4,841 research outputs found
Representativeness of samples from general practice lists in epidemiological studies: case-control study
Ethical constraints often prevent epidemiological studies from evaluating the impact of non-participation. Particular problems may arise when subjects fail to respond to an approach by researchers or when they cannot be contacted because of inaccurate contact details or a doctor's refusal to give permission for their patient to be approached. If these subjects differ from those subjects who agree or decline to participate then the validity and generalisability of the study may be compromised. We investigated these issues in a case-control study of acute leukaemia in England
Studies in the acridine series
In the Annual Reports on the Progress of Chemistry
for 1918 under the heading "chromoisomerism" the following
passage may be found: but it seems tine to protest
against one halfpennyworth of practice to this intolerable
deal of theory, and to demand something more than assertion
in proof of the constitutional formulae which are drawn up
so lavishly." It is fairly clear that the reviewer is
referring principally to Hantzsch who originated the term
"chromoisomeric" to describe compounds which exist in two
or more solid modifications with distinctly different colours.
Hantzsch (1) supposed that all such cases are to be explained
in terms of isomerism and he proposed many structures.Hantzsch, however, was right in looking for an
explanation and his reviewers were wrong where they dismissed
the phenomena as being "merely polymorphism" (2).
Polymorphism perhaps; but where, as in some cases, the
colour change is pronounced - say from green to red - this
is hardly a sufficient explanation.In the sequel the purely descriptive term polychromism
will be used,in place of Hantzsch's "chromoisomerism'' to
avoid the implication that isomerism is necessarily involved.We know that electronic absorption is not exclusively a
function of molecular structure; it may be affected by the
polar environment of the molecule and also by more specific
intermolecular forces such as hydrogen bonding or charge
transfer interaction. A crystal lattice may be regarded
as a structure of intermolecular interactions; to change
the lattice is to change these interactions and, in so far
as some of these may influence light absorption, to change
the colour.But the problem remains. If there is no difference
in the structure or conformation of the molecules in
Lifferent polychromic states then there must be a difference
in one or other of those forms of intermolecular interaction
which can have a significant effect on colour and it should
be possible to specify which of these is involved.Most polychromic organic compounds for which no
evident explanation in terms of isomerism exists, are either
salts or aromatic nitro -compounds (usually polynitro-compounds
and usually phenols or amines). The origin of polychromism
in these cases has not been fully established, but it may
well be that comparatively recent work on charge transfer
phenomena will provide at least a partial answer. Charge
transfer might be expected to occur in the solid state with
both of these classes of compounds.It is well known that aromatic polynitro-compounds can
form coloured molecular compounds with aromatic hydrocarbons,
amines,phenols etc. The formation of such compounds has
been attributed to charge transfer interaction between the
components one of which must be a Lewis acid (e.g. a nitro - compound) and the other á Lewis base (e.g. an aromatic
hydrocarbon or amine) (3 a - e).This type of interaction gives rise to a new absorption
band, which has been called the charge transfer spectrum,
in the visible or near ultra -violet regions. It appears
to be due to the occurrence of electronic transitions
between rather than within molecules. Charge transfer
spectra are always broad and without fine structure. This
is probably due (3a) to the weakness of the charge transfer
bonding (2 - 4 K.cals. /mole (4)) allowing thermal vibration
to provide a considerable range of displacements and
orientations between the interacting groups. If the
energy required for these intermolecular transitions does
indeed depend upon the orientation of the interacting groups
in the crystal - and this seems probable - then the colour
of the solid will be a function of crystal structure. Thus
where a compound, which shows charge transfer interaction in
the solid, is polymorphic it will in general be polychromie.This charge transfer hypothesis for polychromism
must be regarded as a more modern version of a theory
put forward by Pfeiffer (8) as early as 1915 in terms of
residual valency. He proposed that the orange and yellow
forms of nitromethoxystilbenes result from the orientations
A and B respectively in the solids.No work appears to have been done in order to decide
whether charge transfer can occur in organic salts: its
occurrence in inorganic salts appears to be very common.
It is too early therefore to judge whether an extension
of the charge transfer idea to cover the numerous cases of
polychromism in organic salts is justifiable. A purely
electrostatic theory has been proposed by Lucas and Kemp (9)
to explain the polychromism of organic and inorganic salts.
Their general conclusion is that the electronic absorption
of an ion in a crystal lattice will depend on its
electrostatic environment, created by the surrounding ions
of opposite charge, and that this will depend on the crystal
structure.Although at present it is not possible to decide which,
if any, of these theories is true, one thing is clear: where
a compound exists in more than one differently coloured solid
form it is not possible to conclude directly from this that
different molecular structures must be assigned to these
forms. On the other hand a knowledge of the general classes
of compounds which show polychromism and an appreciation of
the factors which may influence light absorption in a crystal
will help in deciding, in specific cases, whether it is worth
looking for an explanation in terms of isomerism. For
example if the compound in question is a salt or a polynitrocompound, or indeed if it is a betaine or contains both
Lewis acid and Lewis base functions, any "isomeric" theory
will be somewhat unconvincing. But the converse also is true.According to Kehrmann and Matusinsky (10) 2- hydroxy5- phenylacridine crystallises from hot benzene as fine
yellow needles with a melting point of 264 °C. On
crystallising slowly from cold benzene red prisms are obtained
which may be converted to the yellow form by heating at 135 °C.
The red modification is formed from the yellow slowly on
standing and rapidly by crushing and powdering. From these
observations it was concluded that the yellow is the form
stable at higher temperatures and the red the lower
temperature stable form.In view of the difference of more than 100 °C between
the melting points of the two forms and in view of their
strong difference in colour, Kehrmann suggested that this
was a case of tautomerism between the structures.Neither argument is very convincing and the great ease
with which the red modification can be formed from the yellow
would seem to weigh heavily in favour of an explanation in
terms of polymorphism. But if this example is considered
in the context of polychromism in general and in the light
of work carried out more recently by John (11) and by Albert
and Short (12) on the tautomerism of analogous compounds,
Kehrmann's theory becomes distinctly more probable.In the first place 2- hydroxy- 5- phenylacridine does not
fall into any of the general classes of polychromic compounds:
8.
this appears to be the only published example of a polychromic
free acridine (although polychromism in acridine salts is
very common). On the other hand John has studied a group
of 2- hydroxy -phenazine derivatives.This compound exists in a yellow and a deep violet
modification,these colours corresponding to the colours of
the 0- Methyl and N- Methyl derivatives respectively. In
solution 1:3:4 trimethyl- 2- hyäroxyphenazine is present
as an equilibrium mixture of the structures(IIIa) and (IIIb).
John concluded that the yellow and violet modifications were
to be identified with (IIIa) and (IIIb) respectively. In
this case however interconversion between the solids can
only be brought about by recrystallising from solution or
by heating to 135 °C at which temperature sublimation can be
seen to occur.Albert and Short (12) have shown that 2- hydroxyacridine
also is tautomeric (although not polychromic) existing in
solution as an equilibrium mixture of lactim and lactam
structures analogous to (I) and (II). Here the lactim
structure is yellow and the lactam structure red.Kehrmann and P+iatusinsky's theory might now seem to be
so reasonable as to be hardly worth questioning. But
there remains one difficulty: the yellow crystals of
2- hydroxy- 5- phenylacridine can be converted to the red
modification simply by rub-uing. If this is a tautomeric
change it occurs with remarkable facility.It was therefore decided to investigate more
thoroughly the colour changes of 2- hydroxy -5- phenylacridine
Designing a web-application to support home-based care of childhood CKD stages 3-5: Qualitative study of family and professional preferences
Background: There is a lack of online, evidence-based information and resources to support home-based care of childhood CKD stages 3-5. Methods. Qualitative interviews were undertaken with parents, patients and professionals to explore their views on content of the proposed online parent information and support (OPIS) web-application. Data were analysed using Framework Analysis, guided by the concept of Self-efficacy. Results: 32 parents, 26 patients and 12 professionals were interviewed. All groups wanted an application that explains, demonstrates, and enables parental clinical care-giving, with condition-specific, continously available, reliable, accessible material and a closed communication system to enable contact between families living with CKD. Professionals advocated a regularly updated application to empower parents to make informed health-care decisions. To address these requirements, key web-application components were defined as: (i) Clinical care-giving support (information on treatment regimens, video-learning tools, condition-specific cartoons/puzzles, and a question and answer area) and (ii) Psychosocial support for care-giving (social-networking, case studies, managing stress, and enhancing families' health-care experiences). Conclusions: Developing a web-application that meets parents' information and support needs will maximise its utility, thereby augmenting parents' self-efficacy for CKD caregiving, and optimising outcomes. Self-efficacy theory provides a schema for how parents' self-efficacy beliefs about management of their child's CKD could potentially be promoted by OPIS. © 2014 Swallow et al.; licensee BioMed Central Ltd
Progress report on the geology of 1:50k sheet 64W (Newtonmore)
This report describes the results of solid geology fieldwork in 1:50 000 sheet 64W
(Newtonmore) resulting from the 2002 summer mapping season. A full revision of the solid
geology at 1:10 000 scale was completed in the north-western part of the sheet (Sheet NN69SE)
while rapid mapping/reconnaissance of the solid geology has been carried out in the remaining
85% of the sheet area The superficial geology of the sheet has been completely revised and will
be described in a separate report.
The north-western part of the sheet contains the transition from the deeper water graded sandy
and silty turbidite deposits of the Corrieyairack Subgroup upwards (and south-eastwards) into
the shallow water sand-dominated deposits of the Strathtummel Subgroup. East of the A9 trunk
road, the Gaick region is confirmed as a single lithostratigraphical package in the Strathtummel
Subgroup recording shallow water depositional conditions, greatly thickened by D2 recumbent
folding. Axial surfaces of these folds dip gently east overall with gently east plunging to subhorizontal fold axes. Axial traces are generally N-S trending. The main regional (biotite)
schistosity is axial planar to these folds and locally, can be seen clearly deforming an earlier
bedding near-parallel biotite fabric. The available evidence for stratigraphical younging is
limited to a few well-washed river sections but shows that regional facing is always to the south
in S2 across the Gaick region. No large-scale F1 folds are recognised with the exception of those
at Crubenmore on the A9. Minor undulations of the main regional fabric mean that the sheet dip
varies between gently north to gently east across open upright north-east-plunging folds, in
marked contrast to the conspicuous pattern of reclined, north-west verging D3 folds deforming
the main regional (S2) schistosity in Glen Truim and farther north-west. There appear to be no
other major fold sets across this part of the Gaick region The Drummochter Dome thus takes the
form of a stack of recumbent D2 folds, modified by steep zones to the north-west (Geal CharnOssian Steep Belt) and south-east (Tummel Steep belt and correlatives). The pattern of early
recumbent folds and later steep belts is similar to that seen in the higher structural levels south of
the Boundary Slide which include the Tay Nappe
Self-replication and evolution of DNA crystals
Is it possible to create a simple physical system that is capable of replicating itself? Can such a system evolve interesting behaviors, thus allowing it to adapt to a wide range of environments? This paper presents a design for such a replicator constructed exclusively from synthetic DNA. The basis for the replicator is crystal growth: information is stored in the spatial arrangement of monomers and copied from layer to layer by templating. Replication is achieved by fragmentation of crystals, which produces new crystals that carry the same information. Crystal replication avoids intrinsic problems associated with template-directed mechanisms for replication of one-dimensional polymers. A key innovation of our work is that by using programmable DNA tiles as the crystal monomers, we can design crystal growth processes that apply interesting selective pressures to the evolving sequences. While evolution requires that copying occur with high accuracy, we show how to adapt error-correction techniques from algorithmic self-assembly to lower the replication error rate as much as is required
Temporal variations in English Populations of a forest insect pest, the green spruce aphid (Elatobium abietinum), associated with the North Atlantic Oscillation and global warming
Based on an exceptionally long modern ecological dataset (41 years), it has been possible to show that warm weather in England associated with a positive North Atlantic Oscillation (NAO) index causes the spring migration of the green spruce aphid (Elatobium abietinum), a pest species of spruce trees (Picea) to start earlier, continue for longer and contain more aphids. An upward trend in the NAO index during the period 1966-2006 is associated with an increasing population size of E. abietinum. It is important to understand the mechanisms behind the population fluctuations, because this aphid causes considerable damage to Picea plantations. Present day weather associated fluctuations in forest insect pests may be useful analogues in understanding past pest outbreaks in forests
Intellectual functioning in clinically confirmed fetal valproate syndrome
Background: An increased risk of impaired intelligence (IQ) has been documented in valproate-exposed children, but investigations have not previously focused on those with a clinical diagnosis of Fetal Valproate Syndrome (FVS). Methods: This cross sectional observational study recruited individuals with a diagnosis of FVS and completed standardized assessments of intellectual abilities making comparisons to a normative comparison group. Both mean difference (MD) and prevalence of scores below the lower average range were analyzed. Results: The mean full-scale IQ in 31 individuals with FVS (mean age 14.97; range 6–27 years) was 19 points lower (19.55, 95% CI −24.94 to 14.15), and IQ scores <70 were present in 26%. The mean differences for verbal comprehension (21.07, 95% CI −25.84 to −16.29), working memory (19.77, 95% CI −25.00 to −14.55) and processing speed (16.87, 95% CI −22.24 to −11.50) performances were poorer than expected with the mean differences over one standard deviation from the comparison group. Sixty one percent of cases demonstrated disproportionately lower verbal comprehension ability. There were no significant group differences for IQ in high vs. moderate dose valproate or mono vs. polytherapy. There were no differences in IQ between those with and those without a major congenital malformation. The requirement for educational intervention was high at 74%. Conclusion: Intellectual difficulties are a central feature of FVS and are more severe in their presentation in individuals with a diagnosis of valproate embryopathy. Individuals with FVS who present with the characteristic facial presentation should be considered at high risk of cognitive difficulties regardless of the dose of valproate exposure or the presence of a major congenital malformation
Electron Spin Injection at a Schottky Contact
We investigate theoretically electrical spin injection at a Schottky contact
between a spin-polarized electrode and a non-magnetic semiconductor. Current
and electron density spin-polarizations are discussed as functions of barrier
energy and semiconductor doping density. The effect of a spin-dependent
interface resistance that results from a tunneling region at the
contact/semiconductor interface is described. The model can serve as a guide
for designing spin-injection experiments with regard to the interface
properties and device structure.Comment: 4 pages, 4 figure
High-Energy Aspects of Solar Flares: Overview of the Volume
In this introductory chapter, we provide a brief summary of the successes and
remaining challenges in understanding the solar flare phenomenon and its
attendant implications for particle acceleration mechanisms in astrophysical
plasmas. We also provide a brief overview of the contents of the other chapters
in this volume, with particular reference to the well-observed flare of 2002
July 23Comment: This is the introductory article for a monograph on the physics of
solar flares, inspired by RHESSI observations. The individual articles are to
appear in Space Science Reviews (2011
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