86 research outputs found
Solution structure of Mannobioses unravelled by means of Raman optical activity
Structural analysis of carbohydrates is a complicated endeavour, due to the complexity and diversity of the samples at hand. Herein, we apply a combined computational and experimental approach, employing molecular dynamics (MD) and density functional theory (DFT) calculations together with NMR and Raman optical activity (ROA) measurements, in the structural study of three mannobiose disaccharides, consisting of two mannoses with varying glycosidic linkages. The disaccharide structures make up the scaffold of high mannose glycans and are therefore important targets for structural analysis. Based on the MD population analysis and NMR, the major conformers of each mannobiose were identified and used as input for DFT analysis. By systematically varying the solvent models used to describe water interacting with the molecules and applying overlap integral analysis to the resulting calculational ROA spectra, we found that a full quantum mechanical/molecular mechanical approach is required for an optimal calculation of the ROA parameters. Subsequent normal mode analysis of the predicted vibrational modes was attempted in order to identify possible marker bands for glycosidic linkages. However, the normal mode vibrations of the mannobioses are completely delocalised, presumably due to conformational flexibility in these compounds, rendering the identification of isolated marker bands unfeasible
Isotope shift in the electron affinity of chlorine
The specific mass shift in the electron affinity between ^{35}Cl and ^{37}Cl
has been determined by tunable laser photodetachment spectroscopy to be
-0.51(14) GHz. The isotope shift was observed as a difference in the onset of
the photodetachment process for the two isotopes. In addition, the electron
affinity of Cl was found to be 29138.59(22) cm^{-1}, giving a factor of 2
improvement in the accuracy over earlier measurements. Many-body calculations
including lowest-order correlation effects demonstrates the sensitivity of the
specific mass shift and show that the inclusion of higher-order correlation
effects would be necessary for a quantitative description.Comment: 16 pages, 6 figures, LaTeX2e, amsmat
Many-body-QED perturbation theory: Connection to the Bethe-Salpeter equation
The connection between many-body theory (MBPT)--in perturbative and
non-perturbative form--and quantum-electrodynamics (QED) is reviewed for
systems of two fermions in an external field. The treatment is mainly based
upon the recently developed covariant-evolution-operator method for QED
calculations [Lindgren et al. Phys. Rep. 389, 161 (2004)], which has a
structure quite akin to that of many-body perturbation theory. At the same time
this procedure is closely connected to the S-matrix and the Green's-function
formalisms and can therefore serve as a bridge between various approaches. It
is demonstrated that the MBPT-QED scheme, when carried to all orders, leads to
a Schroedinger-like equation, equivalent to the Bethe-Salpeter (BS) equation. A
Bloch equation in commutator form that can be used for an "extended" or
quasi-degenerate model space is derived. It has the same relation to the BS
equation as has the standard Bloch equation to the ordinary Schroedinger
equation and can be used to generate a perturbation expansion compatible with
the BS equation also for a quasi-degenerate model space.Comment: Submitted to Canadian J of Physic
Atomic Parity Nonconservation: Electroweak Parameters and Nuclear Structure
There have been suggestions to measure atomic parity nonconservation (PNC)
along an isotopic chain, by taking ratios of observables in order to cancel
complicated atomic structure effects. Precise atomic PNC measurements could
make a significant contribution to tests of the Standard Model at the level of
one loop radiative corrections. However, the results also depend upon certain
features of nuclear structure, such as the spatial distribution of neutrons in
the nucleus. To examine the sensitivity to nuclear structure, we consider the
case of Pb isotopes using various recent relativistic and non-relativistic
nuclear model calculations. Contributions from nucleon internal weak structure
are included, but found to be fairly negligible. The spread among present
models in predicted sizes of nuclear structure effects may preclude using Pb
isotope ratios to test the Standard Model at better than a one percent level,
unless there are adequate independent tests of the nuclear models by various
alternative strong and electroweak nuclear probes. On the other hand,
sufficiently accurate atomic PNC experiments would provide a unique method to
measure neutron distributions in heavy nuclei.Comment: 44 pages, INT Preprint DOE/ER/40561-050-INT92-00-1
Calculation of T_ odd effects in $"" sup 205_TIF including electron correlation
A method and codes for two-step correlation calculation of heavy-atom
molecules have been developed, employing the generalized relativistic effective
core potential and relativistic coupled cluster (RCC) methods at the first
step, followed by nonvariational one-center restoration of proper
four-component spinors in the heavy cores. Electron correlation is included for
the first time in an ab initio calculation of the interaction of the permanent
P,T-odd proton electric dipole moment with the internal electromagnetic field
in a molecule. The calculation is performed for the ground state of TlF at the
experimental equilibrium, R_e=2.0844 A, and at R=2.1 A, with spin-orbit and
correlation effects included by RCC. Calculated results with single cluster
amplitudes only are in good agreement (3% and 1%) with recent
Dirac-Hartree-Fock (DHF) values of the magnetic parameter M; the larger
differences occurring between present and DHF volume parameter (X) values, as
well as between the two DHF calculations, are explained. Inclusion of electron
correlation by GRECP/RCC with single and double excitations has a major effect
on the P,T-odd parameters, decreasing M by 17% and X by 22%.Comment: 5 pages, REVTeX4 style Accepted for publication in Phys.Rev.Letter
Probing exotic phenomena at the interface of nuclear and particle physics with the electric dipole moments of diamagnetic atoms: A unique window to hadronic and semi-leptonic CP violation
The current status of electric dipole moments of diamagnetic atoms which
involves the synergy between atomic experiments and three different theoretical
areas -- particle, nuclear and atomic is reviewed. Various models of particle
physics that predict CP violation, which is necessary for the existence of such
electric dipole moments, are presented. These include the standard model of
particle physics and various extensions of it. Effective hadron level combined
charge conjugation (C) and parity (P) symmetry violating interactions are
derived taking into consideration different ways in which a nucleon interacts
with other nucleons as well as with electrons. Nuclear structure calculations
of the CP-odd nuclear Schiff moment are discussed using the shell model and
other theoretical approaches. Results of the calculations of atomic electric
dipole moments due to the interaction of the nuclear Schiff moment with the
electrons and the P and time-reversal (T) symmetry violating
tensor-pseudotensor electron-nucleus are elucidated using different
relativistic many-body theories. The principles of the measurement of the
electric dipole moments of diamagnetic atoms are outlined. Upper limits for the
nuclear Schiff moment and tensor-pseudotensor coupling constant are obtained
combining the results of atomic experiments and relativistic many-body
theories. The coefficients for the different sources of CP violation have been
estimated at the elementary particle level for all the diamagnetic atoms of
current experimental interest and their implications for physics beyond the
standard model is discussed. Possible improvements of the current results of
the measurements as well as quantum chromodynamics, nuclear and atomic
calculations are suggested.Comment: 46 pages, 19 tables and 16 figures. A review article accepted for
EPJ
Prognostic model to predict postoperative acute kidney injury in patients undergoing major gastrointestinal surgery based on a national prospective observational cohort study.
Background: Acute illness, existing co-morbidities and surgical stress response can all contribute to postoperative acute kidney injury (AKI) in patients undergoing major gastrointestinal surgery. The aim of this study was prospectively to develop a pragmatic prognostic model to stratify patients according to risk of developing AKI after major gastrointestinal surgery. Methods: This prospective multicentre cohort study included consecutive adults undergoing elective or emergency gastrointestinal resection, liver resection or stoma reversal in 2-week blocks over a continuous 3-month period. The primary outcome was the rate of AKI within 7 days of surgery. Bootstrap stability was used to select clinically plausible risk factors into the model. Internal model validation was carried out by bootstrap validation. Results: A total of 4544 patients were included across 173 centres in the UK and Ireland. The overall rate of AKI was 14·2 per cent (646 of 4544) and the 30-day mortality rate was 1·8 per cent (84 of 4544). Stage 1 AKI was significantly associated with 30-day mortality (unadjusted odds ratio 7·61, 95 per cent c.i. 4·49 to 12·90; P < 0·001), with increasing odds of death with each AKI stage. Six variables were selected for inclusion in the prognostic model: age, sex, ASA grade, preoperative estimated glomerular filtration rate, planned open surgery and preoperative use of either an angiotensin-converting enzyme inhibitor or an angiotensin receptor blocker. Internal validation demonstrated good model discrimination (c-statistic 0·65). Discussion: Following major gastrointestinal surgery, AKI occurred in one in seven patients. This preoperative prognostic model identified patients at high risk of postoperative AKI. Validation in an independent data set is required to ensure generalizability
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