1,721,667 research outputs found
Fluctuations cutoff in a 1D Hamiltonian model for DNA
No full textConsidering a one dimensional mesoscopic model for DNA, we focus on the upper bound for the base pair fluctuations, a relevant parameter in computer simulations for which contrasting estimates have been reported. Noticing that the free energy of the model can be obtained analytically in the thermodynamic limit, we derive a relation for the fluctuations upper bound in terms of temperature and elastic force constant of the stacking potential. At room temperature, the fluctuation cutoff is constrained to values in fair agreement with the threshold above which hydrogen bonds break and base pairs dissociate
Poverty alleviation in a welfarist framework of optimal linear income taxation
Full text linkThis paper develops a framework that allows to treat the typically ”non welfarist” goal of poverty alleviation as a ”welfarist” one. Such result is obtained by adopting a censored social welfare function, in which only variations in incomes below the poverty line affect social welfare. Optimal linear income tax structures derived assuming different social objective functions are then explicitly compared. Under quite general conditions, we show that the poverty-minimising income taxation is more redistributive than the standard welfare-maximising one. Under more restrictive assumptions on the income distribution, however, we find that the opposite result may occur. Specifically, when the poor are close enough to the poverty line, a smaller redistribution may be required to minimise poverty rather than maximise welfare
First-passage probability: a test for DNA Hamiltonian parameters
Full text linkA method is proposed to select the suitable sets of potential parameters for a one-dimensional mesoscopic Hamiltonian model, first introduced to describe the DNA melting transition and later extended to investigate thermodynamic and dynamical properties of nucleic acids. The DNA base pair fluctuations are considered as time dependent trajectories whose initial condition sets the no crossing constraint enforced in the path integral for the first-passage probability. Performing the path integration at room temperature, relations are established between the cutoff on the amplitude of the base pair fluctuations and the model parameters. In particular, a suitable range of values for the non-linear stacking parameter has been proposed while the effect of the stiffness constant on the first-passage probability has been highlighted. The formalism here developed may be applied to compute the lifetime of open base pairs in three-dimensional helical models for DNA molecules
Base pair fluctuations in helical models for nucleic acids
Full text linkA statistical method is developed to estimate the maximum amplitude of the base pair fluctuations in a three dimensional mesoscopic model for nucleic acids. The base pair thermal vibrations around the helix diameter are viewed as a Brownian motion for a particle embedded in a stable helical structure. The probability to return to the initial position is computed, as a function of time, by integrating over the particle paths consistent with the physical properties of the model potential. The zero time condition for the first-passage probability defines the constraint to select the integral cutoff for various macroscopic helical conformations, obtained by tuning the twist, the bending and the slide motion between adjacent base pairs along the molecule stack. Applying the method to a short homogeneous chain at room temperature, we obtain meaningful estimates for the maximum fluctuations in the twist conformation with base pairs per helix turn, typical of double stranded DNA helices. Untwisting the double helix, the base pair fluctuations broaden and the integral cutoff grows. The cutoff is found to increase also in the presence of a sliding motion which shortens the helix contour length, a situation peculiar of dsRNA molecules
Mesoscopic helical models for DNA
Full text linkNucleic acids physical properties have been investigated by theoretical
methods based both on fully atomistic representations and on coarse grained
models, e.g. the worm-like-chain, taken from polymer physics. In this article,
I present an intermediate (mesoscopic) approach and show how to build a three
dimensional Hamiltonian model which accounts for the main interactions
responsible for the stability of the helical molecules. While the 3D mesoscopic
model yields a sufficiently detailed description of the helix at the level of
the base pair, it also allows one to predict the thermodynamical and structural
properties of molecules in solution. Relying on the idea that the base pair
fluctuations can be conceived as trajectories, I have built a computational
method based on the time dependent path integral formalism to derive the
partition function. While the main features of the method are presented, I
focus here in particular on a newly developed statistical method to set the
maximum amplitude of the base pair fluctuations, a key parameter of the theory.
Some applications to the calculation of DNA flexibility properties are
discussed together with the available experimental data.Comment: Manuscript submitted To European Biophysics Journal. Final version
available at the DO
Stretching DNA in hard-wall potential channels
Full text linkA three-dimensional mesoscopic model is applied to study the properties of short DNA chains in a confining environment. The cylindrical channel is represented by a hard-wall repulsive potential incorporated in the system Hamiltonian. The macroscopic helical parameters are computed performing statistical averages over the ensemble of microscopic base pair fluctuations. The average molecule elongation, measured by the end-to-end distance, is derived as a function of the channel potential parameters both for a homogeneous and a heterogeneous chain. The overall results suggest that the mesoscopic model, with the channel potential term, yields consistent quantitative estimates for the stretching and twisting of short chains
La Teoria della tassazione ottima del reddito: una rassegna critica
Full text linkLa tassazione del reddito costituisce lo strumento fiscale più utilizzato nella maggior parte dei paesi sviluppati, ma è anche uno degli aspetti dei sistemi tributari maggiormente in discussione. Da un lato, essa rappresenta il mezzo più diretto per perseguire le finalità redistributive e soddisfare le esigenze di equità. Dall’altro lato, si ritiene che l’applicazione di tale imposta generi forti disincentivi allo sforzo e all’iniziativa individuali. La teoria della tassazione ottima del reddito mostra come questo trade off influenza la definizione della struttura ottimale di imposta e si propone di individuare lo schema di aliquote sul reddito che consente di soddisfare gli obiettivi equitativi, al costo minimo in termini di perdita di effcienza. In questa sede,viene proposta una rassegna critica dei principali contributi alla letteratura sul tema, iniziando dal lavoro di Mirrlees del 1971, che segna l’avvio della moderna teoria della tassazione ottima del reddito. La ricerca successiva è proceduta mantenendo la stessa struttura concettuale ed espandendo il modello originario, al fine di analizzare come la forma della tax schedule ottima
può essere modificata dall’adozione di assunzioni diverse. Di alcune di queste
assunzioni si è dato conto all’interno della rassegna, nella quale, dopo aver messo in luce gli elementi di maggiore criticità e le differenti soluzioni prospettate, si evidenziano i risultati più rilevanti ottenuti
La sostenibilità del turismo culturale nel Lazio: le basi teoriche e alcuni possibili indicatori
No full textTwist-stretch relations in nucleic acids
Full text linkNucleic acids are highly deformable helical molecules constantly stretched,
twisted and bent in their biological functioning. Single molecule experiments
have shown that double stranded (ds)-RNA and standard ds-DNA have opposite
twist-stretch patterns and stretching properties when overwound under a
constant applied load. The key structural features of the A-form RNA and B-form
DNA helices are here incorporated in a three-dimensional mesoscopic Hamiltonian
model which accounts for the radial, bending and twisting fluctuations of the
base pairs. Using path integral techniques which sum over the ensemble of the
base pair fluctuations, I compute the average helical repeat of the molecules
as a function of the load. The obtained twist-stretch relations and stretching
properties, for short A- and B-helical fragments, are consistent with the
opposite behaviors observed in kilo-base long molecules.Comment: in pres
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