256 research outputs found
Generalised -structures and type IIB superstrings
The recent mathematical literature introduces generalised geometries which
are defined by a reduction from the structure group of the vector
bundle to a special subgroup. In this article we show that
compactification of IIB superstring vacua on 7-manifolds with two covariantly
constant spinors leads to a generalised -structure associated with a
reduction from SO(7,7) to . We also consider compactifications
on 6-manifolds where analogously we obtain a generalised SU(3)-structure
associated with , and show how these relate to generalised
-structures.Comment: 14 pages, v2: Section 4 rewritten and references added, final version
of the paper to appear in JHE
Fluxes in M-theory on 7-manifolds: G_2-, SU(3)- and SU(2)-structures
We consider compactifications of M-theory on 7-manifolds in the presence of
4-form fluxes, which leave at least four supercharges unbroken. Supersymmetric
vacua admit G-structures and we discuss the cases of G_2-, SU(3)- as well as
SU(2)-structures. We derive the constraints on the fluxes imposed by
supersymmetry and determine the flux components that fix the resulting
4-dimensional cosmological constant (i.e. superpotential).Comment: 17 pages, contribution for the proceedings of: BW2003 Workshop, 29
Aug. - 02 Sept., 2003 Vrnjacka Banja, Serbi
N=1,2 supersymmetric vacua of IIA supergravity and SU(2) structures
We consider backgrounds of (massive) IIA supergravity of the form of a warped
product , where is a six-dimensional compact
manifold and is or a four-dimensional Minkowski space. We
analyse conditions for and supersymmetry on
manifolds of SU(2) structure. We prove the absence of solutions in certain
cases.Comment: 24 pages; v2: reference adde
Superpotentials from flux compactifications of M-theory
In flux compactifications of M-theory a superpotential is generated whose
explicit form depends on the structure group of the 7-dimensional internal
manifold. In this note, we discuss superpotentials for the structure groups:
G_2, SU(3) or SU(2). For the G_2 case all internal fluxes have to vanish. For
SU(3) structures, the non-zero flux components entering the superpotential
describe an effective 1-dimensional model and a Chern-Simons model if there are
SU(2) structures.Comment: 10 page
Background geometries in string and M-theory
In this thesis we consider background geometries
resulting from string theory compactifications. In particular,
we investigate supersymmetric vacuum spaces of supergravity
theories and topological twisted sigma models
by means of classical and generalised G-structures.
In the first part we compactify 11d supergravity
on seven-dimensional manifolds due to phenomenological reasons.
A certain amount of supersymmetry forces the internal background
to admit a classical SU(3)- or G2-structure. Especially, in the case
that the four-dimensional space is maximally symmetric and four form fluxes
are present we calculate the relation to the intrinsic torsion.
The second and main part is two-fold. Firstly, we realise
that generalised geometries on six-dimensional manifolds are a natural
framework to study T-duality and mirror symmetry, in particular if the
B-field is non-vanishing. An explicit mirror map is given and we apply this
idea to the generalised formulation of a topological twisted sigma model.
Implications of mirror symmetry are studied,
e.g. observables and topological A- and B-branes.
Secondly, we show that seven-dimensional NS-NS backgrounds in
type II supergravity theories can be described by generalised
G2-geometries. A compactification on six manifolds leads to a new structure.
We call this geometry a generalised SU(3)-structure. We study the
relation between generalised SU(3)- and G2-structures on six- and
seven-manifolds and generalise the Hitchin-flow equations. Finally, we
further develop the generalised SU(3)- and
G2-structures via a constrained variational principle to incorporate
also the remaining physical R-R fields
N=1 domain wall solutions of massive type II supergravity and the issue of mirror symmetry
We report on Domain Wall solution of Calabi-Yau compactifications with
general fluxes and their application to the study of mirror symmetry in
generalized backgrounds. We address, in particular, to the issue of magnetic
NSNS fluxes. We show that the Domain Wall gradient flow equations can be
interpreted as a set of generalized Hitchin's flow equations of a manifold with
SU(3)x SU(3) structure fibered along the direction transverse to the Domain
wall.Comment: To appear in the proceedings of 2nd RTN Network Workshop and Midterm
Meeting: Constituents, Fundamental Forces and Symmetries of the Universe,
Naples, Italy, 9-13 Oct 200
Ultradeep characterisation of translational sequence determinants refutes rare-codon hypothesis and unveils quadruplet base pairing of initiator tRNA and transcript
Translation is a key determinant of gene expression and an important biotechnological engineering target. In bacteria, 5′-untranslated region (5′-UTR) and coding sequence (CDS) are well-known mRNA parts controlling translation and thus cellular protein levels. However, the complex interaction of 5′-UTR and CDS has so far only been studied for few sequences leading to non-generalisable and partly contradictory conclusions. Herein, we systematically assess the dynamic translation from over 1.2 million 5′-UTR-CDS pairs in Escherichia coli to investigate their collective effect using a new method for ultradeep sequence-function mapping. This allows us to disentangle and precisely quantify effects of various sequence determinants of translation. We find that 5′-UTR and CDS individually account for 53% and 20% of variance in translation, respectively, and show conclusively that, contrary to a common hypothesis, tRNA abundance does not explain expression changes between CDSs with different synonymous codons. Moreover, the obtained large-scale data provide clear experimental evidence for a base-pairing interaction between initiator tRNA and mRNA beyond the anticodon-codon interaction, an effect that is often masked for individual sequences and therefore inaccessible to low-throughput approaches. Our study highlights the indispensability of ultradeep sequence-function mapping to accurately determine the contribution of parts and phenomena involved in gene regulation
Chemo-enzymatic cascades to produce cycloalkenes from bio-based resources
Engineered enzyme cascades offer powerful tools to convert renewable resources into value-added products. Man-made catalysts give access to new-to-nature reactivities that may complement the enzyme's repertoire. Their mutual incompatibility, however, challenges their integration into concurrent chemo-enzymatic cascades. Herein we show that compartmentalization of complex enzyme cascades within E. coli whole cells enables the simultaneous use of a metathesis catalyst, thus allowing the sustainable one-pot production of cycloalkenes from oleic acid. Cycloheptene is produced from oleic acid via a concurrent enzymatic oxidative decarboxylation and ring-closing metathesis. Cyclohexene and cyclopentene are produced from oleic acid via either a six- or eight-step enzyme cascade involving hydration, oxidation, hydrolysis and decarboxylation, followed by ring-closing metathesis. Integration of an upstream hydrolase enables the usage of olive oil as the substrate for the production of cycloalkenes. This work highlights the potential of integrating organometallic catalysis with whole-cell enzyme cascades of high complexity to enable sustainable chemistry
Supersymmetric D-branes and calibrations on general N=1 backgrounds
We study the conditions to have supersymmetric D-branes on general {\cal N}=1
backgrounds with Ramond-Ramond fluxes. These conditions can be written in terms
of the two pure spinors associated to the SU(3)\times SU(3) structure on
T_M\oplus T^\star_M, and can be split into two parts each involving a different
pure spinor. The first involves the integrable pure spinor and requires the
D-brane to wrap a generalised complex submanifold with respect to the
generalised complex structure associated to it. The second contains the
non-integrable pure spinor and is related to the stability of the brane. The
two conditions can be rephrased as a generalised calibration condition for the
brane. The results preserve the generalised mirror symmetry relating the type
IIA and IIB backgrounds considered, giving further evidence for this duality.Comment: 23 pages. Some improvements and clarifications, typos corrected and
references added. v3: Version published in JHE
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