17,871 research outputs found
Molecular similarity of MDR inhibitors
Everyone is free to re-use the published material if proper accreditation/citation of the original publication is given. http://creativecommons.org/licences/by/3.0/The molecular similarity of multidrug resistance (MDR) inhibitors was evaluated using the point centred atom charge approach in an attempt to find some common features of structurally unrelated inhibitors. A series of inhibitors of bacterial MDR were studied and there is a high similarity between these in terms of their shape, presence and orientation of aromatic ring moieties. A comparison of the lipophilic properties of these molecules has also been conducted suggesting that this factor is important in MDR inhibition.Peer reviewe
Dilaton Black Holes Near the Horizon
Generic 4-d black holes with unbroken supersymmetry are shown
to tend to a Robinson-Bertotti type geometry with a linear dilaton and doubling
of unbroken supersymmetries near the horizon. Purely magnetic dilatonic black
holes, which have unbroken supersymmetry, behave near the horizon as a
2-d linear dilaton vacuum . This geometry is invariant under 8
supersymmetries, i.e. half of the original supersymmetries are unbroken.
The supersymmetric positivity bound, which requires the mass of the 4-d dilaton
black holes to be greater than or equal to the central charge, corresponds to
positivity of mass for a class of stringy 2-d black holes.Comment: 10 pages, SU-ITP-92-2
The Geometry of Small Causal Diamonds
The geometry of causal diamonds or Alexandrov open sets whose initial and
final events and respectively have a proper-time separation
small compared with the curvature scale is a universal. The corrections from
flat space are given as a power series in whose coefficients involve the
curvature at the centre of the diamond. We give formulae for the total 4-volume
of the diamond, the area of the intersection the future light cone of
with the past light cone of and the 3-volume of the hyper-surface of
largest 3-volume bounded by this intersection valid to .
The formula for the 4-volume agrees with a previous result of Myrheim.
Remarkably, the iso-perimetric ratio depends only on the energy density at the centre and is bigger
than unity if the energy density is positive. These results are also shown to
hold in all spacetime dimensions. Formulae are also given, valid to next
non-trivial order, for causal domains in two spacetime dimensions. We suggest a
number of applications, for instance, the directional dependence of the volume
allows one to regard the volumes of causal diamonds as an observable providing
a measurement of the Ricci tensor.Comment: 17 pages, no figures; Misprints in eqs.(62), (65), (66) and (81)
corrected; a new note on page 13 (with 2 new equations) adde
Branes as BIons
A BIon may be defined as a finite energy solution of a non-linear field
theory with distributional sources. By contrast a soliton is usually defined to
have no sources. I show how harmonic coordinates map the exteriors of the
topologically and causally non-trivial spacetimes of extreme p-branes to BIonic
solutions of the Einstein equations in a topologically trivial spacetime in
which the combined gravitational and matter energy momentum is located on
distributional sources. As a consequence the tension of BPS p-branes is
classically unrenormalized. The result holds equally for spacetimes with
singularities and for those, like the M-5-brane, which are everywhere
singularity free.Comment: Latex, 9 pages, no figure
The Action of Instantons with Nut Charge
We examine the effect of a non-trivial nut charge on the action of
non-compact four-dimensional instantons with a U(1) isometry. If the instanton
action is calculated by dimensionally reducing along the isometry, then the nut
charge is found to make an explicit non-zero contribution. For metrics
satisfying AF, ALF or ALE boundary conditions, the action can be expressed
entirely in terms of quantities (including the nut charge) defined on the fixed
point set of the isometry. A source (or sink) of nut charge also implies the
presence of a Misner string coordinate singularity, which will have an
important effect on the Hamiltonian of the instanton.Comment: 25 page
Bohm and Einstein-Sasaki Metrics, Black Holes and Cosmological Event Horizons
We study physical applications of the Bohm metrics, which are infinite
sequences of inhomogeneous Einstein metrics on spheres and products of spheres
of dimension 5 <= d <= 9. We prove that all the Bohm metrics on S^3 x S^2 and
S^3 x S^3 have negative eigenvalue modes of the Lichnerowicz operator and by
numerical methods we establish that Bohm metrics on S^5 have negative
eigenvalues too. We argue that all the Bohm metrics will have negative modes.
These results imply that higher-dimensional black-hole spacetimes where the
Bohm metric replaces the usual round sphere metric are classically unstable. We
also show that the stability criterion for Freund-Rubin solutions is the same
as for black-hole stability, and hence such solutions using Bohm metrics will
also be unstable. We consider possible endpoints of the instabilities, and show
that all Einstein-Sasaki manifolds give stable solutions. We show how Wick
rotation of Bohm metrics gives spacetimes that provide counterexamples to a
strict form of the Cosmic Baldness conjecture, but they are still consistent
with the intuition behind the cosmic No-Hair conjectures. We show how the
Lorentzian metrics may be created ``from nothing'' in a no-boundary setting. We
argue that Lorentzian Bohm metrics are unstable to decay to de Sitter
spacetime. We also argue that noncompact versions of the Bohm metrics have
infinitely many negative Lichernowicz modes, and we conjecture a general
relation between Lichnerowicz eigenvalues and non-uniqueness of the Dirichlet
problem for Einstein's equations.Comment: 53 pages, 11 figure
Fixed Scalars and Suppression of Hawking Evaporation
For an extreme charged black hole some scalars take on a fixed value at the
horizon determined by the charges alone. We call them fixed scalars. We find
the absorption cross section for a low frequency wave of a fixed scalar to be
proportional to the square of the frequency. This implies a strong suppression
of the Hawking radiation near extremality. We compute the coefficient of
proportionality for a specific model.Comment: 10 pages, late
Gravitational Instantons, Confocal Quadrics and Separability of the Schr\"odinger and Hamilton-Jacobi equations
A hyperk\"ahler 4-metric with a triholomorphic SU(2) action gives rise to a
family of confocal quadrics in Euclidean 3-space when cast in the canonical
form of a hyperk\"ahler 4-metric metric with a triholomorphic circle action.
Moreover, at least in the case of geodesics orthogonal to the U(1) fibres, both
the covariant Schr\"odinger and the Hamilton-Jacobi equation is separable and
the system integrable.Comment: 10 pages Late
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