Stochastically perturbed Landau-Ginzburg equations

We analyze several aspects of a reaction-diffusion equation in two space dimensions with cubic nonlinearity, stochastically perturbed by white noise in time and in space. This equation needs renormalization, and physical implications of this circumstance are discussed. In particular, for sufficiently large coupling constant the effective potential becomes a double well and rare transitions from one minimum to the other are possible. These, however, are revealed only by large-scale fluctuations which exhibit a bimodal distribution. Fluctuations below a critical scale have unimodal distribution and do not see the double well. This phenomenon is connected with the singular character of local fluctuations in two or more space dimensions. The theoretical results are confirmed by numerical simulations. The possible physical relevance of our results is illustrated in connection with the analysis of certain observations of atmospheric fields.     

A stationary criticality problem in generalL p -space for energy dependent neutron transport in cylindrical geometry

The energy-dependent neutron transport integral equation in a homogeneous cylinder of radiusR and infinite height with isotropic scattering is studied as an abstract equationf=K f in the spaceL ₁((0, 1)×(E _m ,E _M )). By means of techniques based on the theory of positive operators in Banach spaces, we prove that the eigenvalue problem for the integral operatorK admits as a solution a unique a. e. positive eigenfunction to which the leading eigenvalue _o corresponds.After establishing continuity and strictly increasing monotonicity of _o inR we discuss and solve the criticality problem under the assumption of subcriticality for a non-multiplying medium.The formulation of the eigenvalue problem forK is finally extended to anyL _p space, 1p< . Recalling thatK is a Riesz operator inL _p , we prove, as a general result, that the spectrum ofK, acting onL _p , is independent ofp.

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Résumé On étudie un faisceau de neutrons d'une énergieE (comprise entre deux bornesM _e ), dans un cylindre de rayon 0rR de hauteur infinie; on considère la diffraction comme isotrope. L'èquation intègrale du transport de neutrons est formuleeé abstraitement parf=K f, oùK est un opèrateur dans l'espaceL ₁ (r,E) des fonctions intégrables. La théorie des opérateurs positifs dans les espaces de Banach nous permet de démontrer que l'opérateur intégralK possède une fonction propre unique, positive presque partout, correspondant à la valeur propre dominante _o.Après avoir démontré que _o est continue et strictement croissante par rapport àR, on discute et résout le problème critique sous une hypothèse bien motivée physiquement.La formulation du problème aux valeurs propres est généralisée dans un espaceL _p ,p quelconque,K étant un opérateur de Riesz; on obtient comme résultat que le spectre deK dansL _p est indépendant dep.

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Work performed under the auspices of C. N. R. (Gruppo Nazionale per la Fisica-Matematica) and partially supported by M. P. I.

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The research leading to this article was completed while the third author was visiting the University of Florence in the summer of 1983.     

New hints on the Ph-driven tautomeric equilibria of the topotecan anticancer drug in aqueous solutions from an integrated spectroscopic and quantum-mechanical approach

The equilibria between the different forms of the topotecan anticancer drug have been studied at moderately acidic and physiological pH by an integrated computational tool rooted in the density functional theory and its time-dependent extension together with the polarizable continuum model. The results allow an unbiased selection between the different possible tautomeric forms and provide invaluable complements to experimental data. The ultraviolet-visible topotecan spectrum, recorded at moderately acidic pH, is accurately reproduced only by TD-DFT computations including solvent effects. Comparison of the experimental and calculated bands of the UV-vis spectrum at physiological pH indicates the presence of an equilibrium among different forms that is tuned by the microenvironment embedding the drug. The quantitative agreement between TD-DFT/PCM computations and experiments allows the identification of unequivocal spectroscopic signatures for different forms of topotecan.     

An artificial antenna complex containing four Ru(bpy)3(2+)-type chromophores as light-harvesting components and a Ru(bpy)(CN)4(2-) subunit as the energy trap. A structural motif which resembles the natural photosynthetic systems

A cyclam subunit bearing four light-harvesting Ru(bpy)3(2+)-type species is connected to a Ru(II) compound having low-lying excited states (energy trap) by hydrogen bonding: the light absorbed by the light-harvesting elements is funnelled to the energy trap with high efficiency.     

On the α-ketols obtained from α,β-unsaturated aromatic aldehydes and bakers'yeast

From α-methylcinnamaldehyde (1b), acetaldehyde and fermenting bakers'yeast, at pH 5, the $\text{R}$ α-ketol (6) is obtained in $\text{ca}$. 15% yield, togheter with unreacted (1b); from cinnamaldehyde (1a), under identical conditions, optically inactive (5) has been isolated as the only transformation product.     

Achieving linear-scaling computational cost for the polarizable continuum model of solvation

This work describes a new and low-scaling implementation of the polarizable continuum model (PCM) for computing the self-consistent solvent reaction field. The PCM approach is both general and accurate. It is applicable in the framework of both quantum and classical calculations, and also to hybrid quantum/classical methods. In order to further extend the range of applicability of PCM we addressed the problem of its computational cost. The generation of the finite-elements molecular cavity has been reviewed and reimplemented, achieving linear scaling for systems containing up to 500 atoms. Linear scaling behavior has been achieved also for the iterative solution of the PCM equations, by exploiting the fast multipole method (FMM) for computing electrostatic interactions. Numerical results for large (both linear and globular) chemical systems are discussed.     

Enzyme‐Mediated Preparation of Chiral 1,3‐Dioxane Odorants

The enantiomerically enriched diastereoisomers of the chiral 1,3‐dioxane odorants Floropal^® ( 1 ) and Magnolan^® ( 2 ) were prepared by an enzyme‐mediated approach. Their olfactory properties were evaluated to investigate differences in the odor perception for the stereoisomers.     

QUANTITATIVE ANALYSIS OF INTRACELLULAR BEHAVIOUR OF PORPHYRINS

Abstract Fluorometric analysis performed on L 1210 cells after treatment with Photofrin indicated that the interactions with cellular structures induce a significant modification of the equilibria among the different porphyrin species. This modification turned out to be dependent on the uptake and release processes. Thus, a comparative analysis of the dynamic aspects of the drug accumulation process was performed on cells treated with hematoporphyrin, Photofrin and Photofrin II. The results obtained were interpreted taking into account the different chemical composition of the drugs employed. The porphyrin species mainly released seem to be the monomeric ones and 'unfolded oligomers'. The release process results in further modifications of the aggregation and/or configu-rational state of intracellular porphyrins due to altered internal equilibrium.     

A new synthesis of 5-methylene-2(5H)-furanone derivatives

5-methylene-2(5H)-furanone derivatives are easily obtained by treatment of tertiary 2-furylcarbinols with pyridinium dichromate in dimethylformamide solution. Through this procedure, a natural product, the thiophene lactone isolated from Chamaemelum Nobile L., has been synthesized.     

Electron impact mass spectrometry of 4, 5-dihydro-1,2,4-benzothiadiazepine-1,1-dioxide derivatives

The fragmentation pathways of some 4,5-dihydro-1,2,4-benzothiadiazepine-1,1-dioxide derivatives are discussed on the basis of metastable transition data, exact mass measurements and labelling experiments. These compounds show several primary fragmentation processes: M ? SO₂, M ? OH and one involving an intramolecular oxygen migration (M ? CH₃NO).