Thermodynamic response functions of fluids: a microscopic approach based on NpT Monte Carlo.

On the basis of NpT Monte Carlo simulations, a detailed analysis on the microscopic origins of some specific features of thermodynamic response functions of fluids is performed. Specifically, the residual isobaric heat capacity C(p) (res), the isobaric thermal expansivity alpha(p), and the isothermal compressibility kappa(T) for Lennard-Jones methane and optimized potential for liquid simulations (OPLS) methanol have been determined via standard techniques. For the former, data along the liquid, gas, and supercritical regions are presented, while a wide temperature range at a single supercritical pressure is covered for the latter. They have been obtained by computing the various pairwise fluctuations contributing to each property. Attention is mainly focused on isothermal and isobaric maxima found for both C(p) (res) and alpha(p), which have been rationalized at a molecular level using qualitative arguments. It is encountered that maxima emerge as a natural consequence of the destruction of fluid structure as temperature is increased or as pressure is decreased. The results for Lennard-Jones methane reveal the competition of energetic and volumetric effects, while those for OPLS methanol evidence that hydrogen-bonding is dominant as energetic effects are concerned. Further discussion on previous results and alternative approaches using equations of state as well as on closely related topics such as "maxima and critical phenomena" is included.

Heat capacity of associated systems. Experimental data and application of a two-state model to pure liquids and mixtures.

The predictions from a recently reported (J. Chem. Phys. 2004, 120, 6648) two-state association model (TSAM) have been tested against experimental data. The temperature, T, and pressure, p, dependence of the isobaric heat capacity, C(p), for three pure alcohols and the temperature dependence at atmospheric pressure of the excess heat capacity, C(p)(E), for four alcohol + ester mixtures have been measured. The branched alcohols were 3-pentanol, 3-methyl-3-pentanol, and 3-ethyl-3-pentanol, and the mixtures were 1-butanol and 3-methyl-3-pentanol mixed with propyl acetate and with butyl formate. These data, together with literature data for alcohol + n-alkane and alcohol + toluene mixtures, have been analyzed using the TSAM. The model, originally formulated for the C(p) of pure liquids, has been extended here to account for the C(p)(E) of mixtures. To evaluate its performance, quantum mechanical ab initio calculations for the H-bond energy, which is one of the model parameters, were performed. The effect of pressure on C(p) for pure liquids was elucidated, and the variety of C(p)(E)(T) behaviors was rationalized. Furthermore, from the C(p) data at various pressures, the behavior of the volume temperature derivative, (deltaV/deltaT)(p), was inferred, with the existence of a (deltaV/deltaT)(p) versus T maximum for pure associated liquids such as the branched alcohols being predicted. It is concluded that the TSAM captures the essential elements determining the behavior of the heat capacity for pure liquids and mixtures, providing insight into the macroscopic manifestation of the association phenomena occurring at the molecular level.

Atrofia muscular bulbo espinhal recessiva ligada ao cromossomo X (Doença de Kennedy) / Recessive bulbospinal muscular atrophy X-linked (Kennedy's disease)

A doença de Kennedy (DK) é forma rara de doença do neurônio motor caracterizada por mutaçao na regiao codificadora do gene do receptor androgênico localizado no braço longo do cromossoma X (Xq 11-12). Há expansao das sequências de trinucleotídeos CAG que nos pacientes deve atingir número maior do que 347 repetiçoes de pares de bases. Apresentamos quatro geraçoes de uma família com dez indivíduos acometidos. Avaliamos três pacientes do sexo masculino com idade variando entre 50 e 60 anos que desenvolveram sintomatologia por volta de 30 anos de idade caracterizada por fraqueza muscular progressiva associada a disfagia e disartria. O exame demonstrou ginecomastia, atrofia testicular, amiotrofia, fasciculaçoes, paresia, aboliçao de reflexos e tremor postural. A análise do DNA pela técnica do PCR demonstrou número de repetiçoes CAG aumentado no locus Xq 11-12 nos três pacientes e em uma mulher assintomática da família. Demonstramos a primeira família brasileira com diagnóstico de DK através de genética molecular. A DK deve fazer parte do diagnóstico diferencial das doenças do neurônio motor e a identificaçao destes pacientes é importante para o prognóstico e para o aconselhamento genético.