108 research outputs found
Identification of the C2-1H histidine NMR resonances in chloramphenicol acetyltransferase by a 13C-1H heteronuclear multiple quantum coherence method
AbstractChloramphenicol acetyltransferase (CAT) was used to assess the feasibility of study of specific proton resonances in an enzyme of overall molecular mass 75000. [ring2-13C]Histidine was selectively incorporated into the type III chloramphenicol acetyltransferase (CATIII) using a histidine auxotroph of E. coli. Heteronuclear multiple and single quantum experiments were used to select the C2 protons in the histidyl imidazole ring. One- and two-dimensional spectra revealed six signals out of a total of seven histidine residues in CATIII. pH titration, chemical modification and ligand binding were used to demonstrate that the signal from H195, the histidine at the active site, is not among those observed. Nevertheless, this work demonstrates that selective isotopic enrichment and multiple quantum coherence techniques can be used to distinguish proton resonances in a protein of high molecular mass
Soft Gluon Approach for Diffractive Photoproduction of J/psi
We study diffractive photoproduction of by taking the charm quark as
a heavy quark. A description of nonperturbative effect related to can
be made by using NRQCD. In the forward region of the kinematics, the
interaction between the -pair and the initial hadron is due to
exchange of soft gluons. The effect of the exchange can be studied by using the
expansion in the inverse of the quark mass . At the leading order we find
that the nonperturbative effect related to the initial hadron is represented by
a matrix element of field strength operators, which are separated in the moving
direction of in the space-time. The S-matrix element is then obtained
without using perturbative QCD and the results are not based on any model.
Corrections to the results can be systematically added. Keeping the dominant
contribution of the S-matrix element in the large energy limit we find that the
imaginary part of the S-matrix element is related to the gluon distribution for
with a reasonable assumption, the real part can be obtained with
another approximation or with dispersion relation. Our approach is different
than previous approaches and also our results are different than those in these
approaches. The differences are discussed in detail. A comparison with
experiment is also made and a qualitative agreement is found.Comment: 25 pages, 6 figures. Tiny changes in two figures, conclusion and text
unchanged, accpeted by Nucl. Phys.
Diffractive Photoproduction of Eta_c
Diffractive photoproduction of is an important process to study the
effect of Odderon, whose existence is still not confirmed in experiment. A
detailed interpretation of Odderon in QCD, i.e., in terms of gluons is also
unclear.Taking charm quarks as heavy quarks, we can use NRQCD and take
as a bound state. Hence, in the production of a free pair is first produced and this pair is transformed into
subsequently.In the forward region of the kinematics, the pair
interacts with initial hadron through exchanges of soft gluons. This
interaction can be studied with HQET, which provides a systematic expansion in
the inverse of the -quark mass . We find that the calculation of the
-matrix element in the forward region can be formulated as the problem of
solving a wave function of a -quark propagating in a background field of
soft gluons. At leading order we find that the differential cross-section can
be expressed with four functions, which are defined with a twist-3 operator of
gluons. The effect of exchanging a Odderon can be identified with this operator
in our case. We discuss our results in detail and compare them with those
obtained in previous studies. Our results and those from other studies show
that the differential cross-section is very small in the forward region. We
also show that the production through photon exchange is dominant in the
extremely forward region, hence the effect of Odderon exchange can not be
identified in this region.For completeness we also give results for diffractive
photoproduction of .Comment: 20 pages with 3 figures. Text improve
Superconducting String Texture
We present a detailed analytical and numerical study of a novel type of
static, superconducting, classically stable string texture in a renormalizable
topologically trivial massive U(1) gauge model with one charged and one neutral
scalar. An upper bound on the mass of the charged scalar as well as on the
current that the string can carry are established. A preliminary unsuccesful
search for stable solutions corresponding to large superconducting loops is
also reported.Comment: RevTex, 14 pages, 8 figure
Dynamical parton distributions of the nucleon and very small-x physics
Utilizing recent DIS measurements (F_{2,L}) and data on dilepton and
high-E_{T} jet production we determine the dynamical parton distributions of
the nucleon generated radiatively from valence-like positive input
distributions at optimally chosen low resolution scales. These are compared
with `standard' distributions generated from positive input distributions at
some fixed and higher resolution scale. It is shown that up to the next to
leading order NLO(\bar{MS}, DIS) of perturbative QCD considered in this paper,
the uncertainties of the dynamical distributions are, as expected, smaller than
those of their standard counterparts. This holds true in particular in the
presently unexplored extremely small-x region relevant for evaluating ultrahigh
energy cross sections in astrophysical applications. It is noted that our new
dynamical distributions are compatible, within the presently determined
uncertainties, with previously determined dynamical parton distributions.Comment: 21 pages, 2 tables, 16 figures, v2: added Ref.[60], replaced Fig.
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
D* Production in Deep Inelastic Scattering at HERA
This paper presents measurements of D^{*\pm} production in deep inelastic
scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The
data have been taken with the ZEUS detector at HERA. The decay channel
(+ c.c.) has been used in the study. The
cross section for inclusive D^{*\pm} production with
and is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region
{ GeV and }. Differential cross
sections as functions of p_T(D^{*\pm}), and are
compared with next-to-leading order QCD calculations based on the photon-gluon
fusion production mechanism. After an extrapolation of the cross section to the
full kinematic region in p_T(D^{*\pm}) and (D^{*\pm}), the charm
contribution to the proton structure function is
determined for Bjorken between 2 10 and 5 10.Comment: 17 pages including 4 figure
Search for Colour Singlet and Colour Reconnection Effects in Hadronic Z Decays at LEP
A search is performed in symmetric 3-jet hadronic Z decay events for evidence
of colour singlet production or colour reconnection effects. Asymmetries in the
angular separation of particles are found to be sensitive indicators of such
effects. Upper limits on the level of colour singlet production and colour
reconnection effects are established for a variety of models
A Measurement of the Proton Structure Function
A measurement of the proton structure function is reported
for momentum transfer squared between 4.5 and 1600 and
for Bjorken between and 0.13 using data collected by the
HERA experiment H1 in 1993. It is observed that increases
significantly with decreasing , confirming our previous measurement made
with one tenth of the data available in this analysis. The dependence is
approximately logarithmic over the full kinematic range covered. The subsample
of deep inelastic events with a large pseudo-rapidity gap in the hadronic
energy flow close to the proton remnant is used to measure the "diffractive"
contribution to .Comment: 32 pages, ps, appended as compressed, uuencoded fil
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