The adsorption of large proteins in electrofocusing on immobilized pH gradients: II. Dependence on the oligomeric state of Immobiline

Five proteins with molecular mass in excess of 200 kDa were found to adsorb onto gels during isoelectric focusing on immobilized pH gradients (IPGEF). To probe for the mechanism of that adsorption, the homogeneity of the six Immobiline preparations used to make IPGEF gels was tested. Five of these Immobiline preparations appear homogeneous in gel filtration of Sephadex G-10. The sixth Immobiline (pK 9.3) exhibits a minor component eluting ahead of the major peak and comprising less than 4% of the total Immobiline absorbing at 226 nm. The proportion of the minor component increases with column load. Major and minor components when isolated appear to equilibrate with one another. Judging by the results of mass spectrometry, all 6 preparations are free of small aggregates of less than 500-600 Da molecular mass. Ultrafiltration of the Immobiline preparations through a membrane with 500 Da nominal cutoff leads to partial desorption of only 3 of the 5 adsorbed proteins. CHAPS is ineffectual in desorbing the 5 proteins from the IPG gel made with ultrafiltered Immobilines. None of the 6 Immobiline preparations used precipitates ferritin. All large proteins that adsorb onto IPGEF gels in the pH range 4-9.5 also adsorb onto commercial IPGEF gels in the pH range 4-7.     

Geometric and electronic structures of peroxomanganese(III) complexes supported by pentadentate amino-pyridine and -imidazole ligands

Three peroxomanganese(III) complexes [Mn(III)(O(2))(mL(5)(2))](+), [Mn(III)(O(2))(imL(5)(2))](+), and [Mn(III)(O(2))(N4py)](+) supported by pentadentate ligands (mL(5)(2) = N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine, imL(5)(2) = N-methyl-N,N',N'-tris((1-methyl-4-imidazolyl)methyl)ethane-1,2-diamine, and N4py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) were generated by treating Mn(II) precursors with H(2)O(2) or KO(2). Electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD data demonstrate that these complexes have very similar electronic transition energies and ground-state zero-field splitting parameters, indicative of nearly identical coordination geometries. Because of uncertainty in peroxo (side-on η(2) versus end-on η(1)) and ligand (pentadentate versus tetradentate) binding modes, density functional theory (DFT) computations were used to distinguish between three possible structures: pentadentate ligand binding with (i) a side-on peroxo and (ii) an end-on peroxo, and (iii) tetradentate ligand binding with a side-on peroxo. Regardless of the supporting ligand, isomers with a side-on peroxo and the supporting ligand bound in a tetradentate fashion were identified as most stable by >20 kcal/mol. Spectroscopic parameters computed by time-dependent (TD) DFT and multireference SORCI methods provided validation of these isomers on the basis of experimental data. Hexacoordination is thus strongly preferred for peroxomanganese(III) adducts, and dissociation of a pyridine (mL(5)(2) and N4py) or imidazole (imL(5)(2)) arm is thermodynamically favored. In contrast, DFT computations for models of [Fe(III)(O(2))(mL(5)(2))](+) demonstrate that pyridine dissociation is not favorable; instead a seven-coordinate ferric center is preferred. These different results are attributed to the electronic configurations of the metal centers (high spin d(5) and d(4) for Fe(III) and Mn(III), respectively), which results in population of a metal-peroxo σ-antibonding molecular orbital and, consequently, longer M-O(peroxo) bonds for peroxoiron(III) species.     

Thermospray liquid chromatography/mass spectrometry of quaternary ammonium saltst

Various esters of choline and carnitine have been shown to undergo fragmentation dependent on both the structures of the molecules and the vapor and block temperatures when subjected to thermospray ionization mass spectrometry. The fragments observed vary with the structure of the ester portion and the backbone of the molecules, and are consistent with the compounds undergoing three types of reactions: (1) carbon—nitrogen bond cleavage to tertiary amines and alkyl compounds, (2) hydrolysis of the ester linkage, forming alcohols and organic acids, and (3) deacylation of the molecules to form acids and the corresponding quaternary alkene. The general applicability of the carbon—nitrogen bond cleavage mechanism was studied with other classes of quaternary compounds, but the sole products consistently detected for pyridinium, benzyldimethyldodecylammonium and tetraalkylammonium salts were amines. The fragments observed for the esters of choline and carnitine and also the anticholinergic 2,2-diphenyl-4-diisopropylaminobutyramide methiodide suggest that the mechanism of the thermospray fragmentation pathway is similar to that of the Hofmann elimination reaction.     

Free mobility determination by electrophoresis in polyacrylamide containing agarose at a nonrestrictive concentration

In the determination of the free mobility, related to the surface net charge, by quantitative gel electrophoresis, the previous arbitrary extrapolation of Ferguson plots from the lowest gel concentrations that give a mechanically stable gel to 0% T has recently been replaced by measurement of mobilities across that concentration range, using the addition of 0.5% agarose to polyacrylamide at the various low concentrations in application to a DNA fragment 155 bp in size (Orbán, L. et al., in preparation). The present study applies that approach to several proteins and DNA fragments smaller than 1300 bp, using 0.4% agarose in polyacrylamide gels of varying concentration. The intercepts of the plots with the mobility axis provide experimental data by which the free mobility in polyacrylamide gel electrophoresis can be estimated for molecules not significantly retarded in their migration at the agarose concentration admixed to polyacrylamide. Across the gel concentration range below 3% T, in the presence of agarose, the Ferguson plots of proteins and DNA fragments are convex. It was shown by mass spectrometry that this convex curvature of the plots in the mixed polymer is not significantly due to low polymerization efficiency in the concentration range of liquid polyacrylamide (below 3%T).     

A Kinetic Model for the Acid-Catalyzed Decomposition of Delrin

Abstract

A kinetic model is derived for the acid-catalyzed decomposition of Delrin, an acetal-formaldehyde resin. The kinetic model proposed assumes the hydrolysis of Delrin to form polyformaldehyde and the subsequent decomposition of polyformaldehyde to formaldehyde.