206 research outputs found
Improved flux limits for neutrinos with energies above 10 eV from observations with the Westerbork Synthesis Radio Telescope
Particle cascades initiated by ultra-high energy (UHE) neutrinos in the lunar
regolith will emit an electromagnetic pulse with a time duration of the order
of nano seconds through a process known as the Askaryan effect. It has been
shown that in an observing window around 150 MHz there is a maximum chance for
detecting this radiation with radio telescopes commonly used in astronomy. In
50 hours of observation time with the Westerbork Synthesis Radio Telescope
array we have set a new limit on the flux of neutrinos, summed over all
flavors, with energies in excess of eV.Comment: Submitted to Phys. Rev. Let
Experimental and modeling study of the autoignition of 1-hexene/iso-octane mixtures at low temperatures
Autoignition delay times have been measured in a rapid compression machine at
Lille at temperatures after compression from 630 to 840 K, pressures from 8 to
14 bar, \Phi = 1 and for a iso octane/1 hexene mixture containing 82%
iso-octane and 18% 1 hexene. Results have shown that this mixture is strongly
more reactive than pure iso-octane, but less reactive than pure 1 hexene. It
exhibits a classical low temperature behaviour, with the appearance of cool
flame and a negative temperature coefficient region. The composition of the
reactive mixture obtained after the cool flame has also been determined. A
detailed kinetic model has been obtained by using the system EXGAS, developed
in Nancy for the automatic generation of kinetic mechanisms, and an acceptable
agreement with the experimental results has been obtained both for autoignition
delay times and for the distribution of products. A flow rate analysis reveals
that the crossed reactions between species coming from both reactants (like
H-abstractions or combinations) are negligible in the main flow consumption of
the studied hydrocarbons. The ways of formation of the main primary products
observed and the most sensitive rate constants have been identified
Two highly divergent alcohol dehydrogenases of melon exhibit fruit ripening-specific expression and distinct biochemical characteristics
Alcohol dehydrogenases (ADH) participate in
the biosynthetic pathway of aroma volatiles in fruit by
interconverting aldehydes to alcohols and providing substrates
for the formation of esters. Two highly divergent
ADH genes (15% identity at the amino acid level) of
Cantaloupe Charentais melon (Cucumis melo var. Cantalupensis)
have been isolated. Cm-ADH1 belongs to the
medium-chain zinc-binding type of ADHs and is highly
similar to all ADH genes expressed in fruit isolated so far.
Cm-ADH2 belongs to the short-chain type of ADHs. The
two encoded proteins are enzymatically active upon
expression in yeast. Cm-ADH1 has strong preference for
NAPDH as a co-factor, whereas Cm-ADH2 preferentially
uses NADH. Both Cm-ADH proteins are much more active
as reductases with Kms 10–20 times lower for the conversion
of aldehydes to alcohols than for the dehydrogenation
of alcohols to aldehydes. They both show strong preference
for aliphatic aldehydes but Cm-ADH1 is capable of
reducing branched aldehydes such as 3-methylbutyraldehyde,
whereas Cm-ADH2 cannot. Both Cm-ADH genes are
expressed specifically in fruit and up-regulated during
ripening. Gene expression as well as total ADH activity are
strongly inhibited in antisense ACC oxidase melons and in
melon fruit treated with the ethylene antagonist 1-methylcyclopropene
(1-MCP), indicating a positive regulation by
ethylene. These data suggest that each of the Cm-ADH
protein plays a specific role in the regulation of aroma
biosynthesis in melon fruit
Functional characterization of a melon alcohol acyl-transferase gene family involved in the biosynthesis of ester volatiles. Identification of the crucial role of a threonine residue for enzyme activity
Volatile esters, a major class of compounds contributing to the aroma of many fruit, are synthesized by
alcohol acyl-transferases (AAT). We demonstrate here that, in Charentais melon (Cucumis melo var.
cantalupensis), AAT are encoded by a gene family of at least four members with amino acid identity ranging
from 84% (Cm-AAT1/Cm-AAT2) and 58% (Cm-AAT1/Cm-AAT3) to only 22% (Cm-AAT1/Cm-AAT4).
All encoded proteins, except Cm-AAT2, were enzymatically active upon expression in yeast and show
differential substrate preferences. Cm-AAT1 protein produces a wide range of short and long-chain acyl
esters but has strong preference for the formation of E-2-hexenyl acetate and hexyl hexanoate. Cm-AAT3
also accepts a wide range of substrates but with very strong preference for producing benzyl acetate.
Cm-AAT4 is almost exclusively devoted to the formation of acetates, with strong preference for cinnamoyl
acetate. Site directed mutagenesis demonstrated that the failure of Cm-AAT2 to produce volatile esters is
related to the presence of a 268-alanine residue instead of threonine as in all active AAT proteins. Mutating
268-A into 268-T of Cm-AAT2 restored enzyme activity, while mutating 268-T into 268-A abolished
activity of Cm-AAT1. Activities of all three proteins measured with the prefered substrates sharply increase
during fruit ripening. The expression of all Cm-AAT genes is up-regulated during ripening and inhibited in
antisense ACC oxidase melons and in fruit treated with the ethylene antagonist 1-methylcyclopropene
(1-MCP), indicating a positive regulation by ethylene. The data presented in this work suggest that the
multiplicity of AAT genes accounts for the great diversity of esters formed in melon
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