The Visibility Graph: a new method for estimating the Hurst exponent of fractional Brownian motion

Fractional Brownian motion (fBm) has been used as a theoretical framework to study real time series appearing in diverse scientific fields. Because its intrinsic non-stationarity and long range dependence, its characterization via the Hurst parameter H requires sophisticated techniques that often yield ambiguous results. In this work we show that fBm series map into a scale free visibility graph whose degree distribution is a function of H. Concretely, it is shown that the exponent of the power law degree distribution depends linearly on H. This also applies to fractional Gaussian noises (fGn) and generic f^(-b) noises. Taking advantage of these facts, we propose a brand new methodology to quantify long range dependence in these series. Its reliability is confirmed with extensive numerical simulations and analytical developments. Finally, we illustrate this method quantifying the persistent behavior of human gait dynamics.Comment: 5 pages, submitted for publicatio

Impact of alpha-synuclein spreading on the nigrostriatal dopaminergic pathway depends on the onset of the pathology

Misfolded alpha-synuclein spreads along anatomically connected areas through the brain, prompting progressive neurodegeneration of the nigrostriatal pathway in Parkinson's disease. To investigate the impact of early stage seeding and spreading of misfolded alpha-synuclein along with the nigrostriatal pathway, we studied the pathophysiologic effect induced by a single acute alpha-synuclein preformed fibrils (PFFs) inoculation into the midbrain. Further, to model the progressive vulnerability that characterizes the dopamine (DA) neuron life span, we used two cohorts of mice with different ages: 2-month-old (young) and 5-month-old (adult) mice. Two months after a-synuclein PFFs injection, we found that striatal DA release decreased exclusively in adult mice. Adult DA neurons showed an increased level of pathology spreading along with the nigrostriatal pathway accompanied with a lower volume of alpha-synuclein deposition in the midbrain, impaired neurotransmission, rigid DA terminal composition, and less microglial reactivity compared with young neurons. Notably, preserved DA release and increased microglial coverage in the PFFs-seeded hemisphere coexist with decreased large-sized terminal density in young DA neurons. This suggests the presence of a targeted pruning mechanism that limits the detrimental effect of alpha-synuclein early spreading. This study suggests that the impact of the pathophysiology caused by misfolded alpha-synuclein spreading along the nigrostriatal pathway depends on the age of the DA network, reducing striatal DA release specifically in adult mice

Power density of a bare electrodynamic tether generator

The maximum performance of bare electrodynamic tethers as power generating systems under OML-theory is analyzed. Results show that best performance in terms of power density is achieved by designing the tether in such a way to increase ohmic impedance with respect to plasma contact impedance, hence favoring longer and thinner tethers. In such condition the corresponding optimal value of the load impedance is seen to approach the ohmic impedance of the conducting tether. At the other extreme, when plasma contact impedance dominates (which is not optimal but can be relevant for some applications) optimum power generation is found by matching the load impedance with an effective tether-plasma contact impedance whose expression is derived

Towards accurate solvation free energies of large biological systems

Continuum solvation models like PCM or COSMO are the standard tool to calculate solvation free energies in a quantum level, but have been typically limited to small biological molecules due to its large computational cost. Recently, a new implementation of COSMO based on a domain decomposition strategy (ddCOSMO) [1] has been presented, which speeds up calculations by several orders of magnitude, thus paving the way for its application to very large systems. Here, we report the parameterization of ddCOSMO to the prediction of hydration free energies based on the MST solvation model developed in Barcelona, [2][3]. The parameterization is based on the PM6 semi-empirical Hamiltonian, on a set of over 200 experimental hydration free energies. The new model opens the way to the accurate prediction of hydration free energies of very large biomolecules, thus going beyond the usual classical MM-PBSA or MM-GBSA approaches

Classification of qubit entanglement: SL(2,C) versus SU(2) invariance

The role of SU(2) invariants for the classification of multiparty entanglement is discussed and exemplified for the Kempe invariant I_5 of pure three-qubit states. It is found to being an independent invariant only in presence of both W-type entanglement and threetangle. In this case, constant I_5 admits for a wide range of both threetangle and concurrences. Furthermore, the present analysis indicates that an SL^3 orbit of states with equal tangles but continuously varying I_5 must exist. This means that I_5 provides no information on the entanglement in the system in addition to that contained in the tangles (concurrences and threetangle) themselves. Together with the numerical evidence that I_5 is an entanglement monotone this implies that SU(2) invariance or the monotone property are too weak requirements for the characterization and quantification of entanglement for systems of three qubits, and that SL(2,C) invariance is required. This conclusion can be extended to general multipartite systems (including higher local dimension) because the entanglement classes of three-qubit systems appear as subclasses.Comment: 9 pages, 10 figures, revtex

On the geometry of a class of N-qubit entanglement monotones

A family of N-qubit entanglement monotones invariant under stochastic local operations and classical communication (SLOCC) is defined. This class of entanglement monotones includes the well-known examples of the concurrence, the three-tangle, and some of the four, five and N-qubit SLOCC invariants introduced recently. The construction of these invariants is based on bipartite partitions of the Hilbert space in the form ${\bf C}^{2^N}\simeq{\bf C}^L\otimes{\bf C}^l$ with $L=2^{N-n}\geq l=2^n$. Such partitions can be given a nice geometrical interpretation in terms of Grassmannians Gr(L,l) of l-planes in ${\bf C}^L$ that can be realized as the zero locus of quadratic polinomials in the complex projective space of suitable dimension via the Plucker embedding. The invariants are neatly expressed in terms of the Plucker coordinates of the Grassmannian.Comment: 7 pages RevTex, Submitted to Physical Review

Exploring the protonation properties of photosynthetic phycobiliprotein pigments from molecular modeling and spectral line shapes

In photosynthesis, specialized light harvesting pigment- protein complexes (PPCs) are used to capture incident sunlight and funnel its energy to the reaction center. In Cryptophyte algae these complexes are suspended in the lumen, where the pH ranges between ~5-7, depending on the prolongation of the incident sunlight. However, the pKa of the several kinds of bilin chromophores encountered in these complexes and the effect of its protonation state on the energy transfer process is still unknown. Here, we combine quantum chemical and continuum solvent calculations to estimate the intrinsic aqueous pKas of different bilin pigments. We then use Propka and APBS classical electrostatic calculations to estimate the change in protonation free energies when the bilins are embedded inside five different phycobiliproteins (PE545, PC577, PC612, PC630 and PC645), and critically asses our results by analysis of the changes in the absorption spectral line shapes measured within a pH range from 4.0 to 9.4. Our results suggest that each individual protein environment strongly impacts the intrinsic pKa of the different chomophores, being the final responsible of their protonation state

Probing background ionization: Positive streamers with varying pulse repetition rate and with a radioactive admixture

Positive streamers need a source of free electrons ahead of them to propagate. A streamer can supply these electrons by itself through photo-ionization, or the electrons can be present due to external background ionization. Here we investigate the effects of background ionization on streamer propagation and morphology by changing the gas composition and the repetition rate of the voltage pulses, and by adding a small amount of radioactive Krypton 85. We find that the general morphology of a positive streamer discharge in high purity nitrogen depends on background ionization: at lower background ionization levels the streamers branch more and have a more feather-like appearance. This is observed both when varying the repetition rate and when adding Krypton 85, though side branches are longer with the radioactive admixture. But velocities and minimal diameters of streamers are virtually independent of the background ionization level. In air, the inception cloud breaks up into streamers at a smaller radius when the repetition rate and therefore the background ionization level is higher. When measuring the effects of the pulse repetition rate and of the radioactive admixture on the discharge morphology, we found that our estimates of background ionization levels are consistent with these observations; this gives confidence in the estimates. Streamer channels generally do not follow the paths of previous discharge channels for repetition rates of up to 10 Hz. We estimate the effect of recombination and diffusion of ions and free electrons from the previous discharge and conclude that the old trail has largely disappeared at the moment of the next voltage pulse; therefore the next streamers indeed cannot follow the old trail.Comment: 30 pages, 13 figure

Valoración naturalística del Corredor Verde del Río Guadiamar Andalucía, España)

En este trabajo aplicamos un método de valoración naturalística, concretamente el índice de interés para la conservación, a cada una de las unidades de un mapa de vegetación previamente realizado a escala 1:10.000. Para el cálculo de los distintos criterios de valoración se tienen en cuenta las comunidades vegetales (asociaciones) presentes en cada unidad, en vez de las especies como suele ser habitual. Las unidades del mapa están agrupadas en series de vegetación, por lo que también pueden extraerse conclusiones sobre el grado de conservación de cada una de ellas. Los resultados se plasman en un mapa de interés para la conservación. El escenario en el que se realizó el trabajo es el corredor verde del río Guadiamar, situado en el sur de España. La zona fue afectada por un vertido tóxico y lo que aquí mostramos son los resultados obtenidos tras una primera limpieza. Después se han iniciado trabajos de restauración, al cabo de los cuales podremos realizar un estudio comparativo con los resultados de este trabajo.In this paper we have applied a naturalistic evaluation strategy, more precisely, the conservation importance index, to each unit of a preplanned 1:10.000scale vegetation map. The estimation of the different evaluation criteria is based on the plant communities (associations) found in each unit rather than on species, as is usual. Since the map units are grouped in vegetation series, the conservation degree of each of them can also be assessed. The results are presented in a conservation interest map. The survey was undertaken in the Green Corridor of the Guadiamar River, in the south of Spain. The area was affected by a toxic spill and our data refl ect the results obtained after the fi rst stage of cleaning work. Subsequently, restoration work has been undertaken after which we will be able to carry out a comparative analysis with the results of this work.Junta de Andalucía. Consejería de Medio Ambiente