Proton transfer reaction studies of multiply charged proteins in a high mass-to-charge ratio quadrupole mass spectrometer

Proton transfer reaction of multiply charged ions at high mass-to-charge ratios were explored with a low frequency quadrupole mass spectrometer. This instrument enabled a qualitative comparison of proton transfer reaction rates at low charge states for ions generated by electrospray ionization (ESI) from different solution conformations and for disulfide-linked versus disulfide-reduced protein ions. Proton transfer reactions that efficiently reduced the number of charges for ESI-generated ions to approximately the number of arginines in the polypeptide sequence were observed. No significant differences in gas-phase reaction rates were noted between different solution conformers. Differences in reaction rates between “native” and disulfide-reduced proteins were much smaller than those observed below m/z 2000 with lower proton affinity reagents or by using lower reagent concentrations. These smaller differences in reaction rates are thought to reflect the reduced electrostatic contributions from widely spaced charge sites and thus, the reduced sensitivity to an ion's three-dimensional structure or “compactness.”     

The first total synthesis of lipid II: the final monomeric intermediate in bacterial cell wall biosynthesis

Bacterial peptidoglycan is composed of a network of beta-[1,4]-linked glyan strands that are cross-linked through pendant peptide chains. The final product, the murein sacculus, is a single, covalently closed macromolecule that precisely defines the size and shape of the bacterial cell. The recent increase in bacterial resistance to cell wall active agents has led to a resurgence of activity directed toward improving our understanding of the resistance mechanisms at the molecular level. The biosynthetic enzymes and their natural substrates can be invaluable tools in this endeavor. While modern experimental techniques have led to isolation and purification of the biosynthetic enzymes utilized in peptidoglycan biosynthesis, securing useful quantities of their requisite substrates from natural substrates has remained problematic. In an effort to address this issue, we report the first total synthesis of lipid II (4), the final monomeric intermediate utilized by Gram positive bacteria for peptidoglycan biosynthesis.     

Fine structure in proton emission from 145Tm discovered with digital signal processing

Fine structure in proton emission from the 3.1(3) mus activity of 145Tm was discovered by using a novel technique of digital processing of overlapping recoil implantation and decay signals. Proton transitions to the ground state of 144Er and to its first excited 2(+) state at 0.33(1) MeV with a branching ratio I(p)(2(+))=9.6+/-1.5% were observed. The structure of the 145Tm wave function and the emission process were analyzed by using particle-core vibration coupling models.     

Alpha decay of 109I and its implications for the proton decay of 105Sb and the astrophysical rapid proton-capture process

An alpha-decay branch of (1.4+/-0.4) x 10(-4) has been discovered in the decay of 109I, which predominantly decays via proton emission. The measured Q(alpha) value of 3918+/-21 keV allows the indirect determination of the Q value for proton emission from 105Sb of 356+/-22 keV, which is approximately of 130 keV more bound than previously reported. This result is relevant for the astrophysical rapid proton-capture process, which would terminate in the 105Sn(p,gamma)106Sb(p,gamma)107Te(alpha decay)103Sn cycle at the densities expected in explosive hydrogen burning scenarios, unless unusually strong pairing effects result in a 103Sn(p,gamma)104Sb(p,gamma)105Te(alpha decay)101Sn) cycle.     

Target isotope effect in high energy photon production atE/A=10 MeV

A comparison has been made between the production of high energy (E _γ≧30 MeV)γ rays from the bombardment of¹²⁴Sn and¹¹²Sn targets withE/A= 10 MeV¹²C ions. The results are well explained by then-p bremsstrahlung model and do not indicate the need for any new processes.     

Force-field dependence of the conformational properties of α,ω-dimethoxypolyethylene glycol

A molecular dynamics (MD) study of α,ω-dimethoxypolyethylene glycol has been carried out under various conditions with respect to solvent composition, ionic strength, chain length, force field and temperature. A previous MD study on a 15-mer of polyethyleneglycol (PEG) suggested a helical equilibrium structure that was stabilised by hydrogen bonding and bridging water molecules. Experiments show that PEG is highly soluble in water, and indicate that clustering is not favoured. In the present study using different force fields, the GROMOS force fields 45A3 and 53A6, a variation on the latter 53A6_OE, and a force field by Smith et al. produced different results. For the GROMOS force fields 45A3 and 53A6 no helical structure was found, but formation of more or less compact random coils in aqueous solution due to hydrophobic interactions was observed. For the other two force fields used, α,ω-dimethoxypolyethylene glycol stayed flexible and more or less elongated in aqueous solution, more in agreement with experimental observations and the previous MD study.     

Calculation of sequence‐dependent free energies of hydration of dipeptides formed by alanine and glycine

The relative free energies of hydration of the dipeptides glycylalanine and alanyl‐glycine in their naturally occurring form have been calculated both for the zwitterionic and protonated species. Emphasis was laid on comparisons between the conventional cutoff method and the Particle Mesh Ewald method to account for possible differences in electrostatic contributions to the free energy. Furthermore, the convergence behavior of the total free energy and its individual contributions were examined. The results, obtained by means of the thermodynamic integration technique as implemented in the free energy module of the AMBER program suite, suggest that in aqueous solution glycylalanine is more stable than alanylglycine by 2.7 kcal/mol in the zwitterionic form and by 3.5 kcal/mol in the protonated form. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 846–860, 2001