Disentangling the reaction mechanisms of weakly bound nuclei

Complete and incomplete fusion cross sections for the ⁹Be + ¹⁴⁴Sm reaction have been measured at near-barrier energies, using the delayed X-ray detection technique. At above-barrier energies these show a suppression of complete fusion for this weakly bound projectile on an intermediate mass target. The suppression factor, attributed to ⁹Be break-up, was deduced from a comparison of complete fusion yields with coupled-channels calculations, and appears consistent with measured incomplete fusion product yields. At ∼10%, it is considerably smaller than the value of ∼30% previously found for a ²⁰⁸Pb target. Simultaneous measurements of elastic and inelastic scattering permit a clearer picture of the reaction mechanisms.     

Structural characterization of Acetobacter diazotropicus levansucrase by matrix-assisted laser desorption/ionization mass spectrometry: identification of an N-terminal blocking group and a free-thiol cysteine residue

High-resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to characterize the primary structure of the levansucrase (EC 2.4.1.10) secreted by Acetobacter diazotropicus SRT4. The technique permitted not only the reading frame of this enzyme, the amino acid sequence of which was deduced from DNA, but also the elucidation of an N-terminal blocking group and the position of a disulfide bridge between Cys309 and Cys365 among the three Cys residues. A free cysteine (Cys127) was identified by modifying an intact molecule with a sulfhydryl reagent, 5-(octyldithio)-2-nitrobenzoic acid, under non-reducing conditions. In addition, the enzyme obtained by site-directed mutagenesis at Asp279 to Asn279 was also identified by the above methods. Post-source decay analysis of the tryptic peptide containing the mutation site unequivocally revealed an Asn residue at position 279.     

Copper and iron interactions with angiotensin-converting enzyme inhibitors. A study by fast-atom bombardment tandem mass spectrometry

Fast atom bombardment, combined with high-energy collision-induced tandem mass spectrometry, has been used to investigate gas-phase metal-ion interactions with captopril, enalaprilat and lisinopril, all angiotensin-converting enzyme inhibitors.Suggestions for the location of metal-binding sites are presented. For captopril, metal binding occurs most likely at both the sulphur and the nitrogen atom. For enalaprilat and lisinopril, binding preferably occurs at the amine nitrogen. Copyright 1999 John Wiley & Sons, Ltd.     

Selective isolation-detection of two different positively charged peptides groups by strong cation exchange chromatography and matrix-assisted laser desorption/ionization mass spectrometry: application to proteomics studies

We report here a procedure for the independent analysis of two groups of peptides by liquid chromatography-matrix-assisted laser desorption/ionization mass spectrometry (LC-MALDI MS/MS), using a selective isolation-detection procedure. In this procedure all primary amino groups of tryptic peptides derived from mouse liver proteins are blocked, restricting their positive charge, at acidic pH, to the presence of histidine and arginine residues. After strong cation exchange chromatography, multiply charged peptides (R + H > 1) are retained on the column and separated with high selectivity from singly (R + H = 1) and neutral peptides (R + H = 0) which are together collected in the flow-through. Using LC-MALDI-MS/MS analysis, the retained fraction displayed a 94% of enrichment of multiply charged peptides while in the flow-through; peptides with at least one arginine or histidine residue were exclusively identified, which suggests that MS detection in this fraction is restricted only to those peptides with ionizable side chains, arginine and histidine amino acids.     

Thermodynamics of the second-stage dissociation of n-(2-acetamido)iminodiacetic acid in water from 5 to 55°c

The thermodynamic second dissociation constants of the protonated form of N-(2-acetamido)iminodiacetic acid were determined at 12 temperatures from 5–55°C by measurement of the electromotive force using a cell without liquid junction, with hydrogen and silver—silver bromide electrodes. At 25°C, pK₂is 6.844. The standard changes in Gibbs energy, enthalpy, entropy and heat capacity were derived from the change of the pK₂ values with temperature. At 25°C, ΔG° = 9335 cal mol^-1, ΔH° = 2928 cal mol^-1, ΔS^o = -21.5 cal K^-1 mol^-1, and ΔC°_p = -34 cal K^-1 mol^-1. The results are interpreted and compared with those of structurally related compounds.