Suchergebnisse

El hecho político de mayor importancia en la coyuntura actual en México es la debilidad del gobierno del presidente Vicente Fox. Se trata de un gobierno débil en varios sentidos: no ha podido poner en práctica el programa de reformas anunciado; no ha conseguido materializar casi ninguna de las promesas fundamentales de su campaña; se ha mostrado incapaz de reactivar la economía; carece de recursos políticos para controlar los conflictos sociales que comienzan a adquirir formas violentas cada vez con mayor frecuencia. Dicha debilidad obedece a varios factores: inexperiencia y falta de capacidad administrativa del equipo de gobierno; falta de una mayoría en el Congreso; desaparición de los mecanismos tradicionales de gestión política del Partido Revolucionario Institucional –PRI-. El resultado es una percepción social cada vez más negativa de la gestión del gobierno y un descenso muy considerable en su popularidad, es decir: un proceso de desencanto que contribuye a acentuar todavía más la debilidad, porque resta apoyo social al presidente.

Analytical ultracentrifugation (AUC) is a first principles based method to determine absolute sedimentation coefficients and buoyant molar masses of macromolecules and their complexes, reporting on their size and shape in free solution. The purpose of this multi-laboratory study was to establish the precision and accuracy of basic data dimensions in AUC and validate previously proposed calibration techniques. Three kits of AUC cell assemblies containing radial and temperature calibration tools and a bovine serum albumin (BSA) reference sample were shared among 67 laboratories, generating 129 comprehensive data sets. These allowed for an assessment of many parameters of instrument performance, including accuracy of the reported scan time after the start of centrifugation, the accuracy of the temperature calibration, and the accuracy of the radial magnification. The range of sedimentation coefficients obtained for BSA monomer in different instruments and using different optical systems was from 3.655 S to 4.949 S, with a mean and standard deviation of (4.304 ± 0.188) S (4.4%). After the combined application of correction factors derived from the external calibration references for elapsed time, scan velocity, temperature, and radial magnification, the range of s-values was reduced 7-fold with a mean of 4.325 S and a 6-fold reduced standard deviation of ± 0.030 S (0.7%). In addition, the large data set provided an opportunity to determine the instrument-to-instrument variation of the absolute radial positions reported in the scan files, the precision of photometric or refractometric signal magnitudes, and the precision of the calculated apparent molar mass of BSA monomer and the fraction of BSA dimers. These results highlight the necessity and effectiveness of independent calibration of basic AUC data dimensions for reliable quantitative studies. ; This work was supported by the Intramural Research Programs of the National Institute of Biomedical Imaging and Bioengineering, the National Heart, Lung, and Blood ...