Top down characterization of larger proteins (45 kDa) by electron capture dissociation mass spectrometry.

The structural characterization of proteins expressed from the genome is a major problem in proteomics. The solution to this problem requires the separation of the protein of interest from a complex mixture, the identification of its DNA-predicted sequence, and the characterization of sequencing errors and posttranslational modifications. For this, the "top down" mass spectrometry (MS) approach, extended by the greatly increased protein fragmentation from electron capture dissociation (ECD), has been applied to characterize proteins involved in the biosynthesis of thiamin, Coenzyme A, and the hydroxylation of proline residues in proteins. With Fourier transform (FT) MS, electrospray ionization (ESI) of a complex mixture from an E. coli cell extract gave 102 accurate molecular weight values (2-30 kDa), but none corresponding to the predicted masses of the four desired enzymes for thiamin biosynthesis (GoxB, ThiS, ThiG, and ThiF). MS/MS of one ion species (representing approximately 1% of the mixture) identified it with the DNA-predicted sequence of ThiS, although the predicted and measured molecular weights were different. Further purification yielded a 2-component mixture whose ECD spectrum characterized both proteins simultaneously as ThiS and ThiG, showing an additional N-terminal Met on the 8 kDa ThiS and removal of an N-terminal Met and Ser from the 27 kDa ThiG. For a second system, the molecular weight of the 45 kDa phosphopantothenoylcysteine synthetase/decarboxylase (CoaBC), an enzyme involved in Coenzyme A biosynthesis, was 131 Da lower than that of the DNA prediction; the ECD spectrum showed that this is due to the removal of the N-terminal Met. For a third system, viral prolyl 4-hydroxylase (26 kDa), ECD showed that multiple molecular ions (+98, +178, etc.) are due to phosphate noncovalent adducts, and MS/MS pinpointed the overall mass discrepancy of 135 Da to removal of the initiation Met (131 Da) and to formation of disulfide bonds (2 x 2 Da) at C32-C49 and C143-C147, although 10 S-S positions were possible. In contrast, "bottom up" proteolysis characterization of the CoaBC and the P4H proteins was relatively unsuccessful. The addition of ECD substantially increases the capabilities of top down FTMS for the detailed structural characterization of large proteins.     

Approach to Equilibrium of a Body Colliding Specularly and Diffusely with a Sea of Particles

We consider a rigid body acted upon by two forces, a constant force and the collective force of interaction with a continuum of particles. We assume that some of the particles that collide with the body reflect elastically (specularly), while others reflect probabilistically with some probability distribution K. We find that the rate of approach of the body to equilibrium is O(t ^?3?p ) in three dimensions where p can take any value in (0, 2], depending on K.     

Linear Stability Analysis of a Hot Plasma in a Solid Torus

This paper is a first step toward understanding the effect of toroidal geometry on the rigorous stability theory of plasmas. We consider a collisionless plasma inside a torus, modeled by the relativistic Vlasov–Maxwell system. The surface of the torus is perfectly conducting and it reflects the particles specularly. We provide sharp criteria for the stability of equilibria under the assumption that the particle distributions and the electromagnetic fields depend only on the cross-sectional variables of the torus.     

Co-crystallization of octahedral and icosahedral fluoroanions in K3(AsF6)(B12F12) and Cs3(AsF6)(B12F12). Rare examples of salts containing fluoroanions with different shapes and charges

Crystals grown from anhydrous HF solutions of CsAsF₆ and Cs₂B₁₂F₁₂ or KAsF₆ and Cs₂B₁₂F₁₂ are shown by Raman spectroscopy and single-crystal X-ray diffraction to be the ternary salts K₃(AsF₆)(B₁₂F₁₂) or Cs₃(AsF₆)(B₁₂F₁₂). Both compounds exhibit a modified version of the anti-perovskite structure. They are rare examples of crystals that simultaneously contain octahedral and icosahedral molecular species and are also rare examples of salts containing fluoroanions with different shapes and charges. The crystallographic results show that both compounds are densely packed.     

A new approach to spectral approximation

A new technique for approximating eigenvalues and eigenvectors of a self-adjoint operator is presented. The method does not incur spectral pollution, uses trial spaces from the form domain, has a self-adjoint algorithm, and exhibits superconvergence.