A computational study of the excited states of bilirubin IX

We have determined the lowest excited states of bilirubin IX by TD-DFT calculations. The lowest pair of excited states, S(1) and S(2), turn out to be of charge-transfer (CT) nature. Although DFT based methods tend to underestimate the energy of CT states, the small oscillator strengths we have computed indicate that such states may actually exist in this spectral region, but would have escaped spectroscopic detection. The next pair of excited states, S(3) and S(4), account for the most prominent spectral feature of bilirubin. They can be accurately described by the exciton coupling model, as we show by a thorough analysis of wavefunctions and properties. This finding therefore supports the interpretation of bilirubin photoisomerization behaviour, based on the exciton coupling model.     

Coordination properties of 3-functionalized beta-cyclodextrins. Thermodynamic stereoselectivity of copper(II) complexes of the A,B-diamino derivative and its exploitation in LECE

The bis-amino AB derivative of beta-cyclodextrin on the secondary rim was synthesised and spectroscopically characterised by different techniques. Its binary systems both with protons and copper(II) were thermodynamically characterised by pH-metric potentiometry. In addition the ternary systems with each of the enantiomers of tryptophan and alanine were investigated. A thermodynamic stereoselectivity was observed for the tryptophan enantiomers and this was exploited to separate them by capillary electrophoresis through a ligand exchange mechanism (LECE). LECE separation of racemates of phenylalanine and tyrosine was also obtained.     

Assessing the accuracy of metadynamics

Metadynamics is a powerful technique that has been successfully exploited to explore the multidimensional free energy surface of complex polyatomic systems and predict transition mechanisms in very different fields, ranging from chemistry and solid-state physics to biophysics. We here derive an explicit expression for the accuracy of the methodology and provide a way to choose the parameters of the method in order to optimize its performance.     

Orientational dynamics in the isotropic phase of a calamitic liquid-crystal model

We report a molecular dynamics simulation study on the isotropic phase of an idealized calamitic liquid crystal model with a length-to-width ratio of approximately 5-6. The study focuses on the characterization of single-particle and collective orientational dynamics on approaching the phase transition to the nematic phase. Recent experimental and simulation works have suggested that a power law behavior exists at relatively short times in the decay of the time derivative of the orientational correlation functions. Qualitatively, our simulation data are consistent with these findings. Both single-particle and collective time correlation function derivatives possess, in their respective log-log plots, a linear region at very short times, whose slope is essentially independent from the thermodynamic state. Nevertheless, the single-particle orientational correlation functions are better described by a function which is the sum of a fast exponential, an intermediate stretched-exponential and a slow exponential, while the collective orientational correlation functions are satisfactorily described by a sum of two exponentials, at higher density, or by just one exponential, at lower density.     

Bifurcations from infinity in asymmetric nonlinear oscillators

We study the existence of large-amplitude periodic or almost periodic solutions of second order differential equations with asymmetric nonlinearities, when the system is close to "nonlinear resonance". Received September 1998     

Compatti olomorficamente convessi di uno spazio analitico

Riassunto In questo lavoro si considerano sottoinsiemi compatti olomorficamente convessi di uno spazio analitico non ridotto e si prova che questi sono caratterizzati, come nel caso ridotto, dagli analoghi dei teoremiA eB di Cartan. Si dà inoltre una caratterizzazione di certi morfismi in un compatto olomorficamente convesso.

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Summary In this paper we consider compact holomorphically convex subsets of a non reduced analytic space and we prove that they are characterized, as in the reduced case, by the analogous of Cartan's theoremsA andB. We give also a characterization of some morphisms in a compact holomorphically convex set.

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Lavoro eseguito nell'ambito del G.N.S.A.G.A. del C.N.R.     

Chaetoglobosin L,a New Metabolite of Diplodia macrospora

From cultures of Diplodia macrospora a new metabolite of the cytochalasan family, Chaetoglobosin L (3) , has been isolated and its structure elucidated together with the known compounds diplosporin (1) and Chaetoglobosin K (2) .