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E Curotto

Publications and source records attributed to E Curotto.

10 recordsLinked to original sources
PubMed2006

Stereographic projection path-integral simulations of (HF)n clusters.

We perform several quantum canonical ensemble simulations of (HF)(n) clusters. The HF stretches are rigid, and the stereographic projection path-integral method is employed for the simulation in the resulting curved configuration space. We make use of the reweighted random series techniques to accelerate the convergence of the path-integral simulation with respect to the number of path coefficients. We develop and test estimators for the total energy and heat capacity based on a finite difference approach for non-Euclidean spaces. The quantum effects at temperatures below 400 K are substantial for all sizes. We observe interesting thermodynamic behaviors in the quantum simulations of the octamer and the heptamer.

Journal Article↗
PubMed2005

Parallel tempering simulations of the 13-center Lennard-Jones dipole-dipole cluster (muD=0-->0.5 a.u.).

We investigate the thermodynamic behavior of the thirteen center uniform Lennard-Jones dipole-dipole cluster [(LJDD)(13)] for a wide range of dipole moment strengths. We find a relatively wide range of potential parameters where solid-solid coexistence manifests itself. Using structural characterization methods we determine the shape of the few isomers that contribute to the solid-solid coexistence region. The thermal distributions of the size of the net dipole moment are broad even at the coldest temperatures of the simulation where the (LJDD)(13) cluster is solid.

Journal Article↗
PubMed2005

A reweighted random series method for stereographic projection path integrals.

A set of general reweighted random series methods for metric affine spaces is developed. The extension of the theorems to metric affine spaces demands the introduction of a configuration-independent reference metric tensor; this geometric object is used to treat the path expansion coefficients beyond the core path, in both the partial averaging and the reweighted random series approach. Numerical tests are conducted by simulating a particle in a ring. The reweighted random series results show better convergence properties and better statistical quality at a fraction of the cost compared with the related partial averaging simulation.

Journal Article↗
PubMed2005

Partial averaging and the centroid virial estimator for stereographic projection path-integral simulations in curved spaces.

We develop and test three different partial averaging theories for the stereographic projection path integral in curved spaces. Additionally, we adapt and test the centroid virial estimator for the kinetic energy in curved spaces. We tested both a confining as well as a nonconfining potential for the particle in a ring. All three partial averaging theories are demonstrated to converge linearly in the asymptotic region with k(-2)max, the number of Fourier coefficients. We use an error estimator to determine the optimal parameters for an extrapolation to infinite kmax. We verify that the centroid virial estimator (derived from the primitive DeWitt path-integral formula) converges to the kinetic energy for both confining and nonconfining potentials.

Journal Article↗
PubMed2004

Parameter space minimization methods: applications to Lennard-Jones-dipole-dipole clusters.

The morphology of the uniform Lennard-Jones-dipole-dipole cluster with 13 centers (LJDD)13 is investigated over a relatively wide range of values of the dipole moment. We introduce and compare several necessary modifications of the basin-hopping algorithm for global optimization to improve its efficiency. We develop a general algorithm for T=0 Brownian dynamics in curved spaces, and a graph theoretical approach necessary for the elimination of dissociated states. We find that the (LJDD)13 cluster has icosahedral symmetry for small to moderate values of the dipole moment. As the dipole moment increases, however, its morphology shifts to an hexagonal antiprism, and eventually to a ring.

Journal Article↗
PubMed2004

Stereographic projections path integral for inertia ellipsoids: applications to Arn-HF clusters.

The DeWitt formula for inertia ellipsoids mapped by stereographic projection coordinates is developed. We discover that by remapping the quaternion parameter space with stereographic projections, considerable simplification of the differential geometry for the inertia ellipsoid with spherical symmetry takes place. The metric tensor is diagonal and contains only one independent element in that case. We find no difficulties testing and implementing the DeWitt formula for the inertia ellipsoids of asymmetric tops mapped by stereographic projections. The path integral algorithm for the treatment of Rm x S2 manifolds based on a mixture of Cartesian and stereographic projection coordinates is tested for small Arn-HF clusters in the n = 2 to n = 5 range. In particular, we determine the quantum effects of the red shift and the isomerization patterns at finite temperatures. Our findings are consistent with previously reported computations and experimental data for small Arn-HF clusters.

Journal Article↗
PubMed1997

Stability and chemical modification of xylanase from Aspergillus sp. (2M1 strain).

The 2M1 strain of Aspergillus sp., which showed high extracellular xylanolytic activities in a pre-screening, was studied. Oat-spelt, birch, eucalyptus and pine xylans were used as xylanolytic inductors. The following activities were found at 50 degrees C in the presence of 1% xylan: 120 units/ml (oat-spelt xylan), 132 units/ml (birch xylan), 107 units/ml (eucalyptus xylan), 67 units/ml (pine xylan) and 137 units/ml (larch-wood xylan). Xylanase induced by pine xylan exhibited a higher stability than those induced by the other xylans. The stability was improved by addition of glycerol. In the crude extract, reagents which were found to affect xylanase activity were 1-ethyl-3-(3-dimethylaminopropyl)carbodi-imide for amidation of carboxylic groups and N-bromosuccinimide at a concentration of 0.5 mM for indole oxidation. Methylene Blue, butane-2,3-dione, N-acetylimidazole, chloramine-T and iodoacetate had little effect on the enzyme activity (more than 97% of the original activity remained).

Aspergillus↗
PubMed1994

Production of extracellular xylanases by Penicillium janthinellum. Effect of selected growth conditions.

Xylanase production by Penicillium janthinellum using 10-100 mM of 2,2-dimethylsuccinate (DMS) buffer, in a range of pH 4.5-6.0 was studied. The enzyme activity was enhanced using oat xylan as the carbon source. Under these conditions a culture produced 1.14 mumol/min (11.4 U/mL or 84.4 U/mg) of beta-xylanase after 5 d of growth in a 10-mM buffer solution at pH 4.5. Protease was absent in the DMS buffer except when 100 mM phosphate buffer at pH 6.0 was used (4 U/mL). beta-Xylosidase was only found at a pH of 4.5 in all the buffer concentrations. At a 50 mM DMS buffer concentration at pH 4.5 beta-xylanases were induced by both oat and birch xylans, having a greater effect with oat spelt xylans. Electrophoretic analyses showed that the birchwood xylan induction exhibited different proteins profiles. No beta-xylosidase or beta-glucosidase was induced until d 5. The beta-xylanases were rapidly inactivated at 50 degrees C, however, birch xylanase appeared to be more stable than oat xylanase. Using oat xylan as an inductor, the beta-xylosidase and beta-glucosidase were 85 and 91 U/L, respectively, on d 7. The xylanase produced by induction from sugar cane bagasse hydrolyzate was used for pulp biobleaching. A 20% decrease on the Kappa value in Kraft pulp using the culture extract was obtained. These selective growth conditions led us to modulate the xylanase production for pulp delignification.

Avena↗
PubMed1993

Quantitative determination of chitosan and the percentage of free amino groups.

Chitosans were quantified with ninhydrin, a reagent normally used for recognizing and quantifying amino groups. The reaction was time dependent and there was no effect when different acids were added to the mixture. This method was used to determine the percentage of free amino groups in chitosans of different origins.

Amines↗