviewed. The availability of computer-coded analytical first and second derivatives of the Hartree-Fock energy makes possible calculations that will be of significant assistance in structure determination of molecules. A short review of recent progress in empirical energy minimization and molecular dynamics is provided.

Models, Chemical

Characterization of functional interactions of imidazoquinoxaline derivatives with benzodiazepine-gamma-aminobutyric acidA receptors.

U-78875 [imidazo[1,5-a]quinoxalin-4(5H)-one, 3-(5-cyclopropyl-1,2,4-oxadiazol-3-yl)-5-(1-methylethyl)] belongs to a series of imidazoquinoxaline derivatives, recently discovered ligands with high affinity for benzodiazepine receptors. In this study, we have examined the drug and its analogs for their modes of interaction with the receptors, with a particular emphasis on finding molecular determinants for their functional properties. Changes in the substituents on N5 and C6 of the heterocyclic ring produced no major effects on binding characteristics but yielded drugs of widely varying efficacy (antagonist to full agonist), measured as gamma-aminobutyric acid (GABA)-mediated 36Cl- uptake and t-butylbicyclophosphoro[35S]thionate binding in rat cerebrocortical membranes. The relative binding affinity and efficacy of the analogs measured in brain membranes were similar to those in cloned GABAA receptors of the alpha 1 beta 2 gamma 2 (type I) and alpha 3 beta 2 gamma 2 (type II) subtypes. The imidazoquinoxalines showed no marked subtype selectivity. Their Ki value against [3H]flunitrazepam binding for type I was only 2-3 times lower than that for type II, and their rank order for agonistic activity was the same in the two subtypes, measured as GABA-mediated Cl- currents in human kidney cells (A293) expressing the subtypes of GABAA receptors. According to computational modeling of the drugs using both molecular and quantum mechanics, the agonistic activity of the imidazoquinoxaline derivatives depends on the presence of a bulky alkyl substituent at N5 and the deformation of the substituted portion of the otherwise planar ring system induced by a bulky moiety at N5 or C6. With a fixed N5 substituent (isopropyl), the relative efficacy in the brain membranes, as well as in the cloned receptors, appeared to be dependent on the degree of the ring deformation. This out-of-plane portion of the imidazoquinoxalines can be assigned to the general region occupied by the 5-phenyl group of diazepam and other agonistic functional groups of several nonbenzodiazepine ligands. It seems that this region, apparently common to various agonistic ligands, interacts with an agonistic pocket in type I and type II subtypes of the benzodiazepine receptors in the brain. Our results also provide direct support for the view that the agonists and nonagonists share largely overlapping binding regions in the benzodiazepine receptor, which has been proposed earlier from in vivo efficacy measurements of other series of ligands.

Animals

The implementation of ab initio quantum chemistry calculations on transporters.

The RHF and geometry optimization sections of the ab initio quantum chemistry code, GAMESS, have been optimized for a network of parallel microprocessors, Inmos T800-20 transputers, using both indirect and direct SCF techniques. The results indicate great scope for implementation of such codes on small parallel computer systems, very high efficiencies having been achieved, particularly in the cases of direct SCF and geometry optimization with large basis sets. The work, although performed upon one particular parallel system, the Meiko Computing Surface, is applicable to a wide range of parallel systems with both shared and distributed memory.

Computers

Quantum simulation of ferrocytochrome c.

The dramatic progress in the understanding of the dynamics of biomolecules has been largely fuelled by computer simulations based on the law of classical mechanics. However in some respects biomolecules are at the borders of the domain of applicability of classical mechanics. The role of quantum mechanical effects in biomolecular structure and function is therefore worth investigating. Here we present preliminary results from a quantum simulation of a protein and contrast them with results from full classical simulations. The most significant differences are found in motions of high frequency, such as bond stretching or the torsional oscillation of groups that bear hydrogen atoms. The amplitudes of such motions are significantly increased by the penetration of atoms into classically forbidden regions. These differences will directly influence the rates of such processes as proton and electron transfer.

Cytochrome c Group

Three-dimensional molecular illustrations I: Isoelectron density contours and isoelectrostatic energy contours.

A method of depicting dimensional illustrations of molecules in vacuo that are sensitive to small electronic perturbations was attempted. This method would be useful in determining the effects of either perturbing groups from other molecules or changes produced by the addition or modification of an existing atom or chemical group on the same molecule. Isoelectron density contours for small molecules such as benzene, ammonia, and formaldehyde were first considered using the CNDO/2 molecular approximation method and then extended to the use of deorthogonalized CNDO/2 eigenvectors. These methods were similar in molecular projections but insensitive to electronic alterations. Therefore, the electrostatic potential energy was considered in developing contour surfaces of several of the molecules studied. In this case, acute and visually discernible changes were evidenced by electron exchange in the three-dimensional illustration of formaldehyde. The effect on the two-dimensional contour map of ammonia was strikingly altered by the addition of a proton, further substantiating the sensitivity of electrostatic contours to perturbing influences. These methods are considered and amplified in this report.

Ammonia

Prediction of the reactivities of cyclopenta-polynuclear aromatic hydrocarbons by quantum mechanical methods.

1. The direction of epoxide ring opening may be predicted using the techniques of theoretical chemistry by comparing the computed total energy of the two possible carbocations formed. 2. To predict the direction of epoxide ring opening and the potential binding of aceanthrylene 1,2-epoxide to biopolymers, quantum mechanical calculations were performed on the two potential hydroxy carbocations. 3. The 2-hydroxy carbocation (II) was favoured over the 1-hydroxy carbocation by 11.8 kcal/mol. Molecule II had more positive charge at the meso carbon group than at the nominally charged 1 position. Both the lowest unoccupied molecular orbital and the molecular electrostatic potential confirm this result, and indicate the possibility of unusual adducts to biopolymers. 4. Similar calculations on the equivalent epoxides of acenaphthylene and acephenanthrylene do not show the same results. 5. Modelling the addition products of II with small nucleophiles indicates that these unusual addition products do not form, and that the interaction is controlled by electronic effects and not electrostatic effects. 6. The calculations on acephenanthrylene demonstrate the importance of including the hydroxyl group when making predictions relative to epoxide ring opening. 7. Molecular descriptors are surrogates for the interaction of that molecule with an often unknown biological target. In cases where molecular descriptors are used without information about the target, small quantitative differences may not be appropriate discriminators.

Chemical Phenomena