Analytical model for ion acceleration by high-intensity laser pulses

We present a general expression for the maximum ion energy observed in experiments with thin foils irradiated by high-intensity laser pulses. The analytical model is based on a radially confined surface charge set up by laser accelerated electrons on the target rear side. The only input parameters are the properties of the laser pulse and the target thickness. The predicted maximum ion energy and the optimal laser pulse duration are supported by dedicated experiments for a broad range of different ions.     

Insight into catalytically relevant correlated motions in human purine nucleoside phosphorylase

The catalytic site of the homotrimeric enzyme human purine nucleoside phosphorylase enzyme (hPNP) features residue F200 and the 241-265 loop directly skirting the purine base and a residue belonging to the adjacent monomer, F159, immediately conterminous to the ribosyl moiety. Crystallographic B-factors of apo human purine nucleoside phosphorylase, and hPNP complexed with substrate/transition state (TS) analogues, show that residue E250 is the centroid of a highly mobile loop region. Furthermore, superimposition of apo hPNP and hPNP complexed with TS analogue Immucillin-H shows a tightening of the active site, caused by the ligand-dependent 241-265 loop rearrangement taking place upon substrate/inhibitor binding, suggesting a putative dynamic role of the loop in binding/catalysis. However, crystallographic structures reveal only average atomic positions, and more detailed information is needed to discern the dynamic behavior of hPNP. The Essential Dynamics (ED) method is used here to investigate the existence of correlated motions in hPNP and consequently proposes mutagenesis assays to estimate the relative importance of these motions in the phosphorolytic efficiency of the reaction catalyzed by hPNP. We compare the concerted motions obtained from multiple molecular dynamics simulations of apo and Michaelis complex of hPNP both in vacuo and in solution. The results of the principal component analysis for the apo hPNP indicate the existence of strong correlations predominantly in the vicinity of residue F159. However, for the Michaelis complex, concerted motions are seen mostly around both active site residue F200 and loop residue E250. Additionally, for a simulation depicting the relaxation of tight complexed hPNP with a TS analogue, toward its relaxed apo form (after removal of the TS analog), a combination of the apo hPNP and Michaelis complex motions is found, with prominent concerted modes centered around neighboring residues F159, F200, and E250. Finally, we probed the extent to which these concerted motions bear an intrinsic catalytic role by performing experimental site-directed mutagenesis on some residues, followed by kinetic analysis. The F159G and F200G mutants displayed a strong increase in K(M) and modest decrease in k(cat), suggesting that these concerted motions may provide dynamical roles in substrate binding and/or catalysis. However, further structural data for the hPNP mutants are needed to confirm our hypothesis.     

Transition states for glucopyranose interconversion

Glucose is a central molecule in biology and chemistry, and the anomerization reaction has been studied for more than 150 years. Transition-state structure is the last impediment to an in-depth understanding of its solution chemistry. We have measured kinetic isotope effects on the rate constants for approach of alpha-glucopyranose to its equilibrium with beta-glucopyranose, and these were converted into unidirectional kinetic isotope effects using equilibrium isotope effects. Saturation transfer 13C NMR spectroscopy has yielded the relative free energies of the transition states for the ring-opening and ring-closing reactions, and both transition states contribute to the experimental kinetic isotope effects. Both transition states of the anomerization process have been modeled with high-level computational theory with constraints from the primary, secondary, and solvent kinetic isotope effects. We have found the transition states for anomerization, and we have also concluded that it is forbidden for the water molecule to form a hydrogen bond bridge to both OH1 and O5 of glucose simultaneously in either transition state.     

Transition-state structure of human 5'-methylthioadenosine phosphorylase

Kinetic isotope effects (KIEs) and computer modeling using density functional theory were used to approximate the transition state of human 5'-methylthioadenosine phosphorylase (MTAP). KIEs were measured on the arsenolysis of 5'-methylthioadenosine (MTA) catalyzed by MTAP and were corrected for the forward commitment to catalysis. Intrinsic KIEs were obtained for [1'-(3)H], [1'-(14)C], [2'-(3)H], [4'-(3)H], [5'-(3)H(2)], [9-(15)N], and [Me-(3)H(3)] MTAs. The primary intrinsic KIEs (1'-(14)C and 9-(15)N) suggest that MTAP has a dissociative S(N)1 transition state with its cationic center at the anomeric carbon and insignificant bond order to the leaving group. The 9-(15)N intrinsic KIE of 1.039 also establishes an anionic character for the adenine leaving group, whereas the alpha-primary 1'-(14)C KIE of 1.031 indicates significant nucleophilic participation at the transition state. Computational matching of the calculated EIEs to the intrinsic isotope effects places the oxygen nucleophile 2.0 Angstrom from the anomeric carbon. The 4'-(3)H KIE is sensitive to the polarization of the 3'-OH group. Calculations suggest that a 4'-(3)H KIE of 1.047 is consistent with ionization of the 3'-OH group, indicating formation of a zwitterion at the transition state. The transition state has cationic character at the anomeric carbon and is anionic at the 3'-OH oxygen, with an anionic leaving group. The isotope effects predicted a 3'-endo conformation for the ribosyl zwitterion, corresponding to a H1'-C1'-C2'-H2' torsional angle of 33 degrees. The [Me-(3)H(3)] and [5'-(3)H(2)] KIEs arise predominantly from the negative hyperconjugation of the lone pairs of sulfur with the sigma (C-H) antibonding orbitals. Human MTAP is characterized by a late S(N)1 transition state with significant participation of the phosphate nucleophile.     

The peninsula of neutron nuclear stability in the vicinity of N = 258

The properties of extremely neutron-excessive nuclei with Z ?? 70, including the region of transuranium elements, are calculated beyond the previously theoretically known neutron drip line (NDL). The calculations are based on the Hartree-Fock approach using Skyrme forces (SkM*, SkI2, SLy4, Ska) with allowance for axial deformation and pairings in the BCS approximation. It is shown that the series of isotones with neutron number N = 258 outside of 2n NDL forms a peninsula of stable nuclei (PSN) with respect to the emission of one neutron. For SkM* forces, a PSN is formed by ³⁴⁴Rn, ³⁴⁶Ra, ³⁴⁸Th, and ³⁵⁰U nuclides.     

Determination of P,B, and Al Concentrations in Si by Photoluminescence (PL)

Calibration curves to determine P, B, and Al concentrations in silicon at 4.2 K from the intensity ratios of bound (BE) and free (FE) exciton related luminescence are presented. The integral relative intensities of the peaks are used besides the peak height ratios. By means of a large number of samples with widely varying concentrations it is shown that the influence of the compensation ratios of the samples on the determination of the dopant concentrations is smaller than the measurement errors. Integral intensity ratios of different BE phonon branches are also derived.     

Comparison of P- and B-Contents in Si Determined by Photoluminescence (PL) at 4.2 K and 2.0 K

Calibration curves to determine the concentrations of shallow impurities in Si were usually established with the help of homogeneous doped samples at 4.2 K. The improvement of the lateral resolution power necessary to characterize inhomogeneous samples can be conveniently achieved by the use of superfluid helium. The calibration curves required for it were established at 2.0 K for P- and B-doped Si and compared with the dependences measured at 4.2 K. Existing differences between these curves are discussed.