Biomedical subjects

W G Schmidt

Publications and source records attributed to W G Schmidt.

At least 19 recordsLinked to original sources

PubMed2005

Attracted by long-range electron correlation: adenine on graphite.

The adsorption of adenine on graphite is analyzed from first-principles calculations as a model case for the interaction between organic molecules and chemically inert surfaces. Within density-functional theory we find no chemical bonding due to ionic or covalent interactions, only a very weak attraction at distances beyond the equilibrium position due to the lowering of the kinetic energy of the valence electrons. Electron exchange and correlation effects are much more important for the stabilization of the adsystem. They are modeled by the local density or generalized gradient approximation supplemented by the London dispersion formula for the van der Waals interaction.

Journal Article↗

PubMed2005

Initial stage of Si(001) surface oxidation from first-principles calculations.

A comprehensive density-functional theory (DFT) study of the atomic structure, electronic properties, and optical response of the Si(001) surface at the initial stages of oxidation is presented. The most favored adsorption position of a single O atom on top of the (4 x 2)-reconstructed Si(001) surface is found at the back-bond of the "down" Si dimer atom. There is no energy barrier for oxygen insertion into this bond. The ionization energy of the surface reaches a maximum when the oxidation of the second Si monolayer starts. Oxidation leads to an increase of the energy gap between occupied and empty surface states. The calculated reflectance anisotropy spectroscopy (RAS) data in comparison with experiment suggest a considerable amount of surface disorder already after oxidation of the first monolayer.

Journal Article↗

PubMed2005

Coulombic amino group-metal bonding: adsorption of adenine on Cu110.

The interaction between molecular amino groups and metal surfaces is analyzed from first-principles calculations using the adsorption of adenine on Cu110 as a model case. The amino group nitrogens are found to adsorb on top of the surface copper atoms. However, the bonding clearly cannot be explained in terms of covalent interactions. Instead, we find it to be largely determined by mutual polarization and Coulomb interaction between substrate and adsorbate.

Adenine↗

PubMed2005

Femtosecond transfer dynamics of photogenerated electrons at a surface resonance of reconstructed InP(100).

Time-dependent two-photon photoemission spectra are used to resolve the femtosecond dynamics of hot electrons at the energetically lowest surface resonance of reconstructed InP(100). Two different cases are studied, where electrons either are lifted into the surface resonance via a direct optical transition or are captured from bulk states. These data are the first of this kind recorded with a time resolution below 70 fs. The microscopic analysis shows that electron-phonon scattering is a major mechanism for electron transfer between surface and bulk states.

Journal Article↗

PubMed2005

Optical absorption of water: coulomb effects versus hydrogen bonding.

The optical spectrum of water is not well understood. For example, the main absorption peak shifts upwards by 1.3 eV upon condensation, which is contrary to the behavior expected from aggregation-induced broadening of molecular levels. We investigate theoretically the effects of electron-electron and electron-hole correlations, finding that condensation leads to delocalization of the exciton onto nearby hydrogen-bonded molecules. This reduces its binding energy and has a dramatic impact on the line shape. The calculated spectrum is in excellent agreement with experiment.

Journal Article↗

PubMed2004

Energetics of Si(001) surfaces exposed to electric fields and charge injection.

We perform density-functional calculations on the influence of external electric fields and electrons or holes injected into surface states on the relative stability of c(4x2) and p(2x2) reconstructed Si(001) surfaces. It is shown that an electric field parallel to the [001] direction or the insertion of electrons into surface states favors the formation of p(2x2) periodicities. Our results explain recent experimental studies reporting changes of surface reconstruction of Si and Ge(001) surfaces induced by the scanning tunneling microscope and the occurrence of p(2x2) reconstructions on (001) surfaces of n-doped Si.

Journal Article↗

PubMed2004

Ground- and excited-state properties of DNA base molecules from plane-wave calculations using ultrasoft pseudopotentials.

We present equilibrium geometries, vibrational modes, dipole moments, ionization energies, electron affinities, and optical absorption spectra of the DNA base molecules adenine, thymine, guanine, and cytosine calculated from first principles. The comparison of our results with experimental data and results obtained by using quantum chemistry methods show that in specific cases gradient-corrected density-functional theory (DFT-GGA) calculations using ultrasoft pseudopotentials and a plane-wave basis may be a numerically efficient and accurate alternative to methods employing localized orbitals for the expansion of the electron wave functions.

DNA↗

PubMed2003

InP(001)-(2 x 1) surface: a hydrogen stabilized structure.

The InP(001)(2 x 1) surface has been reported to consist of a semiconducting monolayer of buckled phosphorus dimers. This apparent violation of the electron counting principle was explained by effects of strong electron correlation. Combining first-principles calculations with reflectance anisotropy spectroscopy and LEED experiments, we find that the (2 x 1) reconstruction is not at all a clean surface: it is induced by hydrogen adsorbed in an alternating sequence on the buckled P dimers. Thus, the microscopic structure of the InP growth plane relevant to standard gas phase epitaxy conditions is resolved and shown to obey the electron counting rule.

Journal Article↗

PubMed2001

Bulk excitonic effects in surface optical spectra.

We calculate the surface optical properties of the passivated Si(110) surface using a real-space multigrid technique and ab initio pseudopotentials. Rather than from the usual eigenvalue representation, the macroscopic polarizability is obtained from the solution of an initial-value problem, which allows inclusion of excitonic and local-field effects in addition to the electronic self-energy in the surface calculations. It is shown that the electron-hole attraction is largely responsible for the peculiar line shape of the surface reflectance anisotropy.

Journal Article↗

PubMed1992

Spontaneous incorporation of the glycosyl-phosphatidylinositol-linked protein Thy-1 into cell membranes.

Thy-1 is a membrane protein that is attached to the plasma membrane by a glycosyl-phosphatidylinositol anchor. Purified rat brain Thy-1 could be reincorporated into the plasma membrane of murine Thy-1- cells directly from aqueous suspension and without the use of detergents. A peripheral staining pattern similar to that observed for endogenous Thy-1 was achieved. Treatment with phosphatidylinositol-specific phospholipase C removed nearly all antibody staining due to either endogenous or inserted Thy-1. Fluorescence recovery after photobleaching (FRAP) was used to compare the lateral mobility of endogenous and inserted Thy-1. Both forms exhibited large lateral diffusion coefficients, but with a substantial immobile fraction (approximately 50%) indicating that the immobile fraction was not due either to chemical differences between inserted and native Thy-1 or to some surface Thy-1 molecules having a protein anchor. However, the inserted Thy-1 failed to activate mouse T lymphocytes upon crosslinking as assayed by [3H]thymidine uptake. Since Thy-1 could be directly labeled with rhodamine, the effect of the size of the labeling ligand on the mobility obtained by the FRAP technique could be explored. Rhodamine-conjugated MRC-OX7 monoclonal antibody or its fragments [R-F(ab)2 or R-Fab] were compared with rhodamine as labels for Thy-1. The measured diffusion coefficients were 1.6 x 10(-9), 2.0 x 10(-9), and 3.2 x 10(-9) cm2/sec for Thy-1 labeled with R-F(ab)2, R-Fab, and rhodamine, respectively; mobile fractions were all in the 40-50% range. Thus, the size of the ligand affects the lateral mobility of this labeled membrane protein to a measurable extent.

Animals↗

PubMed1990

Influence of coronary arterial thrombus location on left ventricular function in acute myocardial infarction.

In 238 patients with acute myocardial infarction studied during intracoronary streptokinase therapy, the circumferential extent of left ventricular hypokinesis was measured by 5 methods and correlated with the location of the infarct-related coronary artery segment and with 1-year survival. Of the 5 methods, 1 focused only on the infarct region, and 4 varied in the complexity of the noise filter. Hypokinetic segment length measurements by all 5 methods correlated significantly with the location of occlusion along the left anterior descending coronary artery. No method yielded measurements that correlated with occlusion location along the right coronary artery. Measurements by all methods correlated significantly with survival, but the method that focused on the infarct region performed least well. Thus, the circumferential extent of hypokinesis in patients with acute myocardial infarction is greater for proximal than mid- or distal occlusions of the left anterior descending but not the right coronary artery. Survival is influenced by the function of periinfarct and noninfarct regions and by the function of the infarct region. Complex noise filters provide no advantage over simpler filters in measuring the extent of hypokinesis.

Coronary Angiography↗

PubMed1989

Evolution of left ventricular function after intracoronary thrombolysis for acute myocardial infarction.

The temporal evolution of left ventricular (LV) function after intracoronary streptokinase therapy for acute myocardial infarction (AMI) was assessed from the data of 264 patients who had complete occlusion of either the left anterior descending or the right coronary artery before treatment. Angiography was performed immediately, and at 3 days and 6 months after AMI in 91%, 71% and 47% of the study group, respectively. Wall motion was measured by the centerline method. In patients with sustained reperfusion, the ejection fraction decreased at 3 days (delta = -2.0 +/- 9.9%, n = 134, p = 0.02) and recovered later (from 54 +/- 12% acutely to 57 +/- 12% at 6 months, n = 82, p less than 0.05). These changes in global function were associated with a marked regression in hyperkinesis in the noninfarcted wall by 3 days, and delayed recovery of wall motion in the infarct region (delta = 0.2 +/- 0.9 at 3 days, p = 0.055; 1.0 +/- 1.2 at 6 months, p less than 0.001). Patients without reperfusion or with reocclusion had a more severe decrease in ejection fraction at 3 days, and little or no subsequent functional recovery. The length of the hypokinetic segment increased significantly by 3 days but subsequently diminished to slightly less than the acute value. It is concluded that full recovery of ischemically impaired myocardium takes greater than 3 days, but compensatory hyperkinesis regresses earlier so that global LV function deteriorates by the third day. Variability or deterioration of LV function early after AMI need not be due to infarct extension; it can reflect regression of hyperkinesis in the noninfarcted region.

Angiography↗

PubMed1988

Early recovery of left ventricular function after thrombolytic therapy for acute myocardial infarction: an important determinant of survival.

Thrombolytic therapy for acute myocardial infarction reduces early mortality, but full recovery of left ventricular function after reperfusion is delayed. Therefore, the relations among reperfusion, survival and the time course of left ventricular functional recovery were examined in 226 patients treated with intracoronary streptokinase; 77% (134 patients) had sustained reperfusion and 31 patients had no reperfusion or had reocclusion by day 3. Wall motion was measured from contrast ventriculograms performed in the acute period and 3 days later in the central and peripheral infarct regions and the noninfarct region by the centerline method in 165 patients. Patients with reperfusion had better survival (p less than 0.05, mean follow-up 4.5 years) and a higher ejection fraction at 3 days (52 +/- 12 versus 46 +/- 10%, p less than 0.02) attributable to a significantly different change in peripheral infarct region function between the acute and 3 day studies (0.1 +/- 1.0 versus -0.3 +/- 0.9 SD, p less than 0.05). These early functional changes were significant in patients with anterior myocardial infarction and showed similar trends in those with inferior myocardial infarction. On Cox regression analysis, function measured at 3 days was more predictive of survival than was function measured acutely (chi square for acute ejection fraction = 11.48 versus 24.59 at 3 days). Although, as previously reported, greater than 45% of total recovery of left ventricular function occurs later, the ejection fraction achieved by day 3 is already predictive of survival. Thus, the mechanism by which successful thrombolytic therapy enhances survival is improvement of regional and global left ventricular function early after acute myocardial infarction.

Coronary Circulation↗

PubMed1987

Residual coronary stenosis after thrombolysis with rt-PA or streptokinase: acute results and 3 weeks follow-up.

Ninety-one patients with acute myocardial infarction were assigned to intravenous treatment with streptokinase or rt-PA as part of the randomized trial carried out by the European Study Group for Recombinant Tissue-Type Plasminogen Activator (rt-PA). A patent coronary artery was found in 37 of 45 (82%) patients treated with rt-PA and in 27 of 46 (59%) patients treated with streptokinase 75-90 minutes after start of infusion. Patients were subsequently anticoagulated with heparin or dicoumarol up to a repeat angiography 3 weeks after the infarction. Of the 64 patients with successful reperfusion, 3 died and 3 suffered reocclusion of the vessel. Quantitative analysis of the coronary stenosis both immediately after thrombolysis and at 3 weeks follow-up was possible in 33 cases. Residual stenosis (percentage narrowing of diameter) decreased from 74 +/- 14% to 56 +/- 17% (P less than 0.05). No difference was observed between the groups of patients treated with streptokinase (74 +/- 9% to 57 +/- 12%, N = 17) and with rt-PA (74 +/- 17% to 56 +/- 21%, N = 16). Despite the significant regression, a coronary stenosis of more than 50% of the diameter persisted in 82% of the patients three weeks after the infarction.

Cineangiography↗

PubMed1987

[Densitometric follow-up of algodystrophy using computerized tomography].

Clinical and radiological findings obtained from diagnosis and follow-up examination of post-traumatic algodystrophy (Morbus Sudeck) are very much open to subjective interpretation. Decisive importance is attributed not only to alteration of soft tissue but also to typical distribution patterns and severity of bone demineralisation. No objectifiable and quantifiable have so far become available for proper assessment but are urgently desirable in view of the great number of therapeutic approaches. Densitometry integrated with computed tomography was applied to nine patients with algodystrophy of hand or foot in the region of spongy bones to determine absorption values which were then compared with those on the clinically intact side. Significant differences between sides proved to be objectifiable and were quantifiable measures by which demineralisation of the effected extremity could be assessed. Repeated examinations were undertaken for follow-up through a period up to nine months.

Adult↗

PubMed1985

[Electrocardiogram in acute anterior wall infarct after restoration of the blood supply].

Changes in QRS complexes and ST-T segments were measured in 70 patients after acute anterior-wall myocardial infarction. Intracoronary streptokinase infusion was undertaken in the acute phase in 58 patients, 12 other patients treated conventionally without infusion serving as controls. Precordial ECG mapping employed 48 unipolar precordial leads. If recanalization of an occluded coronary artery was achieved, there was a significant rise in R amplitude (sum of R-wave amplitudes in the 48 leads) from 12.4 +/- 10.9 to 16.2 +/- 11.2 mV, within a period of up to four months after the acute stage. If thrombolysis failed or only conventional treatment had been practised, there was no rise in R-wave amplitude. Renewed occlusion of a vessel previously re-opened by streptokinase infusion produced a reduction in R-wave amplitude from 14.0 +/- 13.0 to 9.8 +/- 11.0 mV. The S-T segment elevation regressed immediately after recanalization of the coronary artery. The S-T segments were iso-electric after four months. Re-occlusion caused renewed S-T segment elevation up to monophasic form. Precordial ECG mapping is thus well suited for serial controls after various re-perfusion measures.

Electrocardiography↗

PubMed1985

[Residual stenosis following successful selective lysis of complete thrombotic coronary artery occlusion in acute myocardial infarct].

After successful lysis of a thrombotic coronary obstruction in acute myocardial infarction, both PTCA and bypass surgery can be useful in preventing reocclusion and providing long-term success in selected patients. One condition to perform such measures is a high degree of residual narrowing at the previous site of occlusion. Other investigations concerning the extent and further development of these lesions are methodically inhomogenous and different in their results. Following successful intracoronary lysis of a complete thrombotic occlusion, the remaining stenosis was measured in 106 patients using at least 2 angiographic projections both immediately after reperfusion, and 3 days later. The degree and development of the residual lesion were analysed with special regard to its anatomy and to the occlusion time. During the observation period, no mechanical intervention (PTCA) or bypass surgery took place. The 1st angiogram after thrombolysis revealed an average cross section stenosis of 90.5 +/- 6.2%, which decreased up to the control angiogram to 86.3 +/- 10.6% (p less than 0.05). In only 16 cases there was an improvement of 10% or more, in fact it was not relevant (less than 10%) in 66 patients, and in 24 a slight increase in residual narrowing could even be found. The decrease of eccentric (-5.5 +/- 9.2%) and concentric (-3.3 +/- 5.7%) lesions was not statistically different. Stenoses up to 5 mm of length (-5.7 +/- 7.2%), between 5 and 10 mm (-3.4 +/- 6.7%), and over 10 mm (-4.4 +/- 8.2%) again did not differ significantly. There was no linear correlation between degree of stenosis and total time of occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical Trials as Topic↗

PubMed1984

Determinants of infarct size in patients successfully treated by intracoronary thrombolysis.

In 125 patients successfully treated by intracoronary thrombolysis, data were analyzed to determine the amount of regional wall motion impairment. In 85 patients with complete occlusion of the affected vessel and successful recanalization, ventriculographic study could be performed immediately after recanalization of the vessel and repeated 3 days thereafter. Unexpectedly, no correlation could be seen between the amount of wall motion impairment and the time interval of coronary vessel occlusion. For assessment of other influencing factors two subgroups were analyzed, one with large infarction despite short occlusion time and the other with small infarction despite long occlusion time. The significant differences between these two groups was in regard to the occluded coronary vessels: In the first group, most patients had anterior infarctions caused by left anterior descending (LAD) occlusion, whereas in the second group, there were no LAD occlusions at all. According to the data that we compiled, the location of the occluded coronary vessel was the most important factor in determining infarct size. Taking this into account, recovery of the impaired wall motion up to the third day after infarction was separately analyzed in LAD occlusion or inferior infarction and was found to be more pronounced in the first. Immediate recovery after recanalization however, which could be analyzed in 40 patients who had angiographic studies before recanalization as well as afterwards, was more pronounced in inferior infarction.

Journal Article↗