PubMed Health⌕ Search

Biomedical subjects

A K Hassan

Publications and source records attributed to A K Hassan.

At least 19 recordsLinked to original sources
PubMed2005

Electron paramagnetic resonance shift in II1-xMnxVI diluted magnetic semiconductors in the presence of strong exchange coupling.

Electron paramagnetic resonance (EPR) has been investigated in two II1-xMnxVI alloys--Cd1-xMnxSe and Cd1-xMnxS--for a series of high Mn concentrations and at low temperatures T, i.e., under conditions where the spin subsystems in these materials are strongly coupled. We have observed a very significant shift of the resonance field from the EPR position of Mn2+ ions that increases with increasing x and with decreasing T. Furthermore, the use of multiple frequencies has allowed us to attribute the observed shift to an internal field that originates from the spin sublattice within the II1-xMnxVI host.

Journal Article↗
PubMed2005

Pyroelectric and conduction properties of Z-type calix[4] acid Langmuir-Blodgett films.

Non-centrosymmetric Z-type Langmuir-Blodgett (LB) films were prepared by transferring a calix[4] acid monolayer from a subphase of Millipore water (18 Momegacm(-1)) onto aluminised glass substrates. Electrical measurements were performed at room temperature on LB films with a sandwich structure comprising a 50 nm thick thermally evaporated aluminium film. A pyroelectric figure of merit of 2.23 microC m(-2)K(-1) is reported for this LB system. The low voltage value of conductivity is 1.82 x 10(-12) Sm(-1). The electrode-limited Schottky effect is responsible for the conduction mechanism at a relatively high field due to the dc bias and the barrier potential height is determined to be 1.72 eV. The ac conductance for both samples shows a typical power law dependence with a value of approximately 0.86 for the exponent.

Aluminum↗
PubMed2003

Collective character of spin excitations in a system of Mn2+ spins coupled to a two-dimensional electron gas.

We have studied the low energy spin excitations in n-type CdMnTe based dilute magnetic semiconductor quantum wells. For magnetic fields for which the energies for the excitation of free carriers and Mn spins are almost identical, an anomalously large Knight shift is observed. Our findings suggest the existence of a magnetic-field-induced ferromagnetic order in these structures, which is in agreement with recent theoretical predictions [Phys. Rev. Lett. 91, 077202 (2003)]].

Journal Article↗
PubMed2001

Single-ion versus dipolar origin of the magnetic anisotropy in iron(III)-oxo clusters: a case study.

A multitechnique approach has allowed the first experimental determination of single-ion anisotropies in a large iron(III)-oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6ClO4 (1) in which Hpmdbm = 1,3-bis(4-methoxyphenyl)-1,3-propanedione. High-frequency EPR (HF-EPR). bulk susceptibility measurements, and high-field cantilever torque magnetometry (HF-CTM) have been applied to iron-doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12-(pmdbm)6]ClO4, whose synthesis and X-ray structure are also presented. HF-EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard-axis type anisotropy with DFe = 0.43(1) cm(-1) and EFe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = DFe[S2(z) - S(S + 1)/3] + Fe[S2(x) - S2(y)]. HF-CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster, Z. The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Z are negative, DFe(ZZ)= -0.164(4) cm(-1). Due to the dominant antiferromagnetic coupling, a negative DFe(ZZ) value determines a hard-axis molecular anisotropy in 1, as experimentally observed. By adding point-dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D1 = 4.70(9) cm(-1), is in excellent agreement with that determined by inelastic neutron scattering experiments at 2 K, D1 = 4.57(2) cm(-1). Iron-doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm = dibenzoylmethane), have also been investigated by HF-EPR at 525 GHz. The single-ion anisotropy is of the hard-axis type as well, but the DFe parameter is significantly larger [DFe = 0.770(3) cm(-1). EFe = 0.090(3) cm(-1)]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single-ion terms can contribute significantly to the magnetic anisotropy of iron(III)-oxo clusters, which are currently investigated as single-molecule magnets.

Journal Article↗
PubMed2000

Direct determination of the single-ion anisotropy in a one-dimensional magnetic system by high-field EPR spectroscopy; synthesis, EPR, and X-ray structure of NixZn1-x(C2O4)(dmiz)2.

The synthesis, X-ray structure, and EPR measurements of the integer-spin linear-chain antiferromagnet [Ni(ox)(dmiz)2] (where ox = C2O4(2-) and dmiz = 1,2-dimethylimidazole) are presented. The sign and size of the single-ion zero field splitting (Zfs) of the divalent Ni have been determined by high field/high-frequency EPR spectroscopy. The spectra of powder samples of the derivatives [NixZn1-x(C2O4)(dmiz)2] for x = 0.09 and 0.07, at frequencies ranging from 110 to 440 GHz allowed the accurate determination of the zfs parameters D and E, with D = 1.875(4) cm(-1) and E = 0.38 cm(-1). The X-ray structure has been determined from measurements on a single crystal with x = 0.07. Structural parameters are as follows: a = 14.5252(7) A, b = 12.1916(8) A, c = 8.6850(8) A,beta = 97.460(6)degrees in space group C2/c. The zigzag chain contains octahedrally coordinated metal ions with two cis-oriented N-coordinated dmiz ligands and two cis-oriented, tetradentate bridging oxalato(2-) ligands, together resulting in a MN2O4 donor set. The structure was refined to a conventional R value of 0.073 for 1,051 observed reflections. Zn-O distances are 2.167(5) A and Zn-N = 2.098 A. Coordination angles vary for cis angles from 78.4 to 100.7 degrees, with trans angles varying from 163.9 degrees to 165.5 degrees.

Journal Article↗
PubMed2000

Ultrawide band multifrequency high-field EMR technique: A methodology for increasing spectroscopic information.

We report methodology that combines an ultrawide band multifrequency microwave system with technology of high magnetic fields for solving challenging problems in electron magnetic resonance (EMR) spectroscopy. This strategy has been made possible due to a novel EMR facility operating in an exceptionally wide range of microwave frequencies of 24 GHz to 3 THz, at magnetic fields up to 17 T, and in the temperature range of 1.6 to 330 K. The basic configuration of the multifrequency system works in a transmission mode and employs oversized cylindrical waveguides for routing the microwave power. A wide-band, low-noise, liquid helium cooled (4.2 K) InSb bolometer is used for signal detection. This approach results in an extremely wide-band performance, thus making it possible to employ a variety of solid-state millimeter and submillimeter microwave sources in combination with a far infrared laser microwave source for performing multifrequency EMR experiments. A complexity of resonant structures and related technical problems such as microphonics at high magnetic fields is virtually eliminated. The system is simple, yet sensitive, and has been revealed to be extremely advantageous while solving such problems as observation of AFMR transitions in spin-ordered systems, g-factor resolution enhancement in complex organic radicals, and resonance signal detection in EMR-silent spin systems having integer spin and large zero field splitting. A technical description of the multifrequency high-field EMR facility is presented and results of its performance tests are given. The potential utility of using the multifrequency high-field methodology in EMR studies is illustrated with selected examples of its recent applications.

Electron Spin Resonance Spectroscopy↗
PubMed2000

Reasons for tooth extraction among patients in Sebha, Libyan Arab Jamahiriya: a pilot study.

We investigated the causes of tooth extraction among patients in Sebha in southern Libyan Arab Jamahiriya. A total of 600 patients who had undergone tooth extraction were surveyed. We found more males (68%) than females had had teeth extracted and also patients with no education (53%). Caries and periodontal disease were the main causes of tooth extraction. For those patients who opted for tooth extraction over other treatment, they mostly did so for financial reasons. There was generally poor oral hygiene and oral knowledge, which should be tackled.

Adolescent↗
PubMed1999

Regulation of initiation of Bacillus subtilis chromosome replication.

Bacterial chromosome replication is tightly regulated at the initiation stage to coordinate with mass increase. Together with chromosome partition at cell division, this regulation mechanism ensures the proper number of chromosomes in daughter cells at any growth rate. Therefore, elucidation of this regulation mechanism is important for understanding the bacterial cell cycle. Despite much effort in Escherichia coli and Bacillus subtilis for many years, the mechanism remains to be completely elucidated. In E. coli, it is proposed that a critical amount of DnaA protein determines the time of initiation of replication in the cell cycle. Our study strongly suggested that this might not be the case in B. subtilis. Recently, remarkable progress has been made in bacterial cytology. The new techniques enable us to examine the subcellular location of proteins of interest and DNA regions of the chromosome (for example, the replication origin) and, therefore, to determine directly when in the cell division cycle and where within the cell initiation of chromosome replication takes place. Using the techniques, we detected the initiation complex by examining subcellular location of several Dna-initiation proteins in B. subtilis. Based on our new findings, we propose a novel model for regulation of the time of initiation of chromosome replication in the cell cycle.

Bacillus subtilis↗
PubMed1998

A Bacillus subtilis gene-encoding protein homologous to eukaryotic SMC motor protein is necessary for chromosome partition.

We have analysed the function of a gene of Bacillus subtilis, the product of which shows significant homology with eukaryotic SMC proteins essential for chromosome condensation and segregation. Two mutant strains were constructed; in one, the expression was under the control of the inducible spac promoter (conditional null) and, in the other, the gene was disrupted by insertion (disrupted null). Both could form colonies at 23 degree C but not at 37 degree C in the absence of the expression of the Smc protein, indicating that the B. subtilis smc gene was essential for cell growth at higher temperatures. Microscopic examination revealed the formation of anucleate and elongated cells and diffusion of nucleoids within the elongated cells in the disrupted null mutant grown at 23 degree C and in the conditional null mutant grown in low concentrations of IPTG at 37 degree C. In addition, immunofluorescence microscopy showed that subcellular localization of the SpoOJ partition protein was irregular in the smc disrupted null mutant, compared with bipolar localization in wild-type cells. These results indicate that the B. subtilis smc gene is essential for chromosome partition. The role of B. subtilis Smc protein in chromosome partition is discussed.

Bacillus subtilis↗
PubMed1997

Mechanism of anucleate cell production in the oriC-deleted mutants of Bacillus subtilis.

We constructed oriC-deleted mutants of Bacillus subtilis by integrating the minimal replication region of plasmid pLS32 into the proA (115 degrees), spoIIIJ (360 degrees) and thrS (256 degrees) loci of the chromosome, respectively. All three mutants produced anucleate cells and the DNA/protein ratio was lower than that of the wild-type strain when grown in nutrient broth. However, when grown in minimal-glucose medium, the frequency of anucleate cells was reduced in all of them and the DNA/protein ratio was restored to normal. Especially, the oriC-deleted mutant in which the plasmid was integrated near oriC produced almost no anucleate cell. These results indicate that initiation frequency of chromosome replication from the integrated plasmid origin were reduced disproportionately to cell mass increase in rich medium, which in turn disrupted coordination between DNA replication cycle and cell division cycle. The locations of the plasmid origin relative to the natural oriC locus affected the production of anucleate cell remarkably, suggesting that partition mechanism of chromosome was also impaired by the translocation of its replication origin.

Bacillus subtilis↗
PubMed1997

Suppression of initiation defects of chromosome replication in Bacillus subtilis dnaA and oriC-deleted mutants by integration of a plasmid replicon into the chromosomes.

We constructed Bacillus subtilis strains in which chromosome replication initiates from the minimal replicon of a plasmid isolated from Bacillus natto, independently of oriC. Integration of the replicon in either orientation at the proA locus (115 degrees on the genetic map) suppressed the temperature-sensitive phenotype caused by a mutation in dnaA, a gene required for initiation of replication from oriC. In addition, in a strain with the plasmid replicon integrated into the chromosome, we were able to delete sequences required for oriC function. These strains were viable but had a slower growth rate than the oriC+ strains. Marker frequency analysis revealed that both pyrD and metD, genes close to proA, showed the highest values among the markers (genes) measured, and those of other markers decreased symmetrically with distance from the site of the integration (proA). These results indicated that the integrated plasmid replicon operated as a new and sole origin of chromosome replication in these strains and that the mode of replication was bidirectional. Interestingly, these mutants produced anucleate cells at a high frequency (about 40% in exponential culture), and the distribution of chromosomes in the cells was irregular. A change in the site and mechanism (from oriC to a plasmid system) of initiation appears to have resulted in a drastic alteration in coordination between chromosome replication and chromosome partition or cell division.

Aldehyde Oxidoreductases↗
PubMed1996

Structure of the dnaA region of the endosymbiont, Buchnera aphidicola, of aphid Schizaphis graminum.

Buchnera aphidicola is an intracellular prokaryote (endosymbiont) that lives in the body cavity of the aphid. Phylogenetic studies indicated that it is closely related to Escherichia coli and members of Enterobacteria. The gene order of the region containing the dnaA gene is well conserved in many bacteria. Seven genes of the endosymbiont of the aphid Schizaphis graminum, gyrB, dnaN, dnaA, rpmH, rnpA, yidD, and 60K. were found to be homologous in sequence and relative location to those of E. coli. We have further sequenced the region downstream of the 60K gene to elucidate the boundary of the conserved region, and found that one more gene, thdF, is conserved. The comparison of gene organizations of the dnaA region of the related bacteria supported the close phylogenetic relationship of B. aphidicola to E. coli. In addition, we have identified groES and groEL genes next to the thdF gene. GroEL protein was reported to be expressed at an elevated level in the endosymbionts of aphids, and is considered to play an important role in their association with the aphid host. Comparison of the structure of the groE operon with that of the endosymbiont of the aphid Acyrthosiphon pisum revealed the conservation of a sequence resembling the E. coli consensus heat shock promoter, and this sequence may be responsible for the high expression of the groEL gene in aphid endosymbionts.

Animals↗
PubMed1989

Indirect haemagglutination (IHA) and immunoelectrophoresis in the diagnosis of hydatidosis in Sudanese camels.

The efficiency of indirect haemagglutination (IHA) and immunoelectrophoresis (IEP) as means of detection of cystic hydatid infection in camels was investigated. Both IHA and IEP showed very low detection rates of antibody in camel sera. The IEP showed 84 p. 100 specificity and 36 p. 100 sensitivity while the IHA showed 69 p. 100 specificity and 43 p. 100 sensitivity. These results are discussed.

Animals↗
PubMed1986

A technique to obviate the risk of inadvertent infection of cell cultures with bovine viral diarrhoea virus.

When bovine embryonic kidneys collected at the Gorgie Abattoir, Edinburgh were examined for evidence of infection with bovine viral diarrhoea virus (BVDV), 11 out of 26 kidneys were found to be positive. A technique that detected the presence of inadvertent BVDV in cell cultures consisted of processing and digesting the kidneys to produce cell suspensions, adding dimethyl sulphoxide and dispersing the suspensions in small aliquots that were stored frozen at - 114 degrees C. One aliquot was cultured and screened for BVDV by indirect immuno-fluorescence and interference tests. Bovine embryonic kidney cells so processed retained their viability and virus susceptibility for 15 to 18 months. Selected stocks of "clean" cells only are then used for vaccine production or diagnostic purposes. The cytopathic NADL strain of BVDV multiplied in naturally infected cell cultures but the titres attained were significantly lower than in "clean" cell cultures.

Animals↗
PubMedDate not supplied

Root caries among Benghazi patients.

A total of 420 Benghazi patients were clinically examined for root caries. Root caries was not limited to older patients and the proportion of root caries among young adults was similar to that of the older age groups. Most of the lesions were located proximally and underneath restorations rather than labially or lingually. Our results may serve as baseline data for further studies on root caries in the Libyan Arab Jamahiriya.

Adolescent↗