Hydrocarbon acidities calculated with MINDO/3, MNDO,and AM1

Acidities of 32 hydrocarbons have been calculated using MINDO/3, MNDO, and AM1. All three semiempirical procedures have systematic errors and reproduce experimental acidities poorly. A linear correlation, however, does exist between the calculated and experimental results. Correction of the AM1 or MNDO acidities leads to good agreement with literature values even for acids, such as methane and ethylene, whose conjugate bases are small localized anions. Predictions for several hydrocarbons are given.     

Evaluation of multidimensional (ion-exchange/reversed-phase) protein separations using linear and step gradients in the first dimension

The performance characteristics of multidimensional liquid chromatographic protein separations were evaluated using on-line electrospray mass detection, and a novel workflow for automated LC/MS data processing. Two-dimensional ion exchange/reversed-phase LC separations of Escherichia coli cytosol were conducted using either a continuous linear or discontinuous step gradient in the first dimension. Chromatographic profiles of the top 100 most abundant components were characterized to assess overall separation reproducibility within each mode, and to characterize differences in component distribution between the two modes of operation. Analysis of the resulting data indicates that multidimensional separations of complex protein mixtures can be done reproducibly. Furthermore, under the conditions employed within this study, a linear first dimension gradient was more effective at fractionating the protein mixture, distributing fewer major components to multiple second dimension cycles than an equivalent step gradient. The application of on line mass spectrometry, and automated processing of the resulting data, proved valuable for producing component level analysis of multidimensional protein separations.