Biomedical subjects

A J Williamson

Publications and source records attributed to A J Williamson.

9 recordsLinked to original sources

PubMed2005

Electron emission from diamondoids: a diffusion quantum Monte Carlo study.

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.

Journal Article↗

PubMed2004

Self-healing of CdSe nanocrystals: first-principles calculations.

Ab initio calculations of the structural, electronic, and optical properties of CdSe nanoparticles are presented. The atomic structures of the clusters are relaxed both in vacuum and in the presence of surfactant ligands. In both cases, we predict significant geometrical rearrangements of the nanoparticle surface while the wurtzite core is maintained. These reconstructions lead to the opening of an optical gap without the aid of passivating ligands, thus "self-healing" the surface electronic structure. Our calculations also predict the existence of a midgap state responsible for recently observed subband emission.

Journal Article↗

PubMed2003

Structural stability and optical properties of nanomaterials with reconstructed surfaces.

We present density functional and quantum Monte Carlo calculations of the stability and optical properties of semiconductor nanomaterials with reconstructed surfaces. We predict the relative stability of silicon nanostructures with reconstructed and unreconstructed surfaces, and we show that surface step geometries unique to highly curved surfaces dramatically reduce the optical gaps and decrease excitonic lifetimes. These predictions provide an explanation of both the variations in the photoluminescence spectra of colloidally synthesized nanoparticles and observed deep gap levels in porous silicon.

Journal Article↗

PubMed2002

Surface chemistry of silicon nanoclusters.

We employ density functional and quantum Monte Carlo calculations to show that significant changes occur in the gap of fully hydrogenated nanoclusters when the surface contains passivants other than hydrogen, in particular atomic oxygen. In the case of oxygen, the gap reduction computed as a function of the nanocluster size provides a consistent interpretation of several recent experiments. Furthermore, we predict that other double bonded groups also significantly affect the optical gap, while single bonded groups have a minimal influence.

Journal Article↗

PubMed2001

Linear-scaling quantum Monte Carlo calculations.

A method is presented for using truncated, maximally localized Wannier functions to introduce sparsity into the Slater determinant part of the trial wave function in quantum Monte Carlo calculations. When combined with an efficient numerical evaluation of these localized orbitals, the dominant cost in the calculation, namely, the evaluation of the Slater determinant, scales linearly with system size. This technique is applied to accurate total energy calculation of hydrogenated silicon clusters and carbon fullerenes containing 20-1000 valence electrons.

Journal Article↗

PubMed2000

Leaky scanning is the predominant mechanism for translation of human papillomavirus type 16 E7 oncoprotein from E6/E7 bicistronic mRNA.

Human papillomaviruses (HPV) are unique in that they generate mRNAs that apparently can express multiple proteins from tandemly arranged open reading frames. The mechanisms by which this is achieved are uncertain and are at odds with the basic predictions of the scanning model for translation initiation. We investigated the unorthodox mechanism by which the E6 and E7 oncoproteins from human papillomavirus type 16 (HPV-16) can be translated from a single, bicistronic mRNA. The short E6 5' untranslated region (UTR) was shown to promote translation as efficiently as a UTR from Xenopus beta-globin. Insertion of a secondary structural element into the UTR inhibited both E6 and E7 expression, suggesting that E7 expression depends on ribosomal scanning from the 5' end of the mRNA. E7 translation was found to be cap dependent, but E6 was more dependent on capping and eIF4F activity than E7. Insertion of secondary structural elements at various points in the region upstream of E7 profoundly inhibited translation, indicating that scanning was probably continuous. Insertion of the E6 region between Renilla and firefly luciferase genes revealed little or no internal ribosomal entry site activity. However when E6 was located at the 5' end of the mRNA, it permitted over 100-fold-higher levels of downstream cistron translation than did the Renilla open reading frame. Internal AUGs in the E6 region with strong or intermediate Kozak sequence contexts were unable to inhibit E7 translation, but initiation at the E7 AUG was efficient and accurate. These data support a model in which E7 translation is facilitated by an extreme degree of leaky scanning, requiring the negotiation of 13 upstream AUGs. Ribosomal initiation complexes which fail to initiate at the E6 start codon can scan through to the E7 AUG without initiating translation, but competence to initiate is achieved once the E7 AUG is reached. These findings suggest that the E6 region of HPV-16 comprises features that sponsor both translation of the E6 protein and enhancement of translation at a downstream site.

5' Untranslated Regions↗

PubMed1999

Immediate recall of oral contraceptive instructions: implications for providers.

OBJECTIVE: The object of the study was to determine the patient characteristics associated with inadequate recall of oral contraceptive pill-taking instructions. STUDY DESIGN: Sexually active women aged 13 to 40 years (n = 150) attending university-based family planning clinics completed anonymous self-report measures that assessed demographic and reproductive characteristics, understanding of pill-taking instructions, and contraceptive compliance. Logistic regression was used to determine factors associated with inadequate recall for the sample, stratified by minority versus nonminority women. RESULTS: Minority women with inadequate recall were almost 6 times more likely than minority women with adequate recall not to know the name of the prescribed oral contraceptive and were 3 times more likely to have less than a high school education. In addition there were 1-fold and 2-fold increases in likelihood of inadequate recall as certainty of pill-taking instructions and general oral contraceptive knowledge, respectively, decreased. Inadequate recall was associated with poor compliance. CONCLUSION: Women with inadequate recall may be identified at the conclusion of their visit so that interventions to enhance their pill-taking skills can be provided.

Adolescent↗

PubMed1978

Forebrain lesions and retention of imprinting in domestic chicks.

Newly-hatched domestic chicks were reared separately with one of two types of imprinting object hanging in their cages. On the 5th day discrimination tests demonstrated strong preferences for the familiar object. On the 6th day the chicks were subjected to bilateral aspiration lesions of either the anterior or the lateral forebrain and their imprinting objects removed from the cage. Discrimination tests were made on the 9th and 12th days, the imprinting objects replaced, and a final relearning test given on the 14th day. The anterior lesion group was slightly impaired in the retention and relearning tests while the lateral lesion group was drastically affected, being significantly worse than the anterior group and with choices only just significantly above random expectation in the retention tests and no different from random choice in the relearning test. It was concluded that a lateral forebrain region is necessary for retention and relearning of a previously acquired imprinted preference.

Animals↗

PubMed1975

A forebrain lesion preventing imprinting in domestic chicks.

Newly hatched domestic chicks were subjected to bilateral aspiration lesions of the anterior, dorsal, posterior or lateral regions of the forebrain and then were reared separately with one of two types of imprinting object. Between 3 and 10 days later they were tested for imprinting, pecking accuracy, visual looming and auditory startle responses. Lateral and dorsal lesion groups differed from a control group in the number of successful discriminations of their imprinting object and the lateral group was not significantly different from a random expectancy. All groups were impaired in pecking accuracy but the lateral group was the most severely affected. Looming responses and post startle freezing behaviour were reduced only in the posterior and dorsal groups which were also the least responsive in the imprinting tests. It was concluded that the lateral forebrain area is critically and specifically involved in the visual learning of imprinting being necessary for the discrimination of, but not for approach responsiveness to, imprinting objects.

Animals↗