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Biomedical subjects

W Sacks

Publications and source records attributed to W Sacks.

At least 19 recordsLinked to original sources
PubMed2006

Scanning tunneling spectroscopy on the novel superconductor CaC6.

We present scanning tunneling microscopy and spectroscopy of the newly discovered superconductor CaC6. The tunneling conductance spectra, measured between 3 and 15 K, show a clear superconducting gap in the quasiparticle density of states. The gap function extracted from the spectra is in good agreement with the conventional BCS theory with Delta0=1.6+/-0.2 meV. The possibility of gap anisotropy and two-gap superconductivity is also discussed. In a magnetic field, direct imaging of the vortices allows us to deduce a coherence length in the ab plane xiab approximately 33 nm.

Journal Article↗
PubMed2006

Probing the superfluid velocity with a superconducting tip: the Doppler shift effect.

We address the question of probing the supercurrents in superconducting (SC) samples on a local scale by performing scanning tunneling spectroscopy (STS) experiments with a SC tip. In this configuration, we show that the tunneling conductance is highly sensitive to the Doppler shift term in the SC quasiparticle (QP) spectrum of the sample, thus allowing the local study of the superfluid velocity. Intrinsic screening currents, such as those surrounding the vortex cores in a type II SC in a magnetic field, are directly probed. With Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe(2), reveals both the vortex cores, on the scale of the SC coherence length xi, and the supercurrents, on the scale of the London penetration length lambda. A subtle interplay between the SC pair potential and the supercurrents at the vortex edge is observed. Our results open interesting prospects for the study of screening currents in any superconductor.

Journal Article↗
PubMed2001

Two-gap state density in MgB(2): a true bulk property or a proximity effect?

We report on the temperature dependence of the quasiparticle density of states in the simple binary compound MgB(2) directly measured using scanning tunneling microscope (STM). To achieve high quality tunneling conditions, a small crystal of MgB(2) is used as a tip in the STM experiment. The "sample" is chosen to be a 2H- NbSe(2) single crystal presenting an atomically flat surface. At low temperature the tunneling conductance spectra show a gap at the Fermi energy followed by two well-pronounced conductance peaks on each side. They appear at voltages V(S) approximately +/-3.8 mV and V(L) approximately +/-7.8 mV. With rising temperature both peaks disappear at the T(C) of the bulk MgB(2), a behavior consistent with the model of two-gap superconductivity. The possibility of a particular proximity effect is also discussed.

Journal Article↗
PubMed2000

Influence of disorder on the local density of states in high- T(c) superconducting thin films

Using a low temperature scanning tunneling microscope in the spectroscopic mode, we find that the disorder in a Bi(2)Sr(2)CaCu(2)O(8+delta) thin film modifies dramatically the quasiparticle local density of states. Small, but well-defined superconducting regions, coexisting with dominating semiconducting areas, show well-pronounced gap structures, similar to those observed previously in high-quality single crystals. Surprisingly, between these two regions, the detailed shape of the quasiparticle spectrum is virtually identical to the pseudogap previously observed at temperatures T>T(c), or in the vortex core, at 4.2 K. Thus, the role of the disorder in destroying the superconducting phase is comparable to that of the magnetic field or thermal fluctuations.

Journal Article↗
PubMed1995

Molecular characterization of nucleotide sequences encoding the extracellular glycoprotein elicitor from Phytophthora megasperma.

cDNA sequences encoding the 42 kDa glycoprotein elicitor from the oomycete, Phytophthora megasperma, that induces the defense response in parsley have been cloned and sequenced. The 5' end of the mRNA matches a consensus derived from sequences surrounding the transcription initiation sites of seven other oomycete genes. The major transcript of 1802 nucleotides contains a 529-codon open reading frame, which was predicted to encode a 57 kDa precursor protein. On the basis of peptide sequencing, the N-terminus of the mature protein is at position 163, suggesting that proteolytic processing events, in addition to signal peptide cleavage, generate the protein purified from the fungal culture filtrate. Expression studies in Escherichia coli with the cDNA as well as smaller subfragments demonstrated that a region of 47 amino acids located in the C-terminal third of the protein was sufficient to confer elicitor activity. The gene encoding the elicitor was found to be a member of a multigene family in P. megasperma. Homologous families of differing sizes were found in all eight other Phytophthora species tested, but not in other filamentous fungi including other Oomycetes. No significant similarity of the elicitor preprotein to sequences present in the databases has yet been detected.

Amino Acid Sequence↗
PubMed1991

Inhibition of pyruvate dehydrogenase complex (PDHC) by antipsychotic drugs.

The effects of 11 antipsychotic drugs on the pyruvate dehydrogenase complex (PDHC) prepared from bovine heart and rat brain were investigated. All inhibited PDHC to varying extents. With clinically equivalent doses, chlorpromazine and thioridazine inhibited the most and fluphenazine and thiothixene the least. The relationship of degree of inhibition of PDHC by neuroleptics to clinical improvement of 32 outpatients treated with acetazolamide and thiamine (A + T) ancillary therapy for chronic mental illness suggests that patients treated with psychoactive drugs that inhibit PDHC the least are most likely to have a favorable response with A + T treatment.

Acetazolamide↗
PubMed1990

Design of a 13C (1H) RF probe for monitoring the in vivo metabolism of [1-13C]glucose in primate brain.

The design of an RF probe suitable for obtaining proton-decoupled 13C spectra from a subhuman primate brain is described. Two orthogonal saddle coils, one tuned to the resonant frequency of 13C and the other to the resonant frequency of 1H, were used to monitor the in vivo metabolism of [1-13C]glucose in rhesus monkey brain at 2.1 T. Difference spectra showed the appearance of 13C-enriched glutamate and glutamine 30 to 40 min after a bolus injection of [1-13C]glucose.

Animals↗
PubMed1989

Acetazolamide and thiamine: an ancillary therapy for chronic mental illness.

Twenty-four chronic schizophrenic patients were treated successfully with the addition of acetazolamide and thiamine (A + T) to their unchanged existing therapies in a double-blind, placebo-controlled crossover study. Therapeutic effects were measured by the Scale for the Assessment of Positive Symptoms and the Scale for the Assessment of Negative Symptoms. Overall, 50% of the patients showed improvement on all assessment scales. No untoward effects occurred in these patients or in patients in previous studies who have been treated continuously with A + T therapy for as long as 3 years.

Acetazolamide↗
PubMed1985

Evidence for the cerebral uptake in vivo from two pools of glucose and the role of glucose-6-phosphatase in removing excess substrate from brain.

We propose the following scheme for cerebral uptake and overall metabolism of glucose in vivo: that brain selects from two pools of glucose anomers in arterial blood, that it takes up excess glucose, that glucose enters the brain tissue as glucose-6-phosphate through the actions of mutarotase and hexokinase, that some glucose-6-phosphate becomes metabolized to CO2 and some becomes incorporated into brain carbon pools, and that excess glucose-6-phosphate leaves brain through glucose-6-phosphatase and mutarotase activities. This results from our observations in arterio-venous studies for the determination of cerebral metabolism in humans in vivo that the cerebral uptake of [14C]glucose often appeared to differ from that of unlabeled glucose. With rapidly falling arterial radioactivity, unlabeled glucose uptake was more than [14C]glucose. With rising arterial radioactivity, [14C]glucose extraction exceeded unlabeled glucose. Studies with [14C]glucose-6-phosphate suggested that glucose-6-phosphatase in brain removes excess substrate by dephosphorylation. However, when arterial [14C]glucose increased slowly, [14C]glucose uptake varied considerably and the data resembled human cerebral metabolism of glucose anomers. An experiment employing [13C]glucose and NMR provided further support for our proposed scheme.

Animals↗
PubMed1983

A comparison of the cerebral uptake and metabolism of labeled glucose and deoxyglucose in vivo in rats.

Experimental evidence indicated that: 1) [14C]deoxyglucose-6-phosphate (14C-DG-6-P) in brain (and other rat tissues) did not increase with time after injection of 14C-DG, 2) 14C-DG-6-P in rat brain (and other tissues) did not correlate with glucose metabolism, 3) 14C-DG-6-P in rat brain (and other tissues) had a significant negative correlation with glucose-6-phosphatase activity. Further, arterio-venous studies in rats, in which the cerebral uptake and metabolism of labeled glucose were compared directly with those of labeled DG (and labeled fluorodeoxyglucose, FDG), employing double labeled techniques, showed that DG (and FDG) cannot be used to measure glucose uptake and/or metabolism.

Animals↗