Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “quantum computing”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

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

At least 325 records · Page 18Linked to original sources
PubMed2001

Interplay of intrinsic and environmental effects on the magnetic properties of free radicals issuing from H-atom addition to cytosine.

Possible radical reaction products issuing from H-atom addition to cytosine have been characterized and analyzed by means of a comprehensive quantum mechanical approach including density functional computations (B3LYP), together with simulation of the solvent by the polarizable continuum model (PCM), and averaging of spectroscopic properties over the most important vibrational motions. The hyperfine couplings of the semirigid 5,6-dihydrocytos-6yl radical computed at the optimized geometry are in good agreement with their experimental counterparts. On the other hand, vibrational averaging is mandatory for obtaining an effectively planar structure for the 5,6-dihydrocytos-5yl radical with the consequent equivalence of beta-hydrogens. Finally, only proper consideration of environmental effects restores the agreement between computed and experimental couplings for the base anion protonated at N3.

Cytosine↗
PubMed1999

Performance characteristics of a Kodak computed radiography system.

The performance characteristics of a photostimulable phosphor based computed radiographic (CR) system were studied. The modulation transfer function (MTF), noise power spectra (NPS), and detective quantum efficiency (DQE) of the Kodak Digital Science computed radiography (CR) system (Eastman Kodak Co.-model 400) were measured and compared to previously published results of a Fuji based CR system (Philips Medical Systems-PCR model 7000). To maximize comparability, the same measurement techniques and analysis methods were used. The DQE at four exposure levels (30, 3, 0.3, 0.03 mR) and two plate types (standard and high resolution) were calculated from the NPS and MTF measurements. The NPS was determined from two-dimensional Fourier analysis of uniformly exposed plates. The presampling MTF was determined from the Fourier transform (FT) of the system's finely sampled line spread function (LSF) as produced by a narrow slit. A comparison of the slit type ("beveled edge" versus "straight edge") and its effect on the resulting MTF measurements was also performed. The results show that both systems are comparable in resolution performance. The noise power studies indicated a higher level of noise for the Kodak images (approximately 20% at the low exposure levels and 40%-70% at higher exposure levels). Within the clinically relevant exposure range (0.3-3 mR), the resulting DQE for the Kodak plates ranged between 20%-50% lower than for the corresponding Fuji plates. Measurements of the presampling MTF with the two slit types have shown that a correction factor can be applied to compensate for transmission through the relief edges.

Evaluation Studies as Topic↗
PubMed1982

Effect of quantum noise on the detection of incipient proximal caries.

The effect of quantum noise on the radiographyic detectability of proximal carious lesions was studied. Bitewing radiographs obtained from patients considered to be free of caries were digitized and stored in a computer using a sampling interval comparable to screen-film systems. Lesions limited in depth to one half the thickness of the enamel were induced by means of computer simulation. Various amounts of randomly generated quantum noise were added to each radiographic image. The results from observer-performance studies show that the lesions can be detected reliably in the presence of substantial amounts of quantum noise, suggesting that comparable diagnostic performance could be obtained with significantly faster imagining systems than those currently used in dentistry.

Analog-Digital Conversion↗
PubMed2000

QSAR treatment of electronic substituent effects using frontier orbital theory and topological parameters

A methodology for the estimation of Hammett substituent constants from computational-based descriptors utilizing quantitative structure activity/property relationships (QSAR/QSPR) formalism is presented. Electronic descriptors derived from quantum chemical calculations and molecular topology were used to generate computational-based analogues of empirical Hammett substituent constants from statistical analysis. Global quantum chemical reaction indices were drawn from frontier orbital theory and density functional theory and formulated from AM1-based calculations. A localized index based on the electrotopological state index was used to encode information on individual group properties. From a training set consisting of 150 meta and para-substituted benzoic acids, statistical analysis of computational-based descriptors as a function of empirical substituent constants yielded a five-parameter QSAR/QSPR model which generates computational-based constants exhibiting a strong correlation with empirical values (r2 = 0.958). Both internal (PRESS) and external (independent testing set of benzoic acids) validation procedures suggest that the electronic effects QSAR/QSPR model derived in this work from computational-based parameters is a statistically viable paradigm. Both predicted and empirical constants were used in Hammett-type validation analyses as functions of chemical, biological, and spectroscopic data for thirty structurally diverse meta and parasubstituted aromatic testing sets. Statistical measures of ensuing correlations were examined and compared, and the empirical and predicted results were of similar quality. Validation results reveal that a large number of computational-based substituent constants can be accurately estimated from semiempirical AM1 frontier orbital energies and electronic structure information obtained directly from substituted benzoic acids without the aid of empirical parametrization.

Journal Article↗
PubMed1989

The role of hydrated divalent metal ions in the bridging of two anionic groups. An ab initio quantum chemical and molecular mechanics study of dimethyl phosphate and formate bridged by calcium and magnesium ions.

Ab initio quantum chemical (Gaussian82) and molecular mechanics (AMBER2.0) computational techniques are employed to investigate the interaction of two anions (formate an dimethylphosphate) and a central divalent metal cation (magnesium or calcium). These systems are models for the essential GDP binding unit of the G-proteins (e.g., EF-Tu or the ras oncogene proteins) and for protein/phospholipid interactions, both of which are mediated by divalent metal cations. Various levels of hydration are utilized to examine coordination of differences between magnesium and calcium ions. Two different orientations of formate and dimethyl phosphate in direct ion contact with a magnesium ion and two waters of hydration were energy minimized with both quantum and molecular mechanics techniques. The structures and energy differences between the two orientations determined by either of the computational techniques are similar. Magnesium ion has a strong propensity to assume six coordination whereas calcium ion preferentially assumes a coordination greater than six. Likewise, water molecules attached to magnesium ion are held more rigidly than those of calcium ion, thus calcium ion is more accommodating in the exchange of water for negative ligands.

Calcium↗
PubMed1980

Cannabidiol and phenytoin: a structural comparison.

Conformational energy maps have been computed for the antiepileptic agents phenytoin and cannabidiol by the quantum-mechanical method of perturbative configuration interaction with localized orbitals (PCILO). The computation indicates that the spatial relationship between the two rings in the two drugs is similar and close to the respective structures in the crystal. This is supported by 1H and 13C NMR measurements. Hence, both compounds fulfill the stereochemical requirements suggested for anticonvulsant drug action.

Cannabidiol↗
PubMed2002

1,5-Hydride shift in Wolff-Kishner reduction of (20R)-3beta,20, 26-trihydroxy-27-norcholest-5-en-22-one: synthetic, quantum chemical, and NMR studies.

Heating (20R)-3beta,20,26-trihydroxy-27-norcholest-5-en-22-one (1) with hydrazine and KOH at 160 degrees C completely converted the steroid to a diastereoisomeric mixture of the new (20R,22RS)-27-norcholest-5-ene-3beta,20,22-triols (2). Exclusive formation of 2 suggests that the expected Wolff-Kishner reduction to a methylene group at the C-22 ketone in 1 was diverted to the C-26 position by a 1,5-hydride shift. All attempts under acid conditions failed to produce a C-22 phenyl hydrazone from 1. However, reaction of 1 was reacted with phenylhydrazine in hot KOH, gave the C-26 phenylhydrazone 4 as the sole product. Evidently, under alkaline conditions, first a hydride ion undergoes an intramolecular transfer from the C-26 CH(2)OH group to the C-22 ketone in 1, and then the phenylhydrazine traps the newly formed aldehyde. To examine this hypothesis, we constructed computer-simulated transition state models from quantum chemical calculations and then compared data from these models with NMR measurements of the reaction mixtures containing 2. The NMR data showed that the C-22 diastereoisomers of 2 are formed in a nearly 1:1 ratio exactly as predicted from the energy-optimized transition states, which were calculated for intramolecular 1,5-hydride shifts that produced each of the two C-22 diastereoisomers. Accordingly, these results support the hypothesis that an intramolecular 1,5-hydride shift mechanism promotes complete conversion of 1 to 2 under classical Wolff-Kishner reduction conditions.

Aldehydes↗
PubMed1984

[Analysis of factors limiting the applications of binomial statistics in studies of mediator secretion at neuromuscular synapses].

Binomial parameters of transmitter secretion were calculated on the basis of synaptic potential analyses at frog sartorius muscle. In some synapses negative values of parameter of the p were found. These negative values often occurred at low concentrations of Ca2+ ions and small amplitude of miniature end plate potentials. The obtained data were interpreted in terms of a model which presupposes the heterogeneity of the quantum transmitter release at different sites. A computer imitation of transmitter secretion showed that the appearance of the negative p parameter values and erroneous n estimations in the experiment are related with the shape of the distribution of quantum transmitter release probability and the amplitude of miniature end plate potentials.

Animals↗
PubMedDate not supplied

Molecular quantum similarity measures tuned 3D QSAR: an antitumoral family validation study.

In this work, a new methodology to construct a tuned QSAR model is presented, which is based on a convex set formalism. The present procedure continues previous 3D QSAR studies, performed using molecular quantum similarity measures (MQSM). With this new computational tool, the efficiency of MQSM applied to QSAR analysis is significantly improved. A reliable QSAR model is obtained using convex linear combinations of different kinds of MQSM, corresponding to different quantum-mechanical operators related to the quantum similarity integral. The active compounds studied here, as a case study, are a set of antitumor agents, the camptothecin molecule and analogues, and the property evaluated is the topoisomerase-I inhibition activity. Before performing a tuned QSAR analysis with this particular molecular set, a simple QSAR study for all the different possible types of MQSM is carried out. In addition, another application of MQSM is presented, to determine which method can be used to optimize molecular structures in order to reproduce experimental molecular geometries as well as possible.

Animals↗
PubMed1988

Molecular modelling in design of crop protection chemicals.

Specific examples from pesticide research are given which illustrate the types of analysis employed to design optimal inhibitors for a given receptor, based on the assumption that a congeneric series of compounds behave in a related mode in the biosystem. The examples illustrate the complementary role played by computational chemistry, X-ray crystallography and computer graphics and also raise questions as to the current limitations of existing molecular mechanics and quantum mechanics techniques.

Agriculture↗
PubMed1999

Noise power spectra of images from digital mammography detectors.

Noise characterization through estimation of the noise power spectrum (NPS) is a central component of the evaluation of digital x-ray systems. We begin with a brief review of the fundamentals of NPS theory and measurement, derive explicit expressions for calculation of the one- and two-dimensional (1D and 2D) NPS, and discuss some of the considerations and tradeoffs when these concepts are applied to digital systems. Measurements of the NPS of two detectors for digital mammography are presented to illustrate some of the implications of the choices available. For both systems, two-dimensional noise power spectra obtained over a range of input fluence exhibit pronounced asymmetry between the orthogonal frequency dimensions. The 2D spectra of both systems also demonstrate dominant structures both on and off the primary frequency axes indicative of periodic noise components. Although the two systems share many common noise characteristics, there are significant differences, including markedly different dark-noise magnitudes, differences in NPS shape as a function of both spatial frequency and exposure, and differences in the natures of the residual fixed pattern noise following flat fielding corrections. For low x-ray exposures, quantum noise-limited operation may be possible only at low spatial frequency. Depending on the method of obtaining the 1D NPS (i.e., synthetic slit scanning or slice extraction from the 2D NPS), on-axis periodic structures can be misleadingly smoothed or missed entirely. Our measurements indicate that for these systems, 1D spectra useful for the purpose of detective quantum efficiency calculation may be obtained from thin cuts through the central portion of the calculated 2D NPS. On the other hand, low-frequency spectral values do not converge to an asymptotic value with increasing slit length when 1D spectra are generated using the scanned synthetic slit method. Aliasing can contribute significantly to the digital NPS, especially near the Nyquist frequency. Calculation of the theoretical presampling NPS and explicit inclusion of aliased noise power shows good agreement with measured values.

Biophysics↗
PubMed2002

Computational approaches to the prediction of the blood-brain distribution.

This review attempts to summarise present knowledge related to the theoretical modelling of drug transport across the blood-brain barrier. Several computational protocols are described ranging from quantum mechanics-based approaches through molecular mechanics-related techniques to simple and fast procedures based on only the 2-D graph of the investigated structures. Amazingly, few descriptors have been shown to influence the derived relationships in a significant manner and a cornerstone in most of the described models are terms describing hydrogen bonding. A very quick quantitative assessment of the brain partitioning of a compound has also been devised using the following two rules: If N+O (the number of nitrogen and oxygen atoms) in a molecule is less than or equal to five, it has a high chance of entering the brain. The second rule predicts that if log P-(N+O) is positive then log BB is positive.

Algorithms↗
PubMed1991

Molecular structure and dynamics of cis(Z)-and trans(E)-flupenthixol and clopenthixol.

The three-dimensional structures and molecular electrostatic potentials of the cis(Z) and trans(E)-isomers of flupenthixol and clopenthixol were examined by computer graphics and molecular mechanical and quantum mechanical calculations, and their internal molecular motions were studied by molecular dynamics simulations in vacuo and in aqueous solution. The simulations demonstrated that both the side chains and the tricyclic ring systems of clopenthixol and flupenthixol are highly flexible. The angle between the two phenyl ring planes varied between 105 and 171 degrees during the simulations in solution. The electrostatic potentials around the 2-substituent were significantly more negative in the trans(E)-isomers than in the cis(Z)-isomers. The stronger negative potentials may weaken electrostatic receptor interactions and, thereby, cause the trans(E)-isomers to be less active than cis(Z)-isomers. Differences both in three-dimensional structure and in electronic structure may cause the difference in pharmacological activity between cis(Z)- and trans(E)-thioxanthenes.

Clopenthixol↗
PubMed1991

Three-dimensional structure and molecular dynamics of cis(Z)- and trans(E)-chlorprothixene.

cis(Z)-Chlorprothixene has antidopaminergic potency, while trans(E)-chlorprothixene is virtually inactive. In order to reveal the structural features causing the difference in activity, the three-dimensional molecular and electronic structures of cis(Z)- and trans(E)-chlorprothixene were examined by computer graphics and molecular mechanical and quantum mechanical calculations. The internal molecular motions of the isomers were studied by molecular dynamics simulations in vacuo and in aqueous solution. The cis(Z)-isomer had lower potential molecular energy than the trans(E)-isomer, mainly due to electrostatic interactions within the side-chain and between the dimethylamino group and the chlorine atom. During molecular dynamics simulations in aqueous solution, the side-chain of the trans(E)-isomer stayed closer to the central S-C axis of the ring system than did the side-chain of the cis(Z)-isomer. The molecular electrostatic potentials were significantly lower in the vicinity of the chlorine atom in the trans(E)- than in the cis(Z)-isomer. Differences in molecular electrostatic potentials and in three-dimensional structure are suggested to be the main reasons for the difference in pharmacological activities of cis(Z)- and trans(E)-chlorprothixene.

Chlorprothixene↗
PubMed1987

Nuclear magnetic resonance spectroscopic and computer-stimulated structural analyses of a heptapeptide sequence found around the N-glycosylation site of a proline-rich glycoprotein from human parotid saliva.

The proline-rich glycoprotein from human parotid saliva has a common heptapeptide sequence around four of six N-glycosylation sites (Maeda, N., H. S. Kim, E. A. Azen, and O. J. Smithies, 1985, J. Biol. Chem., 20:11123-11130). A synthetic model of the heptamer protein sequence, NH2-Q(1)-G(2)-G(3)-N(4)-Q(5)-S(6)-Q(7)-CONH2, was examined by nuclear magnetic resonance (NMR) spectroscopy and the ECEPP/2-VAO4A (Empirical Conformation Energy Program for Peptides) energy minimization computer algorithm (Scheraga, H. A., 1982, Quantum Chemistry Program Exchange, 454; Powell, M. J. D., 1964, Quantum Chemistry Program Exchange, 60). The NMR spectrum was almost completely assigned in dimethylsulfoxide-d6 (DMSO), and the amide chemical shift temperature dependence, phi dihedral angles, and chi 1 rotamer populations elucidated. These data indicated that a significant population of the heptamer could exist as a type I beta-turn [4----1 between Q(5) and G(2)] and/or a type II' beta-turn [4----1 between (Q)5 and G(2) and/or a gamma-turn [3----1 between Q(5) and G(3)] with the amino acid chi 1 torsion angles weighted toward the gauche- conformation. Starting from these three possible conformations, the ECEPP/2-VAO4A rigid geometry energy minimization program was used to find the localized predominant in vacuo structures of this heptapeptide sequence. The type II' beta-turn conformation best fits the data based on internuclear hydrogen-bonding distances, minimum potential energy considerations, and the NMR parameters.

Computer Simulation↗
PubMed1987

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↗
PubMed2000

Stereoselectivity and chiral recognition in copper(I) olefin complexes with a chiral diamine

Trigonal copper(I) complexes of the chiral bidentate ligand (1S,2S)-N,N'-Bis-(mesitylmethyl)-1,2-diphenyl-1,2-ethanediamine ((S,S)-1) have been prepared with hydrocarbon olefins, as well as with allylic alcohols and ethers. The stereochemistry of the complexes has been investigated by 1H NMR spectroscopy and by combined quantum mechanics and molecular mechanics (QM/MM) computational methods. The coordinated chiral nitrogen atoms can display equal (R, R) or opposite (R, S) configuration, the latter being disfavored if steric hindrance is present above and below the coordination plane. Although the complexes exist as rapidly equilibrated mixtures of stereoisomers, one of these is often dominant, and prochiral olefins are coordinated with high enantioface selection. In addition, the [(S,S)-1]-Cu+ fragment selectively recognizes the R enantiomer of secondary allylic alcohols and ethers, as confirmed by the X-ray crystal structure analysis of the adduct with (R)-1-buten-3-ol. The reasons for the observed selectivities have been elucidated, and lead to some implications which are consistent with the enantioselection observed in catalytic cyclopropanation reactions promoted by copper complexes of the same ligand.

Journal Article↗
PubMed1992

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↗