19,807 research outputs found
Enzymatic functionalization of carbon-hydrogen bonds
The development of new catalytic methods to functionalize carbon–hydrogen (C–H) bonds
continues to progress at a rapid pace due to the significant economic and environmental benefits
of these transformations over traditional synthetic methods. In nature, enzymes catalyze regio- and
stereoselective C–H bond functionalization using transformations ranging from hydroxylation to
hydroalkylation under ambient reaction conditions. The efficiency of these enzymes relative to
analogous chemical processes has led to their increased use as biocatalysts in preparative and
industrial applications. Furthermore, unlike small molecule catalysts, enzymes can be systematically
optimized via directed evolution for a particular application and can be expressed in vivo to
augment the biosynthetic capability of living organisms. While a variety of technical challenges
must still be overcome for practical application of many enzymes for C–H bond functionalization,
continued research on natural enzymes and on novel artificial metalloenzymes will lead to improved
synthetic processes for efficient synthesis of complex molecules. In this critical review, we discuss the
most prevalent mechanistic strategies used by enzymes to functionalize non-acidic C–H bonds, the
application and evolution of these enzymes for chemical synthesis, and a number of potential
biosynthetic capabilities uniquely enabled by these powerful catalysts (110 references)
On the scalar graviton in n-DBI gravity
n-DBI gravity is a gravitational theory which yields near de Sitter inflation
spontaneously at the cost of breaking Lorentz invariance by a preferred choice
of foliation. We show that this breakdown endows n-DBI gravity with one extra
physical gravitational degree of freedom: a scalar graviton. Its existence is
established by Dirac's theory of constrained systems. Firstly, studying scalar
perturbations around Minkowski space-time, we show that there exists one scalar
degree of freedom and identify it in terms of the metric perturbations. Then, a
general analysis is made in the canonical formalism, using ADM variables. It is
useful to introduce an auxiliary scalar field, which allows recasting n-DBI
gravity in an Einstein-Hilbert form but in a Jordan frame. Identifying the
constraints and their classes we confirm the existence of an extra degree of
freedom in the full theory, besides the two usual tensorial modes of the
graviton. We then argue that, unlike the case of (the original proposal for)
Horava-Lifschitz gravity, there is no evidence that the extra degree of freedom
originates pathologies, such as vanishing lapse, instabilities and strong
self-coupling at low energy scales.Comment: 30 pages, 1 figur
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