55,653 research outputs found
Solar-neutrino reactions on deuteron in effective field theory
The cross sections for low-energy neutrino-deuteron reactions are calculated
within heavy-baryon chiral perturbation theory employing cut-off regularization
scheme. The transition operators are derived up to
next-to-next-to-next-to-leading order in the Weinberg counting rules, while the
nuclear matrix elements are evaluated using the wave functions generated by a
high-quality phenomenological NN potential. With the adoption of the
axial-current-four-nucleon coupling constant fixed from the tritium beta decay
data, our calculation is free from unknown low-energy constants. Our results
exhibit a high degree of stability against different choices of the cutoff
parameter, a feature which indicates that, apart from radiative corrections,
the uncertainties in the calculated cross sections are less than 1 %.Comment: 12 pages, 3 figures. Error estimation of higher order corrections
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THEORETICAL STUDIES OF BILIPROTEIN CHROMOPHORES AND RELATED BILE PIGMENTS BY MOLECULAR ORBITAL AND RAMACHANDRAN TYPE CALCULATIONS
Ramachandran calculations have been used to gain insight into steric hindrance in bile
pigments related to biliprotein chromophores. The high optical activity of denatured phycocyanin, as
compared to phycoerythrin, has been related to the asymmetric substitution at ring A, which shifts the
equilibrium towards the P-helical form of the chromophore. Geometric effects on the electronic structures
and transitions have then been studied by molecular orbital calculations for several conjugation
systems including the chromophores of phycocyanin. phytochrome P,, cations, cation radicals and
tautomeric forms. For these different chromophores some general trends can be deduced. For instance,
for a given change in the gross shape (e.g. either unfolding of the molecule from a cyclic-helical to a fully
extended geometry, or upon out-of-plane twists of the pyrrole ring A) of the molecules under study, the
predicted absorption spectra all change in a simikar way. Nonetheless, there are characteristic distinctions
between the different n-systems, both in the transition energies and the charge distribution, which
can be related to their known differences in spectroscopic properties and their reactivity
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