Exciplex Formation in Lipid-bound Escherichia coli Flavohemoglobin

Flavohemoglobins have the particular capability of binding unsaturated and cyclopropanated fatty acids as free acids or phospholipids. Fatty acid binding to the ferric heme results in a weak but direct bonding interaction. Ferrous and ferric protein, in presence or absence of a bound lipid molecule, have been characterized by transient absorption spectroscopy. Measurements have been also carried out both on the ferrous deoxygenated and on the CO bound protein to investigate possible long-range interaction between the lipid acyl chain moiety and the ferrous heme. After excitation of the deoxygenated derivatives the relaxation process reveals a slow dynamics (350 ps) in lipid-bound protein but is not observed in the lipid-free protein. The latter feature and the presence of an extra contribution in the absorption spectrum, indicates that the interaction of iron heme with the acyl chain moiety occurs only in the excited electronic state and not in the ground electronic state. Data analysis highlights the formation of a charge-transfer complex in which the iron ion of the lipid-bound protein in the expanded electronic excited state, possibly represented by a high spin Fe III intermediate, is able to bind to the sixth coordination ligand placed at a distance of at 3.5 Å from the iron. A very small nanosecond geminate rebinding is observed for CO adduct in lipid-free but not in the lipid-bound protein. The presence of the lipid thus appears to inhibit the mobility of CO in the heme pocket.     

NN final-state interaction in two-nucleon knockout from <Superscript>16</Superscript><Emphasis Type="Italic">O</Emphasis>

The influence of the mutual interaction between the two outgoing nucleons (NN-FSI) in electro- and photoinduced two-nucleon knockout from ¹⁶O has been investigated perturbatively. It turns out that the effect of NN-FSI depends on the kinematics and on the type of reaction considered. The effect is generally larger in pp- than in pn-knockout and in electroinduced than in photoinduced reactions. In superparallel kinematics NN-FSI leads in the (e,epp) channel to a strong increase of the cross-section, that is mainly due to a strong enhancement of the -current contribution. In pn-emission, however, this effect is partially cancelled by a destructive interference with the seagull current. For photoreactions NN-FSI is considerably reduced in superparallel kinematics and can be practically negligible in specific kinematics.Received: 1 July 2003, Published online: 10 February 2004PACS: 13.75.Cs Nucleon-nucleon interactions (including antinucleons, deuterons, etc.) - 21.60.-n Nuclear structure models and methods - 25.30.Fj Inelastic electron scattering to continuum     

Electromagnetic proton-neutron knockout off <Superscript>16</Superscript>O : New achievements in theory

Results for the cross-sections of the exclusive ¹⁶O(e, e'pn)¹⁴N and ¹⁶O(γ, pn)¹⁴N knockout reactions are presented and discussed in different kinematics. In comparison with earlier work, a complete treatment of the center-of-mass (CM) effects in the nuclear one-body current is considered in connection with the problem of the lack of orthogonality between initial bound and final scattering states. The effects due to CM and orthogonalization are investigated in combination with different treatments of correlations in the two-nucleon overlap function and for different parametrizations of the two-body currents. The CM effects lead in super-parallel kinematics to a dramatic increase of the ¹⁶O(e, e'pn) cross-section to the 1₂ ⁺ excited state (3.95MeV) of ¹⁴N . In all the situations considered the results are very sensitive to the treatment of correlations. A crucial role is played by tensor correlations, but also the contribution of long-range correlations is important.     

Analysis of new augmented Lagrangian formulations for mixed finite element schemes

Summary. Two new augmented Lagrangian formulations for mixed finite element schemes are presented. The methods lead, in some cases, to an improvement in the order of the approximation. An error analysis is provided, together with some interesting examples of applications. Received July 27, 1994 / Revised version received November 17, 1995     

A Riemann-Hilbert boundary value problem in neutron transport theory

Summary A Riemann-Hilbert boundary value problem, occurring in neutron transport theory, is presented and solved in this note for the case of an infinite straight line and zero index.

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Sommario Viene presentato e risolto in questa nota un problema al contorno di Riemann-Hilbert di indice zero e relativo ad un intervallo infinito. Esso ha origine da un problema di diffusione di neutroni, studiato nell'ambito della teoria del trasporto.

     

Tensorial formulation of neutron diffusion theory

Summary In this paper a tensorial formulation of monoenergetic neutron diffusion theory is presented as can be derived starting from the integral form of the Boltzmann equation. A result of this tensorial approach is that, as a consequence of a spatial variation of the macroscopic cross sections or of the finite dimensions of the body under examination, the diffusion coefficient is no longer a constant scalar quantity, but a second order symmetric tensor, whose components are functions of both the position and the properties of the material the body is made of. The components of the diffusion coefficient tensor are explicitly evaluated in the case of some homogeneous convex bodies.

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Sommario In questa nota viene presentata una formulazione tensoriale della teoria della diffusione di neutroni monoenergetici, quale può essere dedotta a partire dalla forma integrale dell'equazione di Boltzmann. Nell'ambito di questa formulazione si precisa che il coefficiente di diffusione, in conseguenza di una variazione regolare delle sezioni d'urto macroscopiche con la posizione o delle dimensioni finite del mezzo in esame, non è più una quantità scalare costante, ma un tensore simmetrico del secondo ordine, le cui componenti sono funzioni della posizione e delle proprietà del materiale. Queste componenti vengono calcolate esplicitamente per alcuni mezzi omogenei convessi.

     

Modified edge finite elements for photonic crystals

We consider Maxwell’s equations with periodic coefficients as it is usually done for the modeling of photonic crystals. Using Bloch/Floquet theory, the problem reduces in a standard way to a modification of the Maxwell cavity eigenproblem with periodic boundary conditions. Following [8], a modification of edge finite elements is considered for the approximation of the band gap. The method can be used with meshes of tetrahedrons or parallelepipeds. A rigorous analysis of convergence is presented, together with some preliminary numerical results in 2D, which fully confirm the robustness of the method. The analysis uses well established results on the discrete compactness for edge elements, together with new sharper interpolation estimates.     

On the NN final-state interaction in the <Superscript>16</Superscript>O (e,e<Superscript>′</Superscript>pp) reaction

The influence of the mutual interaction between the two outgoing nucleons (NN-FSI) in the ¹⁶O(e, e'pp) reaction has been investigated. Results for various kinematics are discussed. In general, the effect of NN-FSI depends on kinematics and the chosen final state in the excitation spectrum of ¹⁴C. Received: 19 December 2002 / Accepted: 6 February 2003 / Published online: 15 April 2003